IP 1 Green Science - Chemistry
TOPIC 3 – Diversity of Matter by
its Chemical Composition (Elements, Compounds and Mixtures) &
Using Separation Techniques
(Theme: Diversity)
Contents
3.1
Learning Outcomes
3.2
Elements
3.3
3.2.1
Chemical Symbols of Elements
3.2.2
Classification of Elements
Compounds
3.3.1 Formation of Compounds
3.3.2 Decomposition of Compounds
3.4
Mixtures
3.4.1 Comparing between Mixtures and Compounds
3.4.2 Diagrammatic Representation of Elements, Compounds and Mixtures
3.4.3 Examples of Mixtures
3.4.4 Solutions and Suspensions
3.5
•
Comparing between Solutions and Suspensions
•
Concept of Solubility
•
Rate of dissolving
•
Concept of Miscibility
Separation Techniques
3.5.1 Filtration
3.5.2 Simple distillation
3.5.3 Fractional distillation
3.5.4 Use of separating funnel
3.5.5 Paper chromatography
•
Measurement of Retardation or Retention Factor, Rf values
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3.1
Learning Outcomes
By the end of the topic, students will be able to:
a) describe the general physical properties of metals as solids having high melting and boiling
points, malleable, good conductors of heat and electricity in terms of their structure [LSS]
b) describe an alloy as a mixture of a metal with another element, e.g. brass; stainless steel
[LSS]
c) describe the differences between elements, compounds and mixtures [LSS]
d) show an understanding that compounds are substances consisting of two or more
chemically combined elements [LSS]
e) show an understanding that mixtures are made up of two or more elements and/or
compounds that are not chemically combined [LSS]
f) describe the volume composition of gases present in dry air as being approximately 78%
nitrogen, 21% oxygen and the remainder being noble gases (with argon as the main
constituent) and carbon dioxide [O Level]
g) name some common atmospheric pollutants, e.g. carbon monoxide; methane; nitrogen
oxides (NO and NO2); ozone; sulfur dioxide; unburned hydrocarbons [O Level]
h) state the sources of these pollutants as
(i) carbon monoxide from incomplete combustion of carbon-containing substances
(ii) nitrogen oxides from lightning activity and internal combustion engines
(iii) sulfur dioxide from volcanoes and combustion of fossil fuels [O Level]
i) discuss some of the effects of these pollutants on health and on the environment
(i) the toxic nature of carbon monoxide
(ii) the role of nitrogen dioxide and sulfur dioxide in the formation of ‘acid rain’ and its
effects on respiration and buildings [O Level]
j) distinguish between solute, solvent and solution [LSS]
k) show an understanding that solutions and suspensions are mixtures [LSS]
l) describe the factors that affect the rate of dissolving and solubility of substances [LSS]
m) describe and explain methods of separation and purification for the components of
mixtures, to include: magnetic attraction, filtration, evaporation, sublimation, simple
distillation, fractional distillation, use of a separating funnel, paper chromatography [LSS]
[sublimation, fractional distillation, and separating funnel: O Level]
n) state some examples of the applications of the various separation techniques in everyday
life and industries [LSS]
o) Identify and state the functions of the parts of a Bunsen burner. [LSS]
p) Compare and contrast a luminous and non-luminous flame [LSS]
q) Use the Bunsen burner correctly [LSS]
r) suggest suitable separation and purification methods, given information about the
substances involved in the following types of mixtures: solid-solid, solid-liquid, liquid-liquid
(miscible and immiscible) [O Level]
s) interpret paper chromatograms including comparison with ‘known’ samples and the use of
Rf values [O Level]
t) deduce from given melting point and boiling point data the identities of substances and
their purity [O Level]
u) explain the importance of measuring the purity in substances used in everyday life, e.g.
foodstuffs and drugs [O Level]
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3.2
Elements
•
An element is a substance that cannot be broken down into simpler substances by
chemical methods or electricity.
•
Hydrogen and oxygen are elements because they cannot be further broken down into
simpler substances.
