The microscopic structure of water
Water is a very polar molecule due to the significant electronegativity difference between its
elements. Its shape allows for two exposed lone pairs of electrons on the oxygen atom,
enhancing its polarity.
Water molecules undergo hydrogen bonding. A hydrogen bond is a special type of
dipole-dipole attraction between molecules. It arises from the attractive force between a
hydrogen atom covalently bonded to a highly electronegative atom such as N, O, or F.
A hydrogen bond forms between the negative oxygen side of one molecule and the positive
hydrogen side of another molecule.
The effect of hydrogen bonding on the unique physical
properties of water
Water is used all the time, every day, but many of us are unaware of the unusual and unique
properties of water, including:
➔​ Boiling point and freezing point
➔​ Surface tension and vapour pressure
➔​ Viscosity and cohesion
➔​ Specific heat capacity
Boiling and melting point
Look at the trends in the boiling points of some hydrides formed with elements in Group
16(VI). The hydride for tellurium: H2Te (hydrogen telluride) has a boiling point of -4 °C.
Moving up the periodic table, the next hydride is H2Se (hydrogen selenide) with a boiling
point of -42 °C. Further up is H2S (hydrogen sulfide) with a boiling point of -62 °C. The
hydride of oxygen, H2O (water), surprisingly has a much higher boiling point of 100 °C.
Key terms
Melting point - The temperature at which the solid and liquid phases of a substance are at
equilibrium.
Boiling point - The temperature at which the vapour pressure of a substance equals
atmospheric pressure.
Heat of vaporisation is the energy that is needed to change a given quantity of a substance
into a gas.
Surface tension and vapour pressure
Besides mercury, the only metal that's liquid at room temperature, water has the highest
surface tension among liquids. This is due to hydrogen bonding in water molecules. Water
also has a high heat of vaporization.
Vaporization occurs when a liquid becomes a gas, classifying it as an endothermic reaction.
Vapour pressure is inversely related to intermolecular forces; stronger intermolecular forces
result in lower vapour pressure. Water has strong intermolecular forces and a low vapour
pressure, meaning it's not easy for water to become a gas.
Viscosity
Viscosity is the property of a fluid that resists flow. While we often think of honey or motor oil
as viscous, water also has noticeable viscosity. Liquids with strong intermolecular forces are
typically more viscous than those with weaker interactions.
Solid State (ICE)
Substances, including water, become less dense when heated and denser when cooled.
Thus, cooling water makes it dense, forming ice. Water is special because its solid state
floats on its liquid state!
Why? This is due to water's unique shape. As water freezes, molecules slow down, forming
hydrogen bonds and a structured arrangement. Water’s bent shape means its solid form has
spaces between molecules, increasing volume by about 9%. This structure makes ice less
dense than liquid water, where molecules are packed closer. For example, a can of coke can
explode in a freezer as the water in the coke expands when frozen.
Substances, including water, become less dense when heated and denser when cooled.
Thus, cooling water makes it dense, forming ice. Water is special because its solid state
floats on its liquid state!
Why? This is due to water's unique shape. As water freezes, molecules slow down, forming
hydrogen bonds and a structured arrangement. Water’s bent shape means its solid form has
spaces between molecules, increasing volume by about 9%. This structure makes ice less
dense than liquid water, where molecules are packed closer. For example, a can of coke can
explode in a freezer as the water in the coke expands when frozen.
Water as the "universal solvent"
Water is a very polar molecule (dipole) that can dissolve or dissociate many particles.
Oxygen carries a slightly negative charge, while the two hydrogens carry a slightly positive
charge. Negative particles of another compound are attracted to water's hydrogen atoms,
and positive particles are attracted to water's oxygen molecule, causing dissociation. This
ability allows many ionic compounds and polar covalent substances to dissolve in water,
making it essential for making coffee, most cold drinks, and preparing medicines.
Recap of water's important properties
Here is a list of the important properties of water dealt with in the last lesson.
➔​ Water is the only substance on Earth that naturally exists as solid, liquid, and gas.
These changes require heat exchange, crucial for spreading thermal energy in the
atmosphere.
➔​ Water's high specific heat allows it to absorb much heat before warming and releases
it slowly as it cools. This moderates Earth's climate and aids temperature regulation
in organisms.
➔​ Water conducts heat effectively, second only to mercury, resulting in a large body of
water having a uniform temperature.
➔​ As a universal solvent, water dissolves many compounds, allowing it to carry
nutrients for organisms.
Water's high surface tension helps form droplets, allowing water and nutrients to move
through plants and small blood vessels in animals.
Specific heat capacity
The sun is responsible for warming up the Earth through the process of radiation, but if it
wasn’t for water the effect on the sun's radiation would be more dramatic.
Key term
Specific heat capacity is defined by the amount of heat needed to raise the temperature of 1
gram of a substance 1 degree Celsius (°C).
Water has a high specific heat capacity, meaning it takes more energy to increase its
temperature than many other substances. If we heat a pot of water, which heats first: the pot
or the water? The pot heats up faster! Despite applying the same amount of heat, the pot
responds quicker. This is because water requires more energy to change temperature.
Water exists in three phases on Earth: liquid, solid, and gas. A significant amount of energy
is needed to transition between these phases due to water’s high heat capacity.
If we leave a bucket of water in the summer sun, it will warm up, but not boil an egg. Walking
barefoot on black tar roads in South African summers would burn our feet. Dropping an egg
on a car hood on a very hot day might fry it. Metals have a much lower specific heat than
water.
Water allows organisms to endure various weather conditions. Ice expands as it freezes,
which is why it floats on water. In winter, when a lake starts to freeze, the surface forms ice
and moves deeper, allowing skating and support on frozen lakes. If ice sank, lakes would
freeze bottom-up, killing ecosystems.
Because ice floats, fish survive under it in winter. The ice surface insulates the lake from cold
air, maintaining water temperature suitable for lake ecosystems.
https://youtu.be/v9hm1Ym9E-8
Electromagnetic radiation
Light is electromagnetic radiation detectable by our eyes, carried by electromagnetic waves.
These waves travel at light speed (symbol c), which is 3 x 108 m.s-1 (300 000 000 m.s-1) in
a vacuum. Einstein stated nothing exceeds this speed. The visible spectrum has
wavelengths from 400–700 nanometers.
Other electromagnetic radiation includes microwaves, X-rays, radio waves, and ultraviolet
light. These varieties differ in wavelengths and frequencies.
Electromagnetic waves occur when a charged particle accelerates, creating an electric field
that produces a perpendicular magnetic field, and vice versa. The interaction of these fields
propagates the wave.
The electromagnetic spectrum
The electromagnetic spectrum includes a range of waves with different frequencies and
wavelengths, encompassing both visible and large invisible components. Frequency and
wavelength share a special relationship.
A proportionality compares two quantities. An inversely proportional relationship means as
one increases, the other decreases proportionally. As frequency increases, wavelength
decreases at a constant speed of 3x 108m.s-1.
Thus, frequency and wavelength are inversely proportional.
fαλ
where:
f is the frequency
λ is the wavelength.
These waves will range from extremely low frequencies to extremely high frequencies.
The following is the range of electromagnetic waves in order of increasing frequency (or
decreasing wavelength).
➔​ Radio waves (lowest frequency / highest wavelength)
➔​ Microwaves
➔​ Infra-red
➔​ Visible light
➔​ Ultraviolet
➔​ X-rays
➔​ Gamma rays (highest frequency / lowest wavelength)
Figure 1: The electromagnetic Spectrum in order of increasing frequency
Light has all the properties of waves:
➔​ Light travels in straight lines
➔​ Light is reflected
➔​ Light is refracted
➔​ Light is diffracted
Where does light come from?
Light comes from a luminous source like the sun, a light bulb, or a candle.
We also perceive light when it reflects off surfaces, making them illuminated sources, e.g.,
the moon, a mirror, or any visible object. Light is an electromagnetic wave with dual
characteristics: wave and particle nature.
Why is light important?
Light is a form of energy (radiant energy) that plants use to make food in the process of
photosynthesis. No life could exist on Earth without this process.
Light rays
In geometrical optics, which models light travel, light is described by rays. This accurately
represents wave behavior. Geometric optics covers reflection, refraction, and diffraction.
What happens when light strikes an object?
All objects are transparent, translucent, or opaque. Transparent objects let light pass through
with minimal resistance, allowing clear visibility.
Translucent objects let some light through but diffuse or scatter it. Visibility is unclear.
Examples: Some plastics, coloured glass, cotton cloth.
Opaque objects absorb or reflect light, blocking it entirely. Visibility is impossible. Examples:
Mirrors, wood, metal.
