What is a mixture?
lthough the air we breathe seems like a single, or pure, substance, it is actually a
mixture of many substances. Air consists of nitrogen, oxygen, carbon dioxide
and other gases. Like air, all of the natural and manufactured materials on Earth
can be put into one of two categories: pure substances or mixtures. A mixture is a
combination of two or more substances, with each substance having its own
properties. A pure substance is a single substance.
A
There are many different types of
mixture, including solutions,
suspensions and colloids.
Solutions
A solution is a mixture that
contains one or more substances
(the solute) dissolved in another
(the solvent). The solute must
therefore be soluble; that is, it
must be able to dissolve. The
solute in a solution can be a solid,
a liquid or a gas; the solvent is a
liquid. For example, carbon
dioxide gas and solid sugar
dissolved in water form the basis
of a fizzy drink. Water is a good
solvent, because many substances
dissolve in it. Solutions that
contain water as the solvent are
called aqueous solutions.
Concentration
Cordial is a solution you can easily make yourself. In this case, the water
is the solvent and the syrup is the solute. You can alter the flavour of the
cordial by changing the concentration of the solution. The more syrup
(solute) you add, the more concentrated the solution becomes. If you
add too much syrup, you can dilute it by adding more water (solvent).
For some solutions, like cordial, the precise concentration is not very
important. You don’t often measure out the syrup and water before
mixing them. For other mixtures, such as pesticides used on some crops,
the concentration needs to be measured accurately. In this situation, too
high a concentration of pesticide could harm the plant and the
environment.
The cordial in these glasses is a coloured solution. Can you tell which has the greatest
concentration of cordial syrup?
In a solution, the particles of one substance
(the solute) are spread evenly throughout
the other (the solvent). The particles of the
solute are too small to be seen.
You can tell if a mixture is a
solution by holding it up to the light.
If it is a solution, you can see
through it. Some solutions, like
some fizzy drinks, are coloured.
Others, like salt water, are colourless.
Suspensions
When you complete the experiment on the
opposite page, you will notice that not all of the
substances tested dissolve in water. Instead, the
solid particles may sink to the bottom of the test
tube. A suspension is a mixture that contains
visible, insoluble particles in a liquid. If left to
stand, the particles in a suspension sink to the
bottom of the container and form a layer called
sediment.
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Science Alive for VELS Level 5
Soluble or
insoluble?
• Use a spatula to add a very small amount of each
substance to its labelled test tube. Do not use more
than a quarter of a spatula full.
Substances that dissolve are said to be soluble. Those that
do not are insoluble. This experiment investigates the
solubility of some common substances in water.
You will need:
safety glasses and laboratory coat
heatproof mat
7 test tubes
test-tube rack
spatula
samples of:
salt
sugar
flour
coffee
copper sulfate
copper carbonate.
• Draw up a table of your results like this incomplete
one:
Substance
mixed with
water
Clear or
cloudy?
Solution?
(yes/no)
Salt
Sugar
Flour
sand
Coffee
• Half-fill each of the test tubes with cold water.
• Label the test tubes: salt, sugar, flour, coffee, and so on.
...
• Hold each test tube up to the light. Decide whether
the mixture is clear or cloudy. Record your results in
the table.
• Decide whether each mixture is a solution or a
suspension. Record this in the table.
1. Which of the substances dissolved in water?
2. How can you tell if a substance has dissolved?
• Read the information on filtration on pages 46–7.
3. Which of the mixtures could be separated
by filtration?
Muddy water is an example of a mixture that is a
suspension. A sample of muddy water is not clear.
There are visible particles of dirt in the water that do
not dissolve. If you let the muddy water sit undisturbed,
the solid particles will sink to the bottom.
Milk is an example of a liquid suspended in a
liquid. If you let fresh (non-homogenised) milk sit, it
will separate into layers — cream on top and skim
milk underneath.
Instead, the particles of one substance are spread
evenly throughout the other.
There are many types of colloid. The type depends
on whether the substances that form them are solids,
liquids or gases.
Colloids
The muddy water described above is a suspension
because the solid particles in the water eventually
settle to the bottom. In some cases, the particles of
dirt in the water are so fine that they will not settle
out. An example is water in a creek or river that always
appears brown — the particles of clay and dirt in the
water do not settle and give the water a murky, brown
appearance. In this case, the mixture is known as a
colloid. A colloid is not clear like a solution, and the
particles in it do not settle out as in a suspension.