•
Sugar is not an element. Although sugar can be broken down into water and carbon when
heated strongly, the water produced can be broken down even further by electricity to give
the elements hydrogen and oxygen.
The “chemical methods” or chemical reactions
are the core of chemistry learning.
You’ll be learning more about this from IP2 all the
way to JC2. 😊
3.2.1 Chemical Symbols of Elements
•
Elements can be represented using chemical symbols. Each element has a unique
symbol consisting of one or two letters. If the symbol only has one letter, the letter has to
be capitalised. If the symbol has two letters, only the first letter is capitalised.
For example, the symbol for sulfur is S and the symbol for tin is Sn.
•
The chemical symbols of elements are found in the Periodic table. An example of how
carbon appears in the Periodic Table is shown below.
atomic number
6
C
name
chemical symbol
carbon
12.0
relative atomic mass
You will learn more about what “atomic number” and “relative atomic mass” represent in
the topic on Atomic Structure.
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3.2.2 Classification of Elements
•
There are 118 known elements currently. The most recently discovered element
(Tennessine) to be included in the Periodic Table was officially announced in 2010.
•
One way of organising elements would be to classify them into metals and non-metals.
The different physical properties of metals and non-metals are shown in the table below.
Metals
Appearance
Shiny appearance (lustrous)
Non-metals
Usually dull appearance if solid (nonlustrous)
State under
Solids (except for mercury)
Either gases, solids with low melting
room
points (except for carbon, which has a
conditions
high melting point)
Malleability
Malleable (can be hammered into
Brittle if solid (easily broken when
and ductility
different shapes without breaking)
hammered)
Ductile (can be drawn into wires)
Melting and
Usually high melting points and
Low melting points and boiling points
boiling
boiling points
(except for carbon and silicon)
Good conductors of heat
Generally poor conductors of heat
points
Heat
conductivity
(except carbon in the form of graphite)
Electrical
Good conductors of electricity in
Generally poor conductors of
conductivity
all states of matter
electricity (except carbon in the form
of graphite)
In this section, we talk about PHYSICAL PROPERTIES of metals vs non-metals.
On the next page, we will come across the term CHEMICAL PROPERTIES.
Physical properties refer to characteristics of a substance that can be observed or
measured without changing the identity of the substance.
For e.g. the melting point of a substance refers to the constant temperature at which it starts to
change from solid state to liquid state – there is a change of state, but the identity of the substance
DOES NOT CHANGE.
Chemical properties describe the ability of a substance to undergo a specific chemical
change (which involves a change of the substance’s identity).
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•
A more comprehensive way of organising elements would be the Periodic Table. The
Periodic Table is a list of elements arranged in order of their increasing atomic (or proton)
numbers.
non-metals
metals
Periodic Table of Elements
•
Metals are located on the left while non-metals are located on the right of the Periodic
Table. They are separated by the “zig-zag” line shown in the Periodic Table above.
•
The Periodic Table also divides the elements into periods and groups. A Period is a
horizontal row of elements and a Group is a vertical column of elements (Groups 1, 2 and
13 to 18).
For example, elements in Period 3 are:
Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine
(Cl) and Argon (Ar)
For example, elements in Group 17 are:
Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At)
•
Several elements, for example, silicon (Si) and germanium (Ge), are located close to the
dividing line. Because of their positions, these elements have the properties of both a metal
and non-metal. Silicon and germanium are known as metalloids.
•
Certain groups in the Periodic Table are given “special names”:
o Group 1 elements: alkali metals
o Group 2 elements: alkali earth metals
Elements in the same group have
o Group 17 elements: halogens
similar chemical properties
o Group 18 elements: noble gases
•
The uses of common elements can be found in the infographic on the last page of the
notes.
What do elements in the same period have in common? They have the same number of
electron shells. Don’t worry, you will learn more in the topic Atomic Structure.
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3.3
Compounds
•
A compound is a substance containing two or more elements chemically combined
together.
•
The table below shows a list of common compounds and the elements they contain.