Key term
A light ray is a hypothetical model of light, where light is drawn as a straight line with arrows
to represent direction.
A ray diagram is simply a diagram that shows the path of light rays
Figure 1: Ray diagram of light from the bulb to eye.
Key terms
Incident ray - the ray of light coming towards the surface.
Reflected ray - the ray of light moving away from the surface
Normal - The line, which is perpendicular to the plane of the surface.
Angle of incidence (θi) - The angle between the normal to a surface and the incident light ray
Angle of reflection (θr) - The angle between the normal to a surface and the refracted light
ray.
Figure 2: Light reflecting off a mirror.
There are two rules associated with reflection
The angle of incidence is equal to the angle of reflection
The normal, the angle of incidence and angle of reflection all lie on the same plane (nothing
goes through the surface
Applications of reflection in daily life
A parabolic reflector
A parabolic reflector is a mirror or dish with a parabolic shape (similar to the parabola in
mathematics). Parabolic reflectors are used in car headlamps, torches, and telescopes.
Figure 5: Parabolic mirror of a headlamp.
Fun fact
South Africa is a world leader in astronomy and we have some remarkable telescopes.
Gigantic galaxies have been discovered with the MeerKAT telescope which is found near the
small Northern Cape town of Carnarvon.
The SKA (Square Kilometer Array) project aims to create the world’s largest radio telescope,
with a square kilometre of collecting area. It will be situated in Africa and Australia. The SKA
will offer an unmatched scope in observations, surpassing the Hubble Space Telescope's
image resolution by 50 times and can image vast sky areas.
Binoculars and Microscopes
These are apparatuses you might have seen before. Binoculars magnify distant objects,
using glass prisms that act as mirrors. Microscopes observe small or invisible objects. They
magnify images through at least one lens.
Speed of light
When Einstein developed the Theory of Relativity, he found that light travels at a constant
speed of 299 792 458 m.s-1. We round that number up to 3 x 108 m.s-1.
What is a vacuum again? A vacuum in physics is a space with no matter in it. Not even gas
molecules.
The symbol that we use for the speed of light is lowercase c. On the data sheet that you are
given in exams and tests, you will be given this constant as c = 3 x 108 m.s-1.
Refraction
The refraction of light is a very commonly seen phenomenon. Other waves such as water
waves and sound waves can also undergo refraction.
Key term
Refraction of light is the change in direction of a light ray due to a change in speed when
light travels from one medium into the other of a different optical density.
Optical density is a measure of the refracting power of a medium.
When light moves between optical media, it changes speed based on the medium's density.
A denser medium results in slower light speed. This is known as optical density.
The higher the optical density, the slower the speed of light in that medium. We'll later list the
speed of light in various mediums and learn to calculate it ourselves. Let's understand why
light slows in denser mediums. When light travels from a vacuum into air, it slows down.
Light protons are absorbed and re-emitted upon encountering a particle, taking time and
thus slowing down.
In grade 10, we learned about matter phases: gases, liquids, and solids. In gases, particles
are far apart; in liquids, they are closer; and in solids, they are extremely close together.
Therefore, if light travels from a vacuum into air, it slows down, and if it goes from air to
water, it slows down further. What if light enters glass? It would be slower because the
particles are closer together. The greater the speed difference of light between two
mediums, the more it refracts or bends as it transitions between them.
Key term
The refractive index of a material is the ratio of the speed of light in vacuum (c) to the speed
of light in a material (v).
n=c/v
Where:
n is the refractive index
c is the speed of light in a vacuum
v is the speed of light in the other medium
There are no units for the refractive index as the units for the denominator and numerator
would cancel out.
Study tip
You do not need to learn the speeds of light in various mediums or their respective refractive
indices.
Below is a list of refractive indices for some transparent substances. You don't need to
memorize it, but reviewing the table may help you notice trends between the type or phase
of substance and the refractive index.
Table 1: Refractive indices of some optical media
MATERIAL
REFRACTIVE INDEX
Vacuum​
1
Air​
1,0003
Water​ 1,33
Paraffin​
1,44
Glass​ 1,52
Diamond ​
2,42
By examining the table, you'll notice that the refractive index increases as substances
become denser. Diamond, one of the hardest materials on earth, is very dense with a high
refractive index. Remember, a larger refractive index means slower light speed in that
medium.
Refraction in everyday life
A rainbow
A rainbow is caused by refraction and reflection of light. Light entering a water droplet is
refracted into seven colours. It is then reflected off the droplet's back. As it exits, the light is
refracted again at various angles.
Figure 3: Refraction of light in a drop of water
Twinkling Stars
The atmosphere's air varies in thickness. Some areas are denser than others. At night, stars
seem to twinkle because light refracts while passing through the atmosphere's different
layers.
Fish Bowl
If you've seen an aquarium or fish bowl, you might notice the fish appear bigger through the
side. Even your hand seems larger on the opposite side. This is due to light refraction
through the glass and water, making things look slightly lifted and bigger. Try it at home by
passing your hand behind a glass of water to see the effect.
What are refraction ray diagrams?
In the last section, you learned about refraction. Recap: Refraction occurs when light moves
between different optical media. It will slow down or speed up, depending on whether it
travels from a denser to a less dense medium.
To understand this, imagine pushing a shopping trolley onto grass. If aligned straight with the
grass, the trolley slows as wheels hit the grass. Similarly, light approaching from air to glass
slows down. There's no bending or refraction if light meets the surface perpendicularly, as it
is on the normal line.
Figure 1: Light travelling from air to glass perpendicular to the surface.
What would happen to the trolley as it approached the grass from the pavement at an angle?
First, look at the diagram and then read through the explanation
Figure 2: Trolley going from pavement to grass.
When the right front wheel goes into dense grass it slows down, but the left front wheel
continues at the same speed. This causes the trolley to turn slightly to the right, towards the
normal, which is perpendicular to the surfaces. This analogy helps illustrate what happens
when light travels through different mediums and how it can be refracted.
Key terms
Angle of incidence: The angle between the normal to a surface and the incident light ray.
Angle of refraction: The angle between the normal to a surface and the refracted light ray.
Light travelling from less dense to more dense mediums
If light travels from a less dense medium like air into a more dense medium like water, it
slows down. This is because water has a higher refractive index than air. The light bends
towards the normal, making the angle of incidence greater than the angle of refraction.
Figure 3: Light travelling from less dense to more dense medium.
As you can see, the angle of refraction is less than the angle of incidence when light travels
from a less dense to a more dense medium.
θi > θr
In the next module, we will learn about how to calculate various variables related to
refraction using a principle called Snell’s law.
Light travelling from more dense to less dense medium
If light transitions from a denser medium like glass to a less dense medium like air, it speeds
up due to air having a lower refractive index than glass. The light bends away from the
normal, resulting in a refraction angle greater than the incidence angle.
Figure 4: Light travelling from a more dense to a less dense medium.
As you can see the angle of incidence is less than the angle of refraction when light travels
from a more dense to a less dense medium.
θi < θr
Ray diagrams through various prisms
Let's look at some examples of when light passes through different transparent prisms and
draw the respective ray diagrams.
Key term
A prism is a 3D shape which has a constant cross section, i.e. both ends of the solid are the
same shape.
Some revision
In the previous lesson, we learned that light can undergo refraction. Refraction occurs when
light transitions from one optical medium to another with different optical densities. The
light's speed changes, and its direction often alters.
As light passes between different optical media, it slows down or speeds up based on the
medium's density. The denser the medium, the slower the light travels. This concept is
known as optical density.
Key terms
Angle of incidence: The angle between the normal to a surface and the incident light ray.
Angle of refraction: The angle between the normal to a surface and the refracted light ray.
Refractive index ( n ) of a material is the ratio of the speed of light in vacuum (c) to the speed
of light in a material (v).
n=c/v
Where:
n is the refractive index
c is the speed of light in a vacuum (3 x 108 m.s-1)
v is the speed of light in the other medium
Light travelling from less dense to more dense mediums
When light travels from a less dense medium like air into a denser medium such as water, it
slows down because the refractive index of water is higher. The light bends towards the
normal, making the angle of incidence greater than the angle of refraction.
The angle of refraction is less than the angle of incidence in this scenario.
θi>θr
Light travelling from denser to a less dense medium
When light travels from a denser medium like glass to a less dense medium such as air, it
speeds up. This is because the refractive index of air is lower than that of glass. The light
bends away from the normal, making the angle of refraction greater than the angle of
incidence.
The angle of incidence is less than the angle of refraction when light moves from a denser to
a less dense medium.
