40

Science Alive for VELS Level 5

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.

SolutionsA 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.

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.

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?

SuspensionsWhen 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.

 

 

 

 

41

3. Separating mixtures

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.

ColloidsThe 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.

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.

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.

Soluble or

insoluble?

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 sand

copper sulfate copper carbonate.

• Half-fill each of the test tubes with cold water.

• Label the test tubes: salt, sugar, flour, coffee, and so on.

• 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.

• Draw up a table of your results like this incomplete

one:

• 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?

Substance

mixed with

waterClear or

cloudy?

Solution?

(yes/no)

Salt

Sugar

Flour

Coffee

. . .

 

 

 

42

Science Alive for VELS Level 5

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

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 a mixture of

one liquid (cream) spread evenly

through another (skim milk).

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 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.

 

 

 

 

433. Separating mixtures

✓ le

arn

ing 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.

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

Write down a reason for each decision. Share your

results with the class.

Making mayonnaiseTry 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?

 

 

 

54Science Alive for VELS Level 5

Chromatographyor 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 coloursBecause 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.

ChromatographyChromatography 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.

Sample of food-colour mixtureA small amount of food colour is

placed on the paper, above the level

of the solvent.

SolventThe 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.

Separated coloursThe colours that dissolve more

easily are carried further up the

filter paper by the solvent. The

colours become separated along the

paper strip.

A chromatograph automatically separates mixtures by chromatography.

 

 

 

553. Separating mixtures

Toothpick

Food colouring

Water

Strip offilter paper

Separating colours with chromatography

You will need:filter paperscissorsbeakertoothpickfood colouring.

• Cut a strip into the filter paper and place it over a half-filled beaker of water. The strip should just dip into the water.

• Use the flat end of a toothpick to put a small amount of food colouring onto the centre of the filter paper.

• Wait for the water to soak up into the filter paper. Observe the mixture as it separates into different colours.

• 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?

Chromatography in industryChromatography 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.

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.

THINK

5. Why is the mixture placed 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.

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.

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.

✓ le

arn

ing I CAN:

explain, in basic terms, how chromatography worksuse chromatography to separate mixtures of coloursdescribe examples of how chromatography is used in industry.