Compound
Chemical formula
Elements present
sodium chloride
NaCl
sodium, chlorine
carbon dioxide
CO2
carbon, oxygen
calcium carbonate
CaCO3
calcium, carbon, oxygen
copper(II) sulfate
CuSO4
copper, sulfur, oxygen
hydrogen chloride
HCl
hydrogen, chlorine
3.3.1 Formation of Compounds
•
The formation of compounds involves a chemical reaction. A chemical reaction is a
process in which one or more substances (known as reactants) are converted to one or
more different substances (known as products).
•
Chemical reactions usually involve the absorption or release of energy in the form of light
and/or heat.
•
For example, when the element magnesium burns in the element oxygen, a brilliant white
flame is seen and a new substance, magnesium oxide is obtained. The new substance,
magnesium oxide, is called a compound. It has very different properties from the elements
that form it.
3.3.2 Decomposition of Compounds
•
A compound cannot be broken down by physical means (i.e. no chemical reaction
involved). A chemical reaction is involved when a compound is broken down into its
constituent elements or simpler compounds.
•
Energy is taken in when a compound is broken down. This energy can be in the form of
heat or electricity. The chemical process in which heat is used to break down a compound
is known as thermal decomposition. When electricity is used, the process is known as
electrolysis.
•
For example, when mercury(II) oxide is heated strongly, it thermally decomposes to give
the elements mercury and oxygen. For example, water can be broken down into hydrogen
and oxygen using electricity through the process of electrolysis.
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3.4
Mixture
•
A mixture consists of two or more substances that are not chemically combined
together.
•
Mixtures can consist of elements, compounds or both (as long as these components are
not chemically combined). Some examples of mixtures are brass (a type of alloy), crude
oil, air and seawater.
3.4.1 Comparing between Mixtures and Compounds
The following table provides a summary of key differences between compounds and mixtures.
Compound
Melting and
boiling point
Compounds have fixed melting
and boiling points.
Separation
Compounds can be broken down
into their constituent elements
only by chemical methods.
Formation
Compounds are formed by
chemical reactions.
Mixture
Mixtures melt and boil over a
range of temperatures.
Note:
In general, impurities decrease the
melting point and increase the
boiling point of a substance.
Mixtures can be separated into
their individual components
simply by physical means e.g.
filtration, distillation.
The formation of mixtures do not
involve any chemical reactions.
Composition The elements in a compound are
always combined in a fixed
proportion.
The components of a mixture can
be mixed in any proportion.
Properties
The chemical properties of a
mixture are the same as those of its
components.
The chemical properties of a
compound are different from those
of the elements in the compound.
Do You Know?
Just like how substances are classified according to their properties, various
organisms are also classified based on their characteristics.
In the topic of Understanding Diversity of Living Things, we will learn more about
differences between artificial classification and natural classification.
Artificial: Use only external features such as easily observable features.
Natural: Based on morphology (external), anatomy (internal), genetics (DNA / RNA),
cytology (study of cells) & ecology (environment).
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3.4.2 Diagrammatic Representation of Elements, Compounds and Mixtures
Elements, compounds and mixtures can be represented diagrammatically. Some examples
are shown below:
Element
Element
Compound
Mixture of elements
Mixture of
compounds
Mixture of element
and compound
Quick Check 1
a)
b)
Write down the chemical symbols for the elements.
Astatine: ________________
Iron:
________________
Arsenic: ________________
Lead:
________________
Chlorine: ________________
Manganese:
________________
Suggest a similarity between an element and a compound.
Both have _____________________________________________________________
c)
Suggest a difference between an element and a compound.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
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d) True or false
Indicate whether the following statements are true or false.
1. A compound can have variable melting points.
2. A compound can only be made of two constituent elements.
3. An element cannot be further broken down by chemical methods.
4. A compound can only exist as one state.
5. The composition of the constituent elements in a compound can vary.
6. A compound can be formed from two or more compounds.
7. A compound can only be formed from a metal and non-metal.
8. A chemical reaction is necessary for a compound to be formed.
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3.4.3 Examples of Mixtures
Example 1: Alloys
•
An alloy is a mixture of metals with other elements. An example is brass, which is an
alloy of copper and zinc. The table below shows some other common alloys and their
compositions.