θr>θi
Snell's law
This law was discovered in 1621 by the Dutch astronomer and mathematician Willebrord
Snell (also called Snellius). Snell's law remained unpublished until mentioned by Christiaan
Huygens in his treatise on light. Watch the short video to learn more about his impact on
physics and astronomy.
Key term
Snell's Law: The ratio of the sine of the angle of incidence in one medium to the sine of the
angle of refraction in the other medium is constant.
If you look at the table, do you notice a trend? As the material becomes denser (diamond is
denser than air), the refractive index increases.
The formula for Snell's law is:
ni × Sin Θi = nr × Sin Θr
Where:
ni is the refractive index of incident ray
nr is the refractive index of refracted ray
θi is angle of incidence
θr is angle of refraction
Snell's formula is: ni⋅sinΘi=nr⋅sinΘr
The gradient of this graph is:
m=ΔyΔx
If you manipulate Snell's formula, you get:
m=sinΘrsinΘi
sinΘrsinΘi=ninr
So you can see finding the gradient can be useful.
In the last module, we learned how to use Snell’s law to calculate a missing variable when
light is refracted from one optical medium into another using this formula.
ni×Sinθi=nr×Sinθr
Key term
Snell's Law: The ratio of the sine of the angle of incidence in one medium to the sine of the
angle of refraction in the other medium is constant.
Figure 2: Effect of changing the angle of incidence on the angle of refraction.
As you can see the angle of refraction is less than the angle of incidence when light travels
from a less dense to a more dense medium.
θi>θr
So even if the angle of incidence increases to 90°, the angle of refraction would be less than
90°.
Now let’s consider a situation where light is travelling from a denser to a less dense medium.
We know that the light will bend away from the normal thereby making the angle of refraction
greater than the angle of incidence.
Figure 3: Effect of changing the angle of incidence on the angle of refraction.
Figure 4: Increasing the angle of incidence.
You will notice that as the angle of incidence increased, so did the angle of refraction.
If the angle of incidence increased to the point that the angle of refraction became 90°, then
that angle of incidence is called the critical angle.
Figure 5: Increasing the angle of incidence until it reaches the critical angle.
When this happens the refractive ray lies on the boundary between the two mediums and no
light goes into the second medium.
Key term
Critical angle is the angle of incidence in the optically denser medium for which the angle of
refraction in the optically less dense medium is 90°.
Study tip
When calculating the critical angle, light is always travelling from a denser to a less dense
medium and the angle of refraction is 90°.
If the angle of incidence exceeds the critical angle for the medium, a phenomenon called
total internal reflection takes place. For any angle of incidence larger than the critical angle,
you can’t use Snell's law to solve for the angle of refraction, because it will show that the
refracted angle has a sine larger than 1, which is not possible.
Figure 2: Increasing the angle of incidence until it reaches the critical angle.
In that case, all the light is totally reflected off the boundary, obeying the laws of reflection.
So the light does not go through the surface but rather is reflected off the surface.
Figure 3: Increasing the angle of incidence until it reaches the critical angle
Conditions for total internal reflection:
➔​ The light ray moves from a more dense medium to a less dense medium.
➔​ The angle of incidence must be greater than the critical angle.
Application of Total Internal Reflection
Optical fibers
Optical fibers are based entirely on this principle of total internal reflection. An optical fiber is
a flexible strand of glass. A fiber optic cable is usually made up of many of these strands,
each carrying a signal made up of pulses of laser light. The light travels along the optical
fiber, reflecting off the walls of the fiber. With a straight or smoothly bending fiber, the light
will hit the wall at an angle higher than the critical angle and will all be reflected back into the
fiber. Even though the light undergoes a large number of reflections when travelling along a
fiber, no light is lost.
Figure 4: How fiber optics work in a glass tube.
Fiber Optics in Surgery and Dentistry
Fiber optic cables are widely used in the fields of medicine. An important part of
non-intrusive surgical methods is known as an endoscopy. A non-invasive procedure is a
treatment that does not require large incisions (cutting) into the body or the removal of
tissue.
In these applications, a fiber optic light is used to light up the surgery area within the body,
making it possible to reduce the number and size of incisions made. Fiber optics are also
used in microscopy and biomedical research.
Fiber optics in telecommunications
Fiber optic cables have transformed telecommunications and connectivity. Fiber technology
is a game changer. Through fiber cables, signals can be sent across the world at the speed
of light. They are quite often laid on the seabed between countries.
Fun fact
The South Africa Far East cable is an optical fiber submarine communications
cable linking Melkbosstrand and Mtunzini, South Africa to Penang, Malaysia. It
has a total length of 13,104 kilometers and is one of a pair of cables that
provides high-speed digital links between Europe, West and Southern Africa,
and the Far East.
These are only some of the common uses. There are many more applications
of fiber optics. Spend some time after the lesson researching what other
interesting applications of fiber optics there are.
Huygens's principle
What is Huygens's principle?
In 1670, Christiaan Huygens was the first to describe how wave theory may
also account for the principles of geometric optics. Of course, no one was
paying attention to him at that moment. Only later after wave theory was
accepted, was his work rediscovered.
Huygens had a crucial insight into the physics of wave propagation that is
now known as the Huygens principle.
Key term
Huygens's principle states that every point of a wave front serves as a point
source of spherical, secondary waves that move forward with the same speed
as the wave.
To understand Huygens's principle, we will break down the definition into
smaller components.
Every point of a wavefront serves as a point source - on the last page we saw
that if you look at the top of a wave, it is made out of a series of points. These
points all are a source of small circular secondary wavelets, which is simply a
term for small waves.
Figure 11: Wavelets produced on a wavefront
The new wavefront is drawn by drawing a tangent of all the new wavelets
Figure 12: Wavelets produced a new wavefront using a tangent.
Limitations of Huygens's principle
Although it is still used today there are some drawbacks and limitations to
Huygens's principle.
The principles of Huygens generally failed to explain the photoelectric effect
which you will find about in Grade 12.
A serious drawback is that the theory proposes an all-pervading medium
required to propagate light known as the ether which is a theoretical universal
substance believed during the 19th century to act as the medium for
transmission of electromagnetic waves. The ether was later proven not to
exist.
Study tip
A typical exam question that you could get on Huygens' principle is to state
TWO changes that could be made to change the appearance of a diffraction
pattern if green monochromatic light is used.
Wavefronts
We have looked at two properties already that show the wave nature of light reflection and refraction. In this and the next lesson we will look at diffraction.
A quick reminder: Reflection is the bouncing off of light off a surface.
Refraction is the bending of light as it slows down or speeds up as it moves
from one optical medium to another.
Key term
❖​ A transverse wave is a wave in which the particles of the medium
vibrate at right angles to the direction of motion of the wave. A
transverse wave is a succession of transverse pulses.
❖​ Diffraction is the ability of a wave to spread out in wavefronts as the
wave passes through a small aperture or around a sharp edge.
Waves are not stationary so when you look at the image below remember that
the wave is moving.
Figure 2: Transverse wave
What you can understand is that the crest or trough is not a single particle but
rather a collection of many particles or points all on the crest or on the trough.
We will represent these particles next to each other on the crest with dots in
the diagram below. The dots (red for crest) are all the particles that have
reached their maximum displacement from the rest position. The dots
(orange for trough) are all the particles that have reached their maximum
displacement. All the red points on one crest and all the orange points on one
trough are all in their maximum position from the rest position. So these
adjacent points are in phase and next to each other. This is a specific term
called wavefront.
Figure 5: Transverse wave from above.
Key term
A wavefront: An imaginary line joining points on a wave that are in phase.
Make sure that you don’t get confused between wavelength and wavefront. A
wavelength is the distance between two successive points in phase
Superposition
When two or more pulses occur at the same time in the same location, they
will superimpose on each other. This means that their amplitudes will
combine at the points where they occur at the same time.
Pulses can either interfere constructively or destructively depending on
whether the resulting amplitude is greater than or less than the amplitudes of
the original pulses.
After the pulses superimpose on each other they continue in their original
direction and assume their original amplitude.
Key term
The principle of superposition states that when two pulses occur in the same
place at the same time, the resulting pulse is the sum of the two interfering
pulses.
Pulses can have their amplitudes in different directions from each other. For
this lesson we will say that a pulse that goes up will have a positive amplitude
and pulses that go down have a negative amplitude.
Constructive interference
When two pulses constructively interfere, their resulting amplitude is greater
in magnitude than each of the original pulses. This happens when both pulses
have their amplitudes in the same direction.
Key term
Constructive interference occurs when the resulting amplitude of two or more
pulses during interference is greater than the amplitude of the original pulses.
