 Notes by 0625 syllabus content
PHYSICS NOTES
SIMPLE KINETIC MOLECULAR
MODEL OF MATTER
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State the distinguishing properties of solids, liquids and gases
Describe qualitatively the molecular structure of solids, liquids and gases in terms of the
arrangement, separation and motion of the molecules
Relate the properties of solids, liquids and gases to the forces and distances between molecules and
to the motion of the molecules
STATE
ARRANGEMENT
MOTION
Solid
The particles are closely packed in a
regular pattern and held in place by
strong intermolecular forces.
This is also the reason to why solids
retain their shape
The particles vibrate about a fixed
position within the structure. This is
because each particle is in close
contact with all the neighbouring
particles due to the strong forces
therefore leaving no relative space to
move around
Liquid
The particles are packed slightly less
close together compared to a solid
and in an irregular pattern (more
jumbled and disorderly). The
intermolecular forces are also weak.
This is also the reason to why liquids
take up the shape of their container
The particles slide over each other in
a random manner. This is because the
particles less tightly packed due to
the weak forces hence leaving room
to move around.
Gas
The particles are widely spaced and
separated from each other, no longer
in contact unless on collision. The
forces between the molecules are
negligible.
This is why gases fill their container.
The particles are in random motion,
faster than the other states, bouncing
off one another and off the inner
surfaces of their container.
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Interpret the temperature of matter in terms of the motion of its molecules
When matter is heated, energy is given to the molecules. Generally, the molecules gain kinetic energy. In a solid they
vibrate faster and, in a liquid, or gas they move faster. Not all molecules move at the same speed or in the same direction.
The average kinetic energy of the molecules is a measure of the temperature of the material – the higher the average
kinetic energy of the molecules, the higher the temperature.
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Describe qualitatively the pressure of a gas in terms of the motion of its molecules
Explain pressure in terms of the change of momentum of the particles striking the walls creating a
force
A gas is stored in a container. The gas molecules move at random and collide with each other and the container walls and
bounce off. Each time the wall is hit there is a small force on the wall. There are billions of collisions with the wall each
second, and the overall effect of these forces is to produce a pressure on the wall because force is directly proportional to
pressure: 𝑃 =
𝐹
𝐴
If the gas is heated, the molecules move faster. Therefore, they collide with each other and the walls more frequently and
each collision is harder. This means more force and because 𝑃 =
𝐹
𝐴
,the result is that the pressure increases when the
temperature increases.
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Describe evaporation in terms of the escape of more-energetic molecules from the surface of a
liquid
Demonstrate an understanding of how temperature, surface area and draught over a surface
influence evaporation
Relate evaporation to the consequent cooling of the liquid
By Eng. Trish Sakarombe
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Explain the cooling of a body in contact with an evaporating liquid
Distinguish between boiling and evaporation
Evaporation is when a liquid changes to a gas at a temperature below its boiling point. For example, on a sunny day, a
puddle of water will slowly disappear. The liquid water has turned into vapour. Clearly the water has not boiled, it has
evaporated. Evaporation occurs when the most energetic molecules leave the surface of a liquid. This results in the
average kinetic energy of the remaining molecules being reduced. Consequently, the temperature of the remaining liquid
falls. Evaporation increases when:
1.
2.
3.
The temperature increases
The surface area of the liquid is increased
There is a breeze (draught) across the surface of the liquid
Clothes left crumpled up take a long time to dry because the surface area exposed to air is reduced. A sunny, windy day is
best for drying clothes. The raised temperature means that more molecules have sufficient energy to escape and the wind
blows away those molecules which have already escaped, so they cannot go back into the liquid or bounce other molecules
back into it.
Evaporation vs Boiling
Evaporation
Surface effect
Boiling
Occurs throughout the liquid with bubbles of vapour
forming in the liquid
Occurs only at a boiling point
Vigorous process
Occurs at all temperatures
Quiet process
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Describe qualitatively, in terms of molecules, the effect on the pressure of a gas of:
– a change of temperature at constant volume
– a change of volume at constant temperature
Recall and use the equation 𝒑𝑽 = 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕 for a fixed mass of gas at constant temperature
At constant volume, if the temperature of a gas is increased, the molecules gain kinetic energy and begin to move faster.
This increases the chances of collisions between the gas molecules and the walls of the container, creating a force. The
higher the force, the higher the pressure. The opposite is true.
At constant temperature, if the volume of a gas is increased, the molecules will have more space to move around in. This
decreases the chances of collisions between the gas molecules and the walls of the container, resulting in less force
created. The lower the force, the lower the pressure. If the volume is decreased (compressing a gas), the molecules will
have less space to move around in. This increases the chances of collisions between the gas molecules and the walls of the
container, resulting in a higher force. The greater the force, the greater the pressure.
**Note** ONLY MENTION KINETIC ENERGY WHEN THERE IS A CHANGE OF TEMPERATURE NOT WHEN TEMPERATURE IS
KEPT CONSTANT.
𝐵𝑜𝑦𝑙𝑒 ′ 𝑠 𝐿𝑎𝑤 ∶ 𝑝𝑉 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
Where 𝑝 = pressure
And 𝑉 = volume
By Eng. Trish Sakarombe
𝑝∝
1
𝑉
𝑉=
1
𝑝
Boyle’s law states that the volume of a fixed mass of gas is inversely proportional to its pressure, provided its temperature
remains constant.
Similarly, we can make calculations using:
𝑝1 𝑉1 = 𝑝2 𝑉2
Where,
𝑝1 = initial pressure
𝑝2 = final pressure
𝑉1 = initial volume
𝑉2 = final volume
By Eng. Trish Sakarombe