Alloy
Composition (percentage of element by mass)
stainless steel*
iron (73%), chromium (18%), nickel (8%), carbon (1%)
bronze
copper (90%), tin (10%)
duralumin
aluminium (95%), copper (4%), magnesium (1%)
* Note: The percentage of iron and carbon varies for different types of steel
Example 2: Air
•
Air is a mixture of gases. Dry air consists of 78% nitrogen (N2), 21% oxygen (O2), 0.03%
carbon dioxide (CO2) and the remaining gases are referred as trace gases (with argon as
the main constituent).
•
Some common atmospheric pollutants are: nitrogen dioxide (NO2), carbon monoxide (CO),
sulfur dioxide (SO2) and particulate matter.
•
Some sources and harmful effects of these atmospheric pollutants are shown in the
table below:
Pollutants
Oxides of
nitrogen
•
•
•
Sources
Lightning
Forest fires
Vehicle engines
•
•
Harmful effect
Irritates and damages the
lungs
Formation of acid rain
Carbon
monoxide
Incomplete combustion of fossil
fuel
Toxic gas which binds to
haemoglobin in red blood cells,
causing breathing difficulties and
eventually death
Sulfur dioxide
•
•
•
Volcanoes
Combustion of fossil fuels
Emissions from industries
Formation of acid rain
Particulate matter •
Volcanoes, dust storms,
forest and grassland fires
Burning of fossil fuels in
vehicles and power plants
Increased levels of fine particles
in the air are linked to health
problems such as heart disease
and lung cancer
•
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3.4.4 Solutions and Suspensions
•
Most mixtures can be classified into solutions and suspensions, which have different
characteristics.
Solutions
•
When a salt and water are mixed, the solid salt cannot be seen as it has been dissolved
in water. A solution is formed, where water is the solvent while salt is the solute.
•
The solute is the substance that dissolves in a solvent. The solvent is the substance that
dissolves the solute. The solution is the mixture of solute and solvent.
•
Other examples of solutions include dyes in water for fabrics, Ribena concentrate in water
and alcohol (ethanol) as a solvent for perfumes.
Suspensions
•
When sand is mixed with water, the sand remains undissolved and can be still be seen
with our naked eye. A suspension is formed as a result.
•
A suspension is a mixture in which insoluble substances (substances that cannot be
dissolved) are suspended in a liquid or a gas.
•
Examples of suspensions include muddy water, chocolate milk and salad dressing.
Comparing between Solutions and Suspensions
Solutions
Suspensions
Homogeneity Homogeneous. This means that Heterogeneous. This means that the
the physical and chemical physical and chemical properties are
properties
are
the
same not the same throughout the mixture.
throughout the solution.
Separation
on standing
The solute does not separate Insoluble solids settle to the bottom
from the solvent when left on when the suspension is left on
standing.
standing.
Effects of
filtration
No residual particles will remain Insoluble solids will remain on filter
on filter paper when solution is paper when suspension is filtered.
filtered.
Effects of
light
Light can pass through
Light cannot pass through
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Concept of Solubility
•
There is a limit to the amount of solute that can dissolve in a given amount of solvent at a
given temperature. This limit depends on the solubility of the solute.
•
Solubility is the maximum mass of solute that can dissolve in a given volume of
solvent. A substance is said to be soluble in a solvent if it dissolves completely to form a
solution.
•
A saturated solution is formed when the maximum amount of solute is dissolved in a
solvent at a given temperature. It means that no further amount of solute can dissolve in
the solution. A concentrated solution simply means that there are lots of solute present in
the solution. More solute can be added to it before it becomes saturated.
•
If a solid is unable to dissolve in a liquid, the solid is said to be insoluble. A suspension is
formed as a result. For instance, as sand is insoluble in water, a suspension is formed on
mixing.