Figure 6: Constructive interference is shown in this diagram.
Interference
When two pulses destructively interfere their resulting amplitude is smaller
than the amplitudes of the original pulses. This happens when two pulses
have amplitudes in opposite directions.
Key term
Destructive interference occurs when the resulting amplitude of two or more
pulses during interference is less than the amplitude of the original pulses.
Figure 8: Destructive interference is shown in this diagram.
Diffraction around a corner
Have you noticed that sound seems like it can bend around a corner? You
could be standing on the other side of the door and still hear people on the
other side of the door. The bending of the sound wave is because the wave is
diffracted.
If you examine the wave diagram above, you will see that the red points on
wavefront A, produce wavelets. Those wavelets, in turn, produce wavefront B.
When wavefront B collides with the barrier, only some of the wave moves
forward. If you look carefully at wavefront C, the part of the wave closest to the
barrier starts to curve due to the wavelets. Wavefront C now produces more
wavelets but because it is curved, the curved end will still produce wavelets
but they will not be moving forward but at an angle upwards to form
wavefront D.
Diffraction through a gap
When a water wave or a light wave goes through a gap, how will it bend? We
can use Huygens' principle once again to predict the shape of the wave as it
goes through a gap.
.
Figure 15: Sound diffraction through a doorway
When a water wave or a light wave goes through a gap as shown below, how
will it bend or diffract? You guessed it! We can use Huygens's principle once
again to predict the shape of the wave as it goes through a gap.
Figure 16: Wavefront going through a gap
If you examine the wave diagram above, you will see that the red points on
wavefront A, produce wavelets. These wavelets, in turn, produce wavefront B.
When wavefront B collides with the barrier, only some of the wave moves
forward. If you look carefully at wavefront C, the part of the wave closest to the
barriers on both sides starts to curve due to the wavelets. Wavefront C now
produces more wavelets but because it is curved, the curved end will still
produce wavelets but they will not be moving forward but at an angle
outwards to form wavefront D.
When waves collide with barriers, they can undergo diffraction. What causes
this to happen? It becomes evident when we apply Huygens' principle.
Consider a wavefront approaching a barrier with a slit in it: only the points on
the wavefront that move into the slit can continue emitting forward-moving
waves. However, because a large portion of the wavefront is blocked by the
barrier, the points on the hole's edges emit waves that bend around the
edges.
As the wavefront (see Figure 1) approaches the gap it produces wavelets as
Huygens's principle says.
Figure 1: Wavefront approaching gap.
As the wavefront strikes the barrier, only the portion of wave that can go
through the gap will continue. The rest of the wave will be reflected off the
barrier. If you look at figure 2, you can see that as the wavefront passes
through the opening, it produces wavelets and they in turn produce a new
wavefront. This wave is diffracted or bent as shown in the diagram below.
Figure 2: Wavefront going through a gap.
Diffraction patterns
When the light comes through the narrow slit (gap) and if we placed a screen
a distance away from the gap. What do you think you would see on the
screen?
We are going to use Huygens's principle as well as our understanding of
constructive and destructive interference and see the diffraction pattern on
the screen.
Wavefronts are emitted by each point (A and B) source from the slit's edge. A
succession of wavefronts emitted from each point source is seen in the
diagram. The dashed lines represent troughs in the waves emitted by the
point sources, while the black lines represent peaks in the waves emitted by
the point sources.
With a solid red dot, constructive interference (peak meets peak or trough
meets trough) occurs, and with a circle, destructive interference (trough
meets a peak) occurs. There will be spots on the barrier where constructive
interference occurs and places where destructive interference occurs when
the wavefronts reach a barrier.
Figure 3: Wavefront going through a gap.
Two properties of waves, diffraction and interference, produce a pattern of
constructive and destructive interference measured at a distance from a
single slit. This pattern is sometimes referred to as an interference pattern and
other times as a diffraction pattern.
For a single narrow slit, the intensity of the diffraction pattern looks like this:
figure 4: Diffraction pattern on a screen.
The peak that is the highest (in the middle) shows the greatest intensity.
Intensity can be understood by the amount of light. It is where many waves
undergo constructive interference and therefore is a bright section on the
screen. We will see this more in a section of the module coming up soon.
Remember that the red dots show where constructive interference takes
place and at that point, the screen will be lighter/brighter. The white dots
show where a crest and a trough meet and destructive interference takes
place. Those areas will be darker.
Note that this also works for sound waves. If two peaks had to meet with a
sound wave it would result in a bigger wave but if a crest and trough had to
meet then the waves would cancel out .
Effect of slit width and wavelength on diffraction patterns
Have you ever attempted to push plasticine through a small hole? Maybe not,
but if you have, you've probably observed that it bulges when it passes
through the hole. When waves pass through small gaps or push past
obstructions, the same thing happens.
When we consider waves, there are a couple of factors that cause the
diffraction to be more or less. We are going to look at them now.
Wavelength
The amount of diffraction depends on the wavelength, with longer
wavelengths being diffracted at a greater angle than shorter ones. The longer
the wavelength of the wave the larger the amount of diffraction.
Wavelength
The amount of diffraction depends on the wavelength, with longer
wavelengths being diffracted at a greater angle than shorter ones. The longer
the wavelength of the wave the larger the amount of diffraction.
Key terms
Wavelength: The distance between two successive points in phase.
Frequency: The number of wave pulses per second.
Figure 5: Effect of wavelength on diffraction.
Slit width
The amount of diffraction also depends on the size of the aperture, or gap. The
narrower the slit, the more diffraction (bending) will take place.
Therefore: diffraction∝1w
Where: w is the width of the slit.
Figure 6: Effect of slit width on diffraction.
Both the above waves have the same wavelength but the slit width is smaller
on the right. As you can see on the wave on the left, there is a small amount of
diffraction but on the wave on the right there is more diffraction (more
bending) as it goes through the opening.
Which type of wave, a sound or a light wave, will have greater
diffraction?
It is a sound wave because a light wave has much higher frequencies than
sound waves and therefore will experience less diffraction. Light waves have
wavelengths between 400 nm to 700 nm, where n is nano which is x10-9 so
400 nm is 0,0000004 m but an average sound wave has a wavelength of 17 m
and 17 mm which is the reason sound waves diffract considerably more than
light waves.
Figure 7: Comparing diffraction of a light and sound wave.
Wave nature of light
We know from grade 10 that there are range of electromagnetic waves. They
range from long wavelengths with low frequencies to very short waves with
very very frequencies. The graphic below shows the electromagnetic
spectrum.
Radio waves have very long wavelengths (x103 m) but low frequencies (104
Hz). As you move towards the right, the wavelength decreases but the
frequencies increase as they are inversely proportional to each other. The
same applies to visible light in the middle of the spectrum. Red light has the
longest wavelength and will diffract the most but violet light with a shorter
wavelength will diffract the least.
When we shine monochromatic light (light of one frequency and colour)
through a single slit and shine it onto a surface, what can we expect to see?
Remember, what are the factors that affect diffraction? It is the width of the
slit as well as the wavelength of the wave. The greater the slit width, the less
the diffraction, and the longer the wavelength, the greater the diffraction.
We know that red light has a longer wavelength and will experience more
diffraction as it goes through a single slit. Violet light has a higher frequency
and shorter wavelength and will have less diffraction. Green light is in between
violet and red. Below is the diffraction pattern of red, violet, and green light as
it went through a single slit and was projected onto a screen. Look closely at
them and see if you can notice something about the pattern.
Figure 9: Diffraction pattern of violet, green, and red light.
If you look carefully you will observe that the central band for red is wider than
green and violet. That is because red light has a lower frequency and longer
wavelength which means that it will diffract more than both green and violet
light.
The dark bands on the screen are areas where destructive interference takes
place (where a crest and trough meet) and this causes the waves to be
cancelled out causing a dark area.
What is Kinetic molecular theory?
We already know that all matter is made up of particles. The kinetic molecular
theory says that the particles that make up a substance have a certain
amount of kinetic energy (movement energy), meaning that they are all
moving, and the way in which they move is responsible for the physical state
of matter.
Study tip
Do you remember covering the Kinetic molecular theory in grade 10?
How did we make this model?
Whenever we devise a scientific theory that we want to apply, we always have
to justify that the theory agrees with the observations that we make. Let’s
have a look at how scientists verified the Kinetic molecular theory. To do this,
we first have to look at diffusion.