To distinguish a colloid from a solution, hold the mixture up to
a light. If it is clear, the mixture is a solution. Colloids appear
cloudy because light that enters is scattered by larger particles
within the mixture.
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3. Separating mixtures
Fresh pumpkin soup is a colloid.
Small bits of pumpkin are spread
through water and do not settle
in a layer.
Marshmallows are colloids. They are
mixtures of air spread through sugar
and other ingredients.
Homogenised
milk
Another example of an
emulsion is homogenised
milk. Fresh milk straight from
the cow is a suspension.
When left to stand in a cool
spot, fresh milk will settle into
two layers: a layer of cream
and a layer of skim milk.
However, the milk you buy
does not settle to form layers.
The milk you buy is
homogenised. It has been
passed through a small
passage at a high speed to
force the cream to break up
and spread evenly through
the skim milk base.
Homogenised milk is an emulsion. It is made from butterfat (oil)
spread through water. Both parts of this mixture are liquids.
Emulsions
An emulsion is formed when one liquid is spread evenly through another
liquid and does not settle in a layer. Some substances can be forced to
become emulsions. For example, oil and water do not usually mix — the
oil forms a layer across the top of the water. However, if detergent is
added and the mixture is shaken, the oil forms droplets that spread
through the water. The result is an emulsion. In this case, the detergent
acts as an emulsifier, as it has caused the oil and water to become an
emulsion.
Homogenised milk is a mixture of
one liquid (cream) spread evenly
through another (skim milk).
Other types of colloid
Emulsions are just one type of colloid. Here are some other types.
Type of colloid
Type of substance
Examples
Emulsion
Liquid in liquid
Mayonnaise
Sol
Solid in liquid
Murky, brown creek water
Foam
Gas in liquid or solid
Hair mousse, marshmallow
Aerosol
Liquid or solid in gas
Fog, smoke
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Science Alive for VELS Level 5
Making mayonnaise
Try making mayonnaise by mixing a cup of
olive oil with two tablespoons of vinegar, a pinch
of salt and a dash of mustard. Add the oil to the
rest of the mixture very slowly, stirring as you go
(you can use an electric mixer if you have one).
What does the mixture look like? Does it look
like the mayonnaise you would buy?
Now try the recipe again, this time add an egg
yolk to the mixture. Begin by whisking together
the yolk with the vinegar, salt and mustard.
While stirring, add the oil very, very slowly. If
you are patient enough, an emulsion will form.
Does this mixture resemble the mayonnaise
you buy?
Egg yolks contain a substance called lecithin,
which is an emulsifier. If you try to mix vinegar
and oil, the two separate very quickly. Add an
emulsifier and the vinegar spreads evenly
through the oil to form an emulsion. Next time
you are eating chocolate, check the label — you
may find lecithin in the list of ingredients. Can
you suggest why?
REMEMBER
1. What solutes are present in fizzy drinks?
2. Which two types of substance mix to form
emulsions?
3. Label the solvent, solute and solution in this
photo.
4. Is fog a solution, suspension or colloid? Explain
your answer.
THINK
5. Non-homogenised milk separates into two parts if
it is left to stand. But homogenised milk does not
settle. It is an emulsion.
(a) What type of mixture is untreated milk?
(b) Cream and butter are both made from
milk. What types of mixture are cream and
butter?
INVESTIGATE
6. Some substances dissolve better in hot water than
in cold water. Design an experiment to measure
the amount of sugar that can be dissolved in tap
water that is cold and tap water that is hot.
CLASSIFY
7. In groups of three or four, decide whether the
following substances are suspensions, solutions,
colloids or combinations of these.
Muddy water
Cup of coffee
Mayonnaise
Whipped cream
Hot chocolate
Cup of tea with tea-leaves in it
43
3. Separating mixtures
✓ learning
Write down a reason for each decision. Share your
results with the class.
I CAN:
define solution, suspension and
colloid
explain how the concentration of
a mixture can be increased by
adding more solute
explain how an emulsion works.
Chromatography
or thousands of years, people have added colours
to food. The colours made food more appealing.
Early civilisations used saffron and other spices to
colour some foods yellow. Some of the colours added
to the taste, but some were poisonous.
Today, colours are added to food for many reasons.