Do You Know?
The concentration of a solute in a solution can affect the processes of diffusion, osmosis and
active transport. We will learn more about these processes in the topic of Movement of
Substances.
•
The following factors affect solubility of a solute:
1. Nature of solute
Different solutes have different solubility in a solvent. Some are more soluble than
others. For example, table salt has high solubility in water while magnesium oxide has
such low solubility in water that it can be considered as insoluble.
2. Nature of solvent
The type of solvent used affects the solubility of the solute. For example, table salt is
more soluble in water than in oil, while iodine is more soluble in hexane than in water.
An application is the use of nail polish remover which contains organic solvents such
as acetone. Nail polish is soluble in the nail polish remover but is insoluble in water.
3. Temperature of solvent
The solubility of solids changes as the temperature of the solvent changes. In general,
increase in temperature of the solvent will result in increasing solubility of solids.
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Concept of Rate of Dissolving
•
The rate of dissolving is a measure of how fast a substance dissolves in a solvent.
•
The following factors affect the rate of dissolving of a substance:
1. Surface area of solid
Smaller pieces of a solid dissolve faster than larger pieces due to the larger exposed
surface area that comes into contact with the solvent. For example, fine sugar
dissolves faster than rock sugar.
2. Rate of stirring
Stirring the solution vigorously will help to speed up the rate of dissolution.
3. Temperature of solvent
The higher the temperature of the solvent, generally solute dissolves faster.
Concept of Miscibility
•
When two liquids are able to mix together to form a clear solution with no distinct layers,
we do not use the word soluble to describe the liquid added (“soluble” is used only for
solids); instead, we say that the liquids are miscible in each other. For instance, ethanol
and water are miscible in each other forming a clear solution.
•
Two liquids are said to be immiscible if they are unable to mix with each other. Two
distinct layers will be formed. For instance, oil and water are immiscible in each other,
forming two distinct layers when mixed.
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3.5
Separation Techniques
There are some situations where the purity of substances is important. Mixtures have to be
first separated into their constituent components before these components can be used. The
separation technique used depends on the type of mixture:
•
A mixture of solid and liquid
•
A mixture of two or more liquids
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3.5.1 Filtration
Concept behind filtration: Separation of components of a mixture based on
DIFFERENCES IN SIZE (which affects whether components can pass
through the pores of the filter)
•
Filtration is the means of separating an insoluble solid from a liquid.
•
For filtration to occur, the solid component in the mixture must be insoluble in the liquid.
There must also be the presence of a filter that is selective, allowing the liquid to pass
through, retaining the solid behind.
A solid can be separated from a liquid by filtration
because the filter paper acts as a sieve.
The liquid can pass through the pores (small
holes) of the filter paper but the solid cannot do so.
•
Upon filtration, the solid that remains on the
filter paper is called the residue.
•
The liquid or solution that passes through the
filter paper is called the filtrate.
Filter funnel
- Lined with a filter paper to channel liquid
into another container during filtration
- Filter paper: a partially
permeable barrier which
is used to separate fine
solids from a liquid
beaker
Beaker
- Simple glass container for stirring, mixing and heating liquids
- Generally cylindrical in shape with a flat bottom and a lip for pouring
- Size of the beaker chosen depends on the experiment; comes in
various sizes e.g. 100 cm3 or 250 cm3
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3.5.2 Simple distillation
Concept behind distillation: Separation of components of a
mixture based on DIFFERENCES IN BOILING POINTS
•
Simple distillation separates a pure liquid from a solution. A pure solvent can be
separated from a solution by simple distillation. Distillation is the process of boiling a
liquid and condensing the vapour.
•
For example, pure water can be obtained from seawater by simple distillation. The
apparatus used for simple distillation is shown below.