Diffusion is the movement of particles from a high concentration to a low
concentration. Diffusion takes place in both liquids and gases. Have you
noticed sometimes that when someone sprays deodorant in one part of the
room, in a few minutes you can smell it on your side of the room? What
happens is that the gas molecules in the air, which are in constant motion,
collide with the gas molecules of the deodorant and push them around until
you smell them.
Key term
Diffusion is the movement of particles from a high concentration to a low
concentration. Diffusion takes place in both liquids and gases.
In 1828, a scientist named Robert Brown was trying to study pollen under the
microscope. To achieve this he put the pollen in some water, hoping that he
would get a clearer view if the pollen was suspended on the water. To his
surprise, no matter how still he was trying to keep the sample, the pollen kept
moving around on top of the water, not allowing him to have a good look.
Through this, he accidentally verified the particular nature of matter and this
phenomenon became known as Brownian motion.
Phases of matter
The different phases of matter
You know that matter is found in three phases and that most matter can
change from one phase to another by the addition or removal of heat energy.
Let’s revise the properties of solids, liquids, and gases quickly to ensure that
we know how to distinguish between them.
Solids
In a solid, the particles are packed tightly together and have little kinetic
energy. The particles have strong intermolecular forces and they don’t have
enough energy to overcome the forces between them, so the particles vibrate
in place. This explains why solids have a fixed shape and volume. The particles
vibrate (slight motion) but stay in their crystal structure.
Liquids
In a liquid, the particles have more kinetic energy than in a solid. The particles
have enough energy to overcome the forces between them, so they can move
around freely amongst one another. The particles tend to move wherever they
can reach inside the container which is why the liquid takes the shape of the
container. The spaces between the particles are bigger in a liquid than in a
solid. This means that the forces between the particles are weaker than in a
solid; although it is important to note that there are still forces between the
particles, they are just not as strong as they are in a solid. Sometimes particles
on the surface of a liquid have enough energy to overcome the intermolecular
forces of the particles below and can escape from the liquid phase.
Gases
In a gas the particles have even more kinetic energy than in a liquid. The
particles have more than enough kinetic energy to overcome the forces
between them so they move wherever they can reach. This also means that
the spaces between the particles are much bigger than in a liquid, making the
forces between the particles smaller. In a closed container, the particles of a
gas will have so much energy that they collide with the walls of the container.
This is why a balloon will expand when you add more air, because the air
particles collide with the sides of the balloon pushing outwards and
expanding the balloon.
The kinetic theory of gases can be stated as follows:
Gases are made up of particles (e.g. atoms or molecules). The spaces between
the particles are large in comparison to the size of the particles.
These particles are in constant motion due to their kinetic energy (movement
energy). All the particles move in straight lines but at different speeds.
The force of attraction between particles is weakest between gas particles.
The collisions between particles and the walls of the container do not change
the kinetic energy of the system.
Ideal gas properties
Gases can be somewhat difficult to understand. They are made up of billions
upon billions of energetic molecules that might collide and interact. Precisely
because it's difficult to describe a real gas, the concept of an ideal gas was
created. It was developed as a rough estimation that may be used to simulate
and predict the behaviour of real gases.
Key term
An ideal gas has identical particles of zero volume, with no intermolecular
forces between them. The atoms or molecules in an ideal gas move at the
same speed.
The term "ideal gas" describes a hypothetical gas made up of molecules that
obey these rules:
The particles are identical in all ways.
Their molecules occupy no volume.
They have no intermolecular forces between the molecules.
The collisions between the particles and the walls of the container are
perfectly elastic (which means that they do not lose energy).
Ideal gases do not exist in the real world!
Real gases behave more or less like ideal gases except at high pressures and
low temperatures.
The real gases that behave the closest to ideal gases are H2 and He. We will
learn more about how we use ideal gases in the next lesson on Boyle’s law but
for now, let’s discuss how real gases can deviate from the behavior of ideal
gases.
Terms used in gases
First, let's look at some terms that you will need to know for this entire module
and the next.
Pres
sure
Pressure is the force applied over an area. So if you push your flat hand on the
desk, you are applying a force on that area. If you now push with just your
finger, the pressure will increase because you are pushing with the same force
but the area of your finger is smaller than the area of your hand.
Figure 1: Pressure exerted by hand compared to finger.
In gases, it is slightly different. The pressure is caused by the collisions of the
gas particles on the side of the container. More collisions of the particles will
result in a higher pressure.
The unit for pressure is Pa which is named after the scientist Blaise Pascal.
1 pascal is a pressure of one newton per square metre or N.m-2
Figure 2: 1 Pascal of pressure.
For you to get a feel for 1 pascal of pressure, it is roughly the same as the pressure exerted
by one piece of ordinary A4 paper placed on a table. It is not a big force per unit area and
that is why we very often measure pressure in kPa or MPa or even GPa.
You might have heard the term atmospheric pressure before, but what is it? The air
surrounding you is made of molecules, and although each molecule is very, very light, the
combined effect is that billions and billions of molecules push against everything they come
into contact with. Atmospheric pressure, or air pressure, is the name given to this pressure.
The air above a surface exerts a force on it as gravity pulls it to Earth. We measure
atmospheric pressure at sea level because the level of the sea around the world is constant.
The higher you are above sea level, the lower the atmospheric pressure.
An atmosphere corresponds to the pressure applied by a vertical column of mercury (like a
barometer) with a height of 760 mm. Standard atmospheric pressure, also known as an
atmosphere, corresponds to a force of 101,325 pascals (Pa) or Newtons per square meter.
Study tip
101 300 Pa = 1 atm and 101 300 Pa = 760 mm Hg (Hg is the symbol for mercury)
Please take note that in the three gas laws that follow, the unit for pressure can be given in
any of these forms as long as both sides of the equation have the same unit.
Fun fact
Why do your ears pop? Sudden changes in air pressure, such as when flying, scuba diving,
or climbing a mountain, can cause the eardrum to swell and feel blocked. Your ears pop out
to regain the same pressure on the inside of the ear and the outside of the ear.
Temperature
It is easy, even in Physical science, to get terms such as heat and temperature mixed up.
Heat and temperature are not the same things. Heat is a form of energy, and describes the
transfer of thermal energy between molecules. Its unit is Joules. Temperature is how hot or
cold something is or even better, describes the average kinetic energy of molecules in a
system. It is measured in Celsius (°C), Kelvin(K), or Fahrenheit (°F) as is used in the United
States of America.
UCT-KeyTerm
Key term
The temperature of a substance is a measure of the average kinetic energy of the
particles.
The faster the particles move, the more kinetic energy they have and therefore the higher
the temperature. Although we can get very hot temperatures, such as the surface of the sun
at 5800 °C, or as hot as a supernova. A supernova is a transitory event that marks the end
of a star's life. The star's existence comes to a dramatic end with one of the greatest
explosions ever seen in space. During the explosion, temperatures at the core reach 100
billion degrees Celsius.
Although temperatures can vary widely, as you saw above, the coldest temperature we can
go to is -273 °C which is called absolute zero. At this point all substances are solid and the
particles stop vibrating. In this and the next module, we will be using the Kelvin temperature
scale so you will need to know how to convert from degree Celsius to kelvin and back again.
The symbol for temperature is t but for absolute temperature it is T.
0 K = -273 °C
To convert from degrees Celsius to kelvin:
T=t+273
To convert from kelvin to degrees Celsius:
t=T−273
STP in chemistry is the abbreviation for Standard Temperature and Pressure. STP is most
commonly used when performing calculations on gases. The standard temperature is 273 K
(0° Celsius) and the standard pressure is 101 kPa which is atmospheric pressure at sea
level. At STP, one mole of gas occupies 22,4 dm3.
Key term
Volume is how much space an object or substance takes up.
There are 100 cm in 1 m so there are 100 cm x 100 cm in 1 m2
There are 100 cm x 100 cm x 100 cm in 1 m3.
To convert from m3 to cm3, you must multiply by 1 000 000 or x 106.
There are 10 dm in 1 m so there are 10 dm x 10 dm in 1 m2
There are 10 dm x 10 dm x 10 dm in 1 m3.
To convert from m3 to dm3, you must multiply by 1 000 or x 103.
1000 L = 1 m3
There are 10 cm in 1 dm, so there are 10 cm x 10 cm in 1 dm2
There are 10 cm x 10 cm x 10 cm in 1 dm3.
To convert from dm3 to cm3, you must multiply by 1 000 or x 103.
Deviation of a real gas from an ideal gas
The term deviation simply means to depart from. It means that at some point real gases stop
behaving like ideal gases. In most cases real gases obey the ideal gas rules but when do
they stop? Let’s find out what happens when real gases reach high pressure and very low
temperatures.