Some colours protect vitamins that are affected by
sunlight. Other colours replace colours lost during
storage. Some fruits and vegetables are coloured to
give them a more even appearance. Other foods, like
lollies, are coloured just for fun!
F
Safe colours
Because some colours added to
food in the past were actually
poisons, colours and other
additives must pass strict tests
before they can be used in foods.
Foods with added colours can be
tested at any time to identify the
particular additives that have been
used.
One method used for testing is
called chromatography. Even foods
that appear to be just one colour
could be a mixture of colours.
Chromatography separates the
different colours in the mixture.
Chromatography
Chromatography works because different
colours have different solubilities. Some colours
dissolve more easily than others. Water is a very
good solvent for many food colours. However,
to separate the colours, they are not all placed
straight into the water. For paper
chromatography, the food colouring is placed
on paper just above the solvent. The colours
dissolve as the solvent soaks up the paper
column. The colours separate because they are
washed along the paper at different rates. The
less soluble colours move more slowly and travel
less distance up the paper.
Separated colours
The colours that dissolve more
easily are carried further up the
filter paper by the solvent. The
colours become separated along the
paper strip.
Sample of food-colour mixture
A small amount of food colour is
placed on the paper, above the level
of the solvent.
Solvent
The filter paper is hung so that it just dips into the
solvent. The solvent soaks up the strip of filter
paper, taking the food colours with it.
A chromatograph automatically separates mixtures by
chromatography.
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Science Alive for VELS Level 5
Separating colours with
chromatography
REMEMBER
1. Complete the sentence: Chromatography is a
method used to separate parts of a mixture with
different
.
2. Why is water used as a solvent to
separate food colours?
3. List the colours found in this ink,
from:
(a) the fastest moving to the
slowest moving
(b) the most soluble to the least
soluble.
4. List three uses of
chromatography in
industry.
Toothpick
• Cut a strip
Strip of
into the filter
filter paper
paper and
place it over a halffilled beaker of water.
The strip should just dip
into the water.
Food
colouring
Water
• Use the flat end of a toothpick to put a small
amount of food colouring onto the centre of the
filter paper.
THINK
5. Why is the mixture placed
• Wait for the water to soak up into the filter paper.
Observe the mixture as it separates into different
colours.
above the level of the solvent in
chromatography? (Hint: What would happen
if the mixture were placed in the solvent?)
6. Zoe performs a chromatography experiment
on waterproof markers using water as a
solvent. Will her experiment work? Explain
your answer.
• Try this experiment with other food colourings.
1. Are the colours used in food colouring single colours
or mixtures of colours? Explain your answer.
2. How many colours in total are needed to make
each of the different food colourings that you
tested?
CREATE
7. Use chromatography to create colourful designs
that can be displayed as scientific art. Fold the
filter paper and use different colours to make
your designs unique.
Chromatography in industry
Chromatography is used in the food industry to detect
more than just food colours. Food scientists can tell us
what other ingredients have been added to food.
Chromatography can also identify pesticides and
harmful chemicals that have entered our food from
the water in creeks and dams, or from soil pollution.
Forensic scientists use gas chromatography to
detect a range of substances, including traces of
illegal drugs. They can also use chromatography to
compare mixtures found at crime scenes with
those found on suspects. Many mixtures contain
a unique combination of substances. For
example, ink from different pens is slightly
different, even if the colours look the same.
The separating technique of
chromatography is used to detect substances
in blood and urine. In medical laboratories,
samples of blood and urine are tested for
drugs and alcohol. Abnormal levels of
vitamins and hormones in a person’s blood
can also be detected using chromatography.
INVESTIGATE
8. Investigate different types of solvent that could
be used to separate pen ink and waterproof
markers. As a starting point, you may wish to
use methylated spirits. Before running the
experiment, have your choice of solvents
approved by your teacher.
9. Does the colour of food affect whether or not
people choose to buy and eat it? Design an
experiment to test your answer.
10. Write a fake ransom note that could have
been used in a crime. Design an experiment
that someone else could perform to test
markers that may have been used to write
the note.
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3. Separating mixtures
✓ learning
You will need:
filter paper
scissors
beaker
toothpick
food colouring.
I CAN:
explain, in basic terms, how
chromatography works
use chromatography to separate
mixtures of colours
describe examples of how
chromatography is used in industry.