Round-bottom flask
- Round body with a cylindrical
neck
- The round body allows a more
uniform heating and/or boiling
- Must be supported by retort stand
with boss and clamp at the neck
roundbottom flask
Tripod stand
- 3-legged
equipment used
as a platform to
support and hold
various flasks
- A wire gauze is
usually placed on
it to allow support
of the flask being
heated
Liebig condenser
- Consists of a straight
glass tube
surrounded by a
water jacket
- Used to cool and
condense hot vapour
during distillation
Bunsen burner
- Used for providing heat during experiments
- Different parts of a Bunsen burner:
-
Functions of different parts of a Bunsen burner:
o Barrel - Raise the flame to a suitable level for burning
o Collar - Adjust the size of the air-hole
o Air-hole - Allow air to enter the Bunsen burner
o Jet - Allow the gas to rush into the Bunsen burner
o Base - To support the Bunsen burner, making it more stable
o Gas-tap - Control the amount of gas supplied to the Bunsen burner
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More about the Bunsen burner…
•
How to light a Bunsen burner
Step 1: Turn the collar to close the air-hole Step 2: Turn on the gas tap with your free
of the Bunsen burner.
hand.
Step 3: Hold the lighter just above the Step 4: Turn the collar to adjust the size of
barrel and click it.
the air-hole to change the temperature of
the flame.
•
Comparing luminous flame and non-luminous flame
When the air-hole of a Bunsen burner is closed, there is insufficient air for complete
combustion to occur. Hence, a luminous flame which is not so hot is observed.
When the air-hole of a Bunsen burner is opened, there is now plenty of air available for
complete combustion to occur. Hence, a hotter non-luminous flame is observed.
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•
When setting up the distillation apparatus, note the following:
➢ Feature: Boiling chips or anti-bumping granules are usually added to the mixture in
the round-bottom flask during heating.
o Significance: The boiling chips or anti-bumping granules provide a surface for the
bubbles produced during boiling to burst on, hence ensuring smoother or less
vigorous boiling.
➢ Feature: The thermometer should be placed beside the side arm of the distillation flask.
It should not be dipped into the solution.
o Significance: This ensures that the thermometer measures the boiling point of the
substance that is being distilled.
➢ Feature: The condenser consists of two tubes: an inner tube and an outer water jacket.
Cold running water is allowed to enter the water jacket from the bottom of the
condenser and leave from the top.
o Significance: This ensures that the condenser is fully filled with cold water at all
times.
➢ Feature: If the distillate is volatile, the receiver can be placed in a large container filled
with ice.
o Significance: This keeps the temperature of the distillate low, minimising loss of
distillate by evaporation.
•
As the seawater is heated, its temperature increases. When the solution finally boils, the
thermometer records a temperature of 100°C. This is the temperature of the water vapour.
The temperature remains unchanged until all the water has boiled off.
•
Simple distillation is usually not used to separate a mixture of two or more miscible liquids
with similar boiling points. This is because they would boil and condense together during
distillation, resulting in little to no separation.
3.5.3 Fractional distillation
Concept behind distillation: Separation of components of
a mixture based on DIFFERENCES IN BOILING POINTS
(For fractional distillation: components have rather close
boiling points [but there is still a difference])
•
Fractional distillation is the means of separating mixtures of miscible liquids with
differing boiling points.
•
It uses a fractioning column, which is packed with glass beads or filled with plates or a
spiral to provide a large surface area for vapour to condense on. During fractional
distillation, the liquid with the lowest boiling point will distill over to the condenser first
while the vapour of liquids with higher boiling points condense along the
fractionating column and re-enter the round-bottomed flask.
•
For example, a solution of ethanol and water can be separated using fractional distillation.
The apparatus used for fractional distillation is shown below.
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•
Industrial applications of fractional distillation:
✓ Used to separate mixtures of liquids such as crude oil.
✓ Used in industries to obtain nitrogen, argon and oxygen from air.