When we spoke about an ideal gas, we said that an ideal gas has identical particles with
zero volume and there are no intermolecular forces between the particles in the gas.
When the pressures are really high and the molecules are compressed, the volume will
decrease. The volume will approach a very small number but will not be zero because the
molecules will occupy some space that cannot be compressed further. As a result, the
pressure of a real gas is higher than it would be for an ideal gas. You can see this when you
try to compress a syringe but block the end with your finger. You feel the pressure on your
finger, but push as hard as you want, the syringe will not be totally compressed.
Figure 12: Gases deviate from ideal gas behaviour at high pressure.
The intermolecular forces become greater at low temperatures because the molecules slow
down and the molecules are closer together. As the attractive force between molecules
grows, the molecules slow down, and there are fewer collisions. As a result, the gas's
pressure at low temperatures is lower than what would be predicted for an ideal gas. A real
gas will liquefy if the temperature is low enough or the pressure is high enough
Figure 13: Gases deviate from ideal gas behaviour at low temperatures.
Study tip
Real gases deviate from the behaviour of ideal gases under very high pressures and very
low temperatures.
Quick summary
The kinetic molecular theory says that the particles that make up a substance have a certain
amount of kinetic energy (movement energy), meaning that they are all moving, and how
they move is responsible for the physical state of matter.
Matter is found in three phases - solids, liquids, and gases. The distance of the particles in a
gas is greater than in a liquid and solid. The particles in a liquid also have more kinetic
energy as they move faster.
UCT-KeyTerm
Key term
An ideal gas has identical particles of zero volume, with no intermolecular forces between
them. The atoms or molecules in an ideal gas move at the same speed.
The term "ideal gas" describes a hypothetical gas made up of molecules that obeys these
rules:
The particles are identical in all ways
Their molecules occupy no volume
They have no intermolecular forces between the molecules
The collisions between the particles and the walls of the container are perfectly elastic
(which means that they do not lose energy)
There are four factors that are considered in gas laws. Pressure, volume, temperature and
amount of gas.
Pressure exerted by a gas is the collisions of the molecules with the walls of the container.
The temperature of a substance is a measure of the average kinetic energy of the particles.
Volume is how much space an object or substance takes up
Real gases deviate from the behaviour of ideal at very high pressures and very low
temperatures.
What is Boyle’s law?
If you take an inflated balloon and squash it, you know that at some point the balloon will
burst. It is easy to simply say the squashing burst the balloon, but what is taking place inside
the balloon causing it to burst really has to do with the relationship between pressure and
volume of the balloon. Look at the video below of a balloon bursting in slow motion when it is
pricked with a pin.
Key term
Pressure is exerted by a gas in terms of the collision of the molecules with the walls of the
container.
We know that it is fairly easy to compress gases. As the volume decreases as you compress
a gas then the pressure will increase. This happens because now that the particles are
closer together, they have more collisions, and as the number of collisions increases so does
the pressure.
Boyle’s law deals with the relationship between the two variables pressure and volume.
Key term
Boyle's law: The pressure of an enclosed gas is inversely proportional to the volume it
occupies at constant temperature.
There are several important factors to consider for Boyle’s law (as well as for the other gas
laws we will be dealing with soon). The statement talks about "an enclosed gas". What is
meant by an enclosed gas is that the amount of gas or the number of moles remains
constant. Let’s look at some concepts that you should have dealt with in Grade 10.
You learned that the mole is the SI unit for the amount of substance.
Key term
One mole is the amount of substance having the same number of particles as there are
atoms in 12 g carbon-12.
Avogadro's number is defined as the number of elementary particles (molecules, atoms,
compounds, or even Coke cans for that matter) per mole of a substance. It is equal to
6,022×1023 mol-1 and is expressed as the symbol NA.
It was named after Amedeo Avogadro, an Italian physicist who discovered that two gases of
equal volume have the same number of molecules at the same temperature and pressure. In
the early twentieth century, French scientist Jean Perrin coined the term "Avogadro's
number" to describe the number of units in a mole.
This is a truly large number and it is difficult to wrap our heads around really large numbers
sometimes. So, to give some idea about how big the number is, if you had 1 mole of soft
drink cans it would cover the surface of the Earth to a depth of over 320 km. If you were able
to count atoms at the rate of 10 million per second, it would take about 2 billion years to
count the atoms in one mole. This just shows that it is truly a huge number. How many
grams are in one mole of water? Let’s find out.
n=mM
Where:
n = number of moles
m = mass of sample
M is the molar mass of the compound
Water’s formula is H2O. Hydrogen has a molar mass of 1g/mol and oxygen is 16 g/mol, so 1
mole of water is:
n=mM
1=m(2×1)+16
m=18g
A normal teaspoon is 5 ml or 5 g so 1 mole of water is just under 4 teaspoons of water.
Hopefully, this helps you understand why we use moles to count quantities in chemistry
So in Boyle’s law, remember that it is a fixed amount of gas, i.e. the amount does not
change. It is called a constant variable. You might have noticed that after riding a bicycle, the
tyres seem harder even though you did not pump them up more. That is because
temperature can also affect the pressure of a gas. If the temperature of a gas increases the
molecules move faster because they increase in kinetic energy. If they move faster, then the
pressure will also increase.
However, in Boyle’s law, the temperature of the gas must remain the same so it is also a
constant variable.
Graphing Boyle's law
From the definition of Boyle’s law in the last section, we saw they have an inversely
proportional relationship.
Figure 2: Graph showing the inverse relationship between pressure and volume.
Using the kinetic theory of gases to help explain this inverse relationship between the
pressure and volume of a gas is as follows. If the volume of a gas is decreased for a fixed
amount of particles, the particles are now going to collide more frequently with each other
and with the sides of the container. Pressure is a measure of the number of collisions of gas
particles with each other and with the sides of the container. Therefore, if the volume
decreases, the number of collisions increases and so the pressure will also increase. If the
volume of the gas is increased, the gas particles collide less frequently and the pressure will
decrease.
Boyle’s law relationship can be written as:
pα1V
If we used k as a proportionality constant, we could rewrite the equation as:
pαkV
where k is a proportionality constant.
Therefore the constant k:
k=p×V
What this tells us is that if a gas has a constant temperature and amount of moles, then the
pressure multiplied by volume will give a constant value. This gives a formula for Boyle’s law
which says:
p1×V1=p2×V2
Where 1 and 2 refer to readings of a fixed amount of gas at a constant temperature.
Another way to graph the relationship is to consider:
1V
as a variable.
This would be written as; pα1V
This means that the pressure is directly proportional to the inverse of the volume.
Figure 3: The graph of pressure drawn against the inverse of volume.
In Newton’s second law the relationship between the acceleration and mass is also inversely
proportional where the acceleration always depends on the mass of the object, which means
mass will be the independent variable and acceleration will be the dependent variable.
However, if we use the example that we had earlier of a person squashing a balloon. Which
variable, pressure or volume will be the independent variable? It will be the pressure that is
causing the volume to change. The graph will still be an inversely proportional relationship
but with the x and y-axis swapped around.
Figure 4: Graph showing the inverse relationship between pressure and volume.
The graph for volume and the inverse of pressure would be:
Figure 5: Graph showing the inverse relationship between volume and the inverse of
pressure.
Boyle’s law is the inversely proportional relationship between pressure and volume provided
the temperature of the system and the amount of particles or moles remain constant.
When answering Boyle’s law, questions, or the other gas laws, read through the statement
carefully to determine which law you are using. Normally in the statement, if they say the
temperature remains constant then you know you must use Boyle’s law. Let’s try a few.
We have already studied chemical bonding. Take a moment and see if you remember the
three types of chemical bonding. Chemical bonds are the forces that keep molecules and
compounds bonded together. If we want to change the chemical composition of a molecule,
a chemical change needs to take place. A chemical change (chemical reaction) occurs when
the composition of a substance is changed, and that requires the breaking and forming of
chemical bonds during a chemical reaction.
These forces (covalent, ionic and metallic bonding) are called intramolecular forces.
The prefix intra- means "within’’ or inside
Key term
Intramolecular forces are the forces that hold atoms together within a molecule.
Fun fact
Interatomic (between atoms) forces are also sometimes referred to as intramolecular (within
molecules) forces.
A way to remember this is that you might have had inter-house athletics at school which
means that the different houses competed against each other. These are the houses in the
school, not competing with other schools.
Covalent bonds
A covalent bond forms when two atoms come close together and their half-filled orbitals
overlap and the electrons are found in orbitals around both atoms. Simply put, this is when
atoms share electrons to achieve a noble gas state. We use Lewis structures to show how
the atoms share electrons.
Covalent bonds can be single, double, or triple depending on the number of half-filled
orbitals that overlap. Atoms in a single bond share two electrons, atoms in a double bond
share four electrons, and atoms in a triple bond share six electrons.
Key term
A molecule is a particle that consists of atoms covalently bonded together.
A diatomic molecule is a molecule where two of the same type of atoms are chemically
bonded together. E.g. H₂, Cl2 , and O2 are all diatomic molecules.
Study tip
All diatomic molecules have covalent bonds.
Molecules can be described as polar or nonpolar. A polar molecule is one in which there is a
difference in electronegativity between the atoms in the molecule which causes the shared
electron pair to spend more time closer to the atom with the higher electronegativity. This
results in one end of the molecule having a slightly positive charge (+), and the other end of
the molecule having a slightly negative charge (−). Therefore a polar molecule is said to be a
dipole.
The symbol used for a dipole is the lowercase Greek delta (δ).
Figure 1: Representation of a dipole.
Ionic bonds
Key terms
An ion is a particle with a net charge due to losing or gaining an electron.
A cation is a positively charged ion.
An anion is a negatively charged ion.
An ionic bond is formed when there is a transfer of electrons between two atoms. This
happens when the difference in electronegativity between the two atoms is greater than 1,7,
which is often the case between a metal and a nonmetal. This attraction between cations
and anions forms a crystal lattice where ionic bonds hold the atoms together. We don’t find
individual sodium atoms attracted to individual chlorine atoms, but rather a crystal lattice of
ions packed together in a 1:1 ratio. It is mostly referred to as a compound.
Metallic bonds
Metallic bonds occur between metal atoms. Metallic bonds are much different from the
covalent and ionic bonds that we have been looking at so far. These bonds occur when
metal nuclei are packed together with the valence electrons free to move among all the
nuclei. These free valence electrons hold the metallic structure together.
Figure 3: Metallic bonding with free valance electrons.
Intermolecular forces
The term “INTERmolecular forces” is used to describe the forces of attraction between
molecules and ions when they are placed close to each other.
The prefix inter means between. So intermolecular forces are the forces between molecules
and ions.
Key term
Intermolecular forces are forces that exist between molecules.
Intermolecular forces are much weaker than the intramolecular forces (covalent, ionic and
metallic bonding) but are important because they determine the physical properties of
molecules like their boiling point, melting point, density, and vapour pressure. These are
NOT bonding forces.
Key term
Physical properties can be observed or measured without changing the composition of
matter.
Types of intermolecular forces
Intermolecular forces are the forces that act between molecules and are responsible for the
phases that substances are in. The weaker the intermolecular force, the more likely the
substance will be a gas. On this page, you will see all the intermolecular forces. The type of
intermolecular force will be determined by the polarity of the molecule. In previous modules,
you saw that the polarity of a molecule (covalent substance) is due to the shape of the
molecule and the difference in electronegativity.
There are a range of different intermolecular forces. Some are stronger than others and
therefore determine the effect on the properties of substances – which we will learn about in
the next lesson.
Ion-dipole force
As the name implies, this type of intermolecular force exists between an ion and a dipole
molecule. Remember when you hear the term dipole it refers to a polar molecule.
A good way to understand this force is to consider what happens when you dissolve table
salt (NaCl) in water (H2O). We know that there is dissociation.
Key term
Dissociation is the process in which solid ionic crystals are broken up into ions when
dissolved in water.
NaCl(s)→ Na+(aq)+ Cl-(aq)
As you can see, the ionic compound is broken into positive and negative ions. The process
of hydration that you learnt about in Grade 10 takes place where the water molecules
surround the ions. The question is: why does that take place?
Key term
Hydration is the process where ions become surrounded by water molecules.
There is a force of attraction between the negative ion and the positive side of the dipole
(polar molecule) or the positive ion and the negative side of the dipole. This force is called
the ion-dipole force.
Figure 1: Ion-dipole force.
If magnesium chloride (MgCl2(s)) is put into ethanol, will it dissolve? Magnesium chloride is
an ionic compound and ethanol is an alcohol (which is very polar), therefore it will dissolve.
To see if there is an ion-dipole force, check to see if you have an ionic compound and polar
molecules.
Ion-induced dipole force
This is an intermolecular force that comes about between an ion and a nonpolar molecule.
The word "induced" means to bring about or give rise to. When an ion approaches a
nonpolar molecule, it induces polarity in the molecule by distorting the electron cloud.
If you had a negative ion (anion) approach a nonpolar molecule, it would repel the electron
cloud and make one side of the molecule positive. The nonpolar molecule would become a
polar molecule. This leads to a weak, short-lived force which holds the compounds together.
Figure 2: Ion-induced dipole force.
Fun fact
Ion-induced dipole forces in hemoglobin (the molecule that carries oxygen around your
body). Hemoglobin has Fe2+ ions. Oxygen (O2) is attracted to these ions by ion-induced
dipole forces.
Key term
An induced dipole is a momentary dipole in a nonpolar molecule when it attains a polarity.
Here are a few examples of ion-induced dipole forces. In the presence of a nitrate ion
(NO3-), the iodine molecule (I2), which is non-polar, will become a dipole.
An ion-induced dipole force takes place between ions and nonpolar molecules. It is therefore
important to be able to recognise if a molecule is polar or nonpolar due to its shape and
electronegativity.
Van der Waals forces
The previous forces looked at the intermolecular forces between ions and polar and
nonpolar molecules. Let’s look at forces between molecules only. A Dutch physicist
Johannes Diderik van der Waals started his career as a school teacher. He became the first
physics professor at the University of Amsterdam in 1877 and won the 1910 Nobel Prize in
physics. His name is associated with Van der Waals forces (forces between stable
molecules).
There are three van de Waals forces:
Dipole-Dipole forces
Dipole-Induced dipole forces
Induced dipole-induced dipole forces (London forces)
Fun fact
Geckos, insects, and some spiders have setae on the pads of their feet that allow them to
climb extremely smooth surfaces such as glass. A gecko can even hang from a single toe!
Scientists have found out the main reason for the adhesion, more than van der Waals forces
or capillary action, is electrostatic force.
Dipole-Dipole force
This is the intermolecular force that exists between two polar molecules (between two
dipoles). When two polar molecules come close together, they align themselves with
opposite poles (positive and negative poles), and there is a force of attraction between these
poles which is called a dipole-dipole force. This force can exist between different polar
substances such as hydrogen bromide (HBr) and ethanol (C2H5OH). It can also exist
between polar molecules of the same substance such as hydrogen bromide (HBr).
Figure 4: Dipole-Dipole force between hydrogen bromide (HBr) and ethanol
(C2H5OH).
Figure 5: Dipole-Dipole force between hydrogen bromide (HBr) molecule.
Other examples would be between HCl molecules. Why not water (H2O)? It is
also polar but it is considered a special dipole-dipole force called a hydrogen
bond.
Hydrogen bond
Hydrogen bonding is a special type of dipole-dipole attraction between
molecules, not a covalent bond to a hydrogen atom. It results from the
attractive force between a hydrogen atom covalently bonded to a very
electronegative atom such as a N, O, or F atom and another very
electronegative atom. Take note, this is not the force inside the molecule
(covalent bond) but the force between molecules.
Hydrogen bonds are strong intermolecular forces created when a hydrogen
atom bonded to an electronegative atom approaches a nearby
electronegative atom. If you consider hydrogen fluoride molecules, it is a very
polar molecule due to the large difference in electronegativity. There is a
hydrogen bond established between the negative fluorine side of one
molecule and the positive hydrogen side of a second molecule.
Figure 6: Hydrogen bond between hydrogen fluoride (HF) molecules.
If you consider water molecules, it is a very polar molecule due to the large
difference in electronegativity. There is a hydrogen bond established between
the negative oxygen side of one molecule and the positive hydrogen side of a
second molecule.
Figure 7: Hydrogen bond between water (H2O) molecules.
Let’s now look at ammonia molecules. They also have quite polar molecules
but not as polar as with oxygen and fluorine which are more electronegative.
There is a hydrogen bond established between the negative nitrogen side of
one molecule and the positive hydrogen side of a second molecule.
Figure 8: Hydrogen bond between ammonia (NH3) molecules.
Hydrogen bonds are relatively strong intermolecular forces and substances
with hydrogen will have higher boiling and melting points.
A question you might ask is, why does chlorine not form a hydrogen bond if it
has the same electronegativity as nitrogen? As you know chlorine is in a
period below oxygen, nitrogen, and fluorine and is larger than N, F, and O. It
does not make a strong H bond. Chlorine is in a lower period and therefore the
size of the Cl atom is larger. The bond length is therefore greater between HCl
than HF, for example, and it simply produces a dipole-dipole attraction.