Air
cooling &
compression
Liquid air
fractional
distillation
Nitrogen (Boiling point: –196°C)
Argon (Boiling point: –186°C)
Oxygen (Boiling point: –183°C)
There are many different substances that can be extracted
from crude oil, and you will be learning a lot more about them
in Organic Chemistry in IP3-4, as well as in JC. 😊
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3.5.4 Use of Separating funnel
Concept behind use of separating funnel:
Separation of two liquids that are IMMISCIBLE
(cannot mix to form a homogeneous layer)
A separating funnel is used to separate a mixture of immiscible liquids.
Immiscible liquids will form separate layers when mixed together. The denser liquid forms the
bottom layer while the less dense liquid will form the top layer.
For example, a mixture of oil and water can be separated using a separating funnel. As water
and oil are immiscible, they form two separate layers when they are mixed together. Water
being the denser liquid will form the bottom layer, while the less dense oil will form the top
layer. The denser liquid, water, will be separated first using the separating funnel followed by
oil, which is less dense.
Stopper
Separating funnel
Oil (less dense liquid)
Water (denser liquid)
Tap
3.5.5 Paper Chromatography
Concept behind paper chromatography:
Separation of components of a mixture based on
DIFFERENCES IN THEIR SOLUBILITIES IN A SOLVENT
•
The technique of using a solvent to separate a mixture into its components is called
chromatography.
•
Chromatography can be used to achieve the following:
✓ Separate the components in a sample
✓ Identify the number of components in a sample
✓ Identify the components present in a sample
✓ Determine whether a sample is pure
•
Paper chromatography can be used to separate dyes in ink, pigments in plants, amino
acids obtained from proteins, to identify poisons (e.g. pesticides) or drugs, and to detect
traces of banned substances in food.
IP1 GS (Diversity of Matter by its Chemical Composition & Using Separation Techniques)
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IP 1 Green Science - Chemistry
•
How does paper chromatography work?
o A spot of food colouring is applied to the chromatography paper.
o Once the chromatography paper is dipped in the solvent, it soaks up solvent.
o The solvent that is soaked up by the paper dissolves the dyes. It continues to travel up
the paper, carrying the dyes along.
o A dye that is not very soluble in solvent will not be carried far along the paper.
o A dye that is very soluble in solvent will be carried far along the paper.
o If the dyes are identical, they will travel the same distance up the paper.
o Coloured spots are left in different places on the paper at the end of the experiment.
•
The result of chromatography is called a chromatogram. The figure below shows a typical
result when dyes from the food colouring in sweets are separated by paper
chromatography. The chromatogram revealed that the food colouring in the sweet was a
mixture of three dyes.
•
When performing chromatography for colourless substances, the chromatogram has to be
sprayed with a locating agent. A locating agent is a substance that reacts with the
substances on the paper to produce a coloured product.
IP1 GS (Diversity of Matter by its Chemical Composition & Using Separation Techniques)
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IP 1 Green Science - Chemistry
Measurement of Retardation or Retention Factor, Rf values
•
The ratio between the distance travelled by the substance and the distance travelled
by the solvent is called the retardation or retention factor, Rf value.
•
The Rf value of a substance does not change as long as chromatography is carried out
under the same conditions (i.e. same solvent and same temperature). This property allows
us to easily identify a substance on a chromatogram.
IP1 GS (Diversity of Matter by its Chemical Composition & Using Separation Techniques)
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IP 1 Green Science - Chemistry
Hydrogen
- Properties: low density, flammable
- Used as a lifting gas in airships
- Used in the manufacture of
margarine
Uses of common elements
Nitrogen
- Used to make fertilisers
- Used as a cryogenic (ultra-cold) fluid
Iron
- Properties:
high tensile
strength,
malleable
- Used to
make
stainless
steel
- Used to
make cans
and foils
Mercury
- Properties: expands evenly on heating, small
change in temperature results in large expansion
- Used in thermometers
Aluminium
- Properties: low density, malleable
- Used to make aircraft bodies
- Used to make cans and foils
IP1 GS (Diversity of Matter by its Chemical Composition & Using Separation Techniques)
Chlorine
- Used as a
disinfectant
in swimming
pools
- Used to make
bleach
Iodine
- Used as an
antiseptic to
treat wounds
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