Dipole-Induced dipole
This is the intermolecular force that exists between a polar molecule and a
nonpolar molecule.
When a polar molecule comes close to a nonpolar molecule there is a weak
attraction between the polar molecule which induces a dipole in a nonpolar
molecule by disturbing the arrangement of electrons in the non-polar
molecules.
Figure 9: Dipole-induced dipole between HCl and Argon
Figure 10: Dipole-induced dipole between water and an oxygen molecule.
To identify if you have dipole-induced dipole forces, one substance will be
polar and the other will be a nonpolar molecule or atom.
Induced dipole-induced dipole force or London Dispersion
forces
The induced-induced dipole force is the weakest intermolecular force. It is
sometimes called a London dispersion force, which is the term we will use in
Grade 12 while studying organic chemistry, is a temporary attractive force that
results when the electrons in two adjacent atoms occupy positions that make
the atoms form temporary or momentary dipoles.
Due to the constant motion of the electrons, an atom or molecule can develop
a temporary or a momentary dipole when its electrons are all on one side of
the nuclei.
Figure 11: Atoms electrons moving to one side of the nucleus producing a
dipole.
If another atom or molecule comes close to it in turn, it can be distorted by the
dipole in the first atom or molecule. This then causes an electrostatic
attraction between the two atoms or molecules called an induced - induced
dipole force or London force.
Figure 12: The first dipole inducing a dipole in a second atom.
These forces are dominant in the halogens (e.g. F2 and I2) and other nonpolar
molecules such as carbon dioxide and carbon tetrachloride. These are the
weakest intermolecular forces, and therefore substances having these forces
will have the lowest boiling and melting points as it requires the least energy
to break these intermolecular forces.
The intermolecular forces are responsible for the phases that substances are
in. If the intermolecular forces are strong, then the substance's boiling and
melting points will also be high. On the next page, we will go through some
worked examples on intermolecular forces.
What is a physical property?
Key term
Physical property: Any characteristic that can be determined without
changing the substance's chemical identity.
When a physical change takes place, it does not change the actual compound
or molecules. Physical properties can be measured or observed without
changing the chemical nature of the substance. A chemical change would
entail changing the chemical nature of a substance through a chemical
reaction.
Why is knowing physical properties important?
Knowing the physical properties of compounds and molecules enables
scientists and engineers to use materials in different applications due to their
various properties.
What are the properties affected by intermolecular forces?
The physical properties of melting point, boiling point, vapour pressure,
evaporation, viscosity, surface tension, and solubility are related to the strength
of attractive forces between molecules.
The table below will help you to determine what intermolecular force exists
between substances. You must be able to identify the shape as well as
whether a molecule is polar or nonpolar.
Figure 1: Summarising intermolecular forces.
The stronger the intermolecular forces, the greater the effect it will have on
the properties. We are going to study each property individually to understand
the effect that the intermolecular force plays on them. If you remember from
the kinetic theory of matter, the phase of a substance is dependent on the
strength of the forces between its particles. The weaker the forces, the more
chance the substance will be a gas. This is because the particles can move far
apart since they are not held together with a strong force. If the forces are very
strong, the particles are held closely together in a solid structure.
In the next few pages, we will look at different physical properties such as:
❖​ Vapour pressure, melting and boiling points
❖​ Thermal expansion and thermal conductivity
❖​ Viscosity and density
❖​ Surface tension
Vapour pressure
Some liquids seem to change to gases more easily than others. This ease of
change is called vapour pressure.
Key term
Vapour pressure: The pressure exerted by a vapour at equilibrium with its
liquid in a closed system.
The stronger the intermolecular forces, the lower the vapour pressure. Liquids
with a high vapour pressure evaporate easily. Liquids with a low vapour
pressure do not evaporate easily.
Melting and boiling point
Intermolecular forces affect the melting and boiling points of substances.
Substances with weak intermolecular forces will have lower melting and
boiling points than those with strong intermolecular forces, which will have
high melting and boiling points.
Key terms
Boiling point - The temperature at which the vapour pressure of a substance
equals atmospheric pressure.
The stronger the intermolecular forces, the higher the boiling point.
The boiling point is used in many places in industry to separate substances
from each other. A process called fractional distillation is used in the
production of petrol and other fuels from crude oil. What exactly is crude oil?
Crude oil is a mixture of hydrocarbons (molecules made of hydrogen and
carbon atoms) that exists in the liquid phase in natural underground lakes and
remains liquid at atmospheric pressure. It is basically unprocessed oil. We
make petrol, diesel and paraffin from crude oil, so it is quite important.
Fractional distillation is a process that separates a mixture into several
different parts, called fractions. Some of the hydrocarbon molecules have
short molecules and need less energy to break bonds whereas some
molecules have long chains and need more energy to break intermolecular
forces.
Crude oil is heated outside a fractional distillation column (see diagram below)
until it forms mostly gases. It is cooler at the top of the column than at the
bottom. As the gases rise through the fractional column, different substances
condense at different temperatures. The boiling point of petrol is between 25 –
60° C but diesel fuel is 220 – 250 °C. Because they have different boiling points,
they will be taken out of the fractional column at different points. This is
extremely useful for us to produce the fuels we need daily.
Figure 7: Fractional distillation of crude oil.
In the example above, we made a liquid (component of crude oil) into a gas by
heating, but can we do the opposite? To make a liquid into a gas, we must
remove heat and therefore decrease the kinetic energy of the system. We are
going to learn about doing the opposite now. Ammonia is a very useful
substance that is used in many applications. To produce ammonia (NH3), we
need nitrogen and hydrogen. The nitrogen is mostly taken from the air around
us, which is a mixture of gases.
So if we decrease the temperature of air, then it will form a liquid.
Key term
Melting point - The temperature at which the solid and liquid phases of a
substance are at equilibrium.
The stronger the intermolecular forces, the higher the melting point.
The air in Earth's atmosphere consists of nitrogen (78 %), oxygen (21 %), argon
(0,93 %) and carbon dioxide (0,038 %).
Fractional distillation of liquid air.
Figure 8: Fractional distillation of liquid air.
By cooling air to about -200 °C, it becomes a liquid. If the temperature is slowly
increased from -200 °C temperature, nitrogen gas can be taken off at -195,8 °C
and oxygen gas can be removed at -183 °C. This is due to their different boiling
points.
What is the difference between evaporation and boiling?
Evaporation occurs on the surface of a liquid.
Key term
Evaporation is defined as the process of conversion of liquid into vapour from
the surface of the liquid at room temperature.
Evaporation occurs at any temperature while boiling occurs at a fixed
temperature.
Thermal expansion and thermal conductivity
As substances are heated, their molecules start moving more vigorously (their kinetic energy
increases). This causes them to expand on heating. You can observe this in a thermometer.
As the alcohol (or mercury) is heated, it expands and rises up the tube.
Thermal conductivity
Conduction is the transfer of thermal energy between particles that are in contact. Different
materials conduct heat differently. Heat is transferred through the substance from the heated
point to the other end. This is why the bottom of the pot heats up first (if you heat the pot on
the stove). Metals have free, delocalised electrons that help transfer thermal energy through
the metal. In covalent molecular bonds, there are no free, delocalised electrons, and heat
does not pass through the material so easily. If you look at the picture below, the handles of
the pot are made from plastic which is a poor thermal conductor.
Viscosity and density
Viscosity
Some liquids flow more easily than others. Which liquid flows easier? Water, syrup or
honey? The water flows much faster than the syrup or honey.
Key term
Viscosity is the resistance to flow of a liquid.
Density
From previous years, you know that in solids the atoms and molecules are arranged in an
orderly way, whereas in liquids the molecules are more loosely arranged and are further
apart. In gases the molecules are even further apart. These differences in arrangements of
the molecules are due to the intermolecular forces between the molecules.
Key term
➔​ Density is a measure of the mass in a unit volume.
➔​
➔​ These forces pull the molecules together which results in more molecules in one unit
volume than in the liquid or gas phases.
Surface tension is the property of the surface of a liquid that allows it to resist an external
force, due to the cohesive nature of its molecules.What does cohesive mean? A cohesive
force is the action or property of like molecules sticking together, being mutually attractive.
➔​ The cohesive forces between liquid molecules at the surface do not have molecules
all around it and therefore they are more strongly bound together. This forms a thin
layer which makes it more difficult.
➔​ The surface tension of water decreases significantly with temperature. The surface
tension arises from the polar nature of the water molecule.