Grade 7 · Matter & Materials · Lesson 4

Separating Mixtures

Discover the physical techniques scientists and engineers use to separate substances — from filtering water to refining crude oil.

Why do we separate mixtures?

Most natural materials — seawater, crude oil, air, soil — are mixtures. To obtain a useful, pure substance from a mixture, we need to use a separation technique. All separation techniques rely on the fact that the components of a mixture retain their own physical properties, which differ from one another. No chemical reaction takes place during separation.

Key principle: We choose a separation technique based on the difference in physical properties between the components (e.g. particle size, solubility, boiling point, magnetic properties, density).

Methods of Separating Mixtures

Filtration Solid + Liquid

A mixture of an insoluble solid and a liquid is poured through filter paper in a funnel. The solid (residue) is trapped by the filter paper; the liquid (filtrate) passes through into a beaker below. Works because the solid particles are too large to pass through the tiny pores in the filter paper.

Example: Separating sand from water · Removing tea leaves from tea

Evaporation Solid dissolved in Liquid

The solution is heated so that the liquid (solvent) evaporates away, leaving behind the dissolved solid (solute). Best used when the solid does not decompose on heating and when we want to recover the solid, not the liquid.

Example: Recovering salt (NaCl) from salt water · Making salt pans on the coast

Distillation Two or more Liquids

The mixture is heated; the liquid with the lower boiling point evaporates first. The vapour is cooled in a condenser and collected as a pure liquid. Fractional distillation separates liquids with similar boiling points (e.g. crude oil, liquid air) using a fractionating column.

Example: Purifying water · Desalinating seawater · Refining crude oil into petrol, diesel, and jet fuel

Chromatography Dissolved pigments / dyes

A spot of the mixture is placed on chromatography paper. A solvent travels up the paper by capillary action, carrying the dissolved components with it. Components travel at different speeds depending on how strongly they are attracted to the paper vs the solvent — so they separate into distinct bands (spots).

Example: Separating ink pigments · Testing food dyes · Forensic analysis of substances

Magnetic Separation Magnetic + Non-magnetic solids

A magnet is passed over (or through) the mixture. Magnetic materials (iron, nickel, cobalt) are attracted to the magnet and pulled out of the mixture, while non-magnetic materials remain behind.

Example: Separating iron filings from sand · Removing iron from breakfast cereal · Steel recycling in industry

Sieving Solids of different particle sizes

The mixture is passed through a sieve or mesh. Smaller particles pass through the holes; larger particles are retained. Works because of differences in particle size.

Example: Separating fine flour from coarse bran · Grading sand and gravel in construction · Separating pebbles from soil

Summary: Choosing the Right Method

Insoluble solid + liquid → Filtration
Dissolved solid + liquid → Evaporation (or distillation to keep liquid too)
Two or more liquids with different boiling points → Distillation
Mixed colours/dyes in solution → Chromatography
Magnetic + non-magnetic solids → Magnetic separation
Solids of different sizes → Sieving

Real-world application: Drinking water treatment

Water treatment plants use several separation techniques in sequence: sieving (removes large debris), sedimentation (heavy particles sink), filtration (removes fine particles), and chemical treatment. This demonstrates that real mixtures often need more than one technique to fully separate all their components.

Distillation and desalination

South Africa faces water scarcity. Desalination — removing salt from seawater — is a key strategy. One method is distillation: seawater is heated until water evaporates (salt stays behind), the steam is cooled and collected as fresh water. Modern desalination plants near Cape Town and along the coast use this principle alongside reverse osmosis (forcing water through a membrane under pressure).

Separation Technique Simulator

Choose a mixture and a separation method, then press Run to watch the animation. Then test your knowledge with the Match-the-Method game below.

Select a method and press Run Animation
Choose Mixture
Choose Method
Select a mixture and method above, then press Run.
Match the Method
Click a mixture on the left, then click the correct separation method on the right. Get all 6 correct!
Mixture
Separation Method
0/8
Review the explanations above for any you missed.
Answer these questions in your exercise book. Draw diagrams where helpful.
Question 1 · (2 marks)
Explain WHY it is possible to separate a mixture using physical methods. Why can we not use the same physical methods to separate a compound into its elements?
Question 2 · (4 marks)
A learner has a mixture of sand, salt, and iron filings. Describe, in order, the steps she should take to separate all three components. Name the technique used at each step and explain why it works.
Question 3 · (3 marks)
Draw and label a diagram of a filtration setup. Name: (a) the apparatus used to hold the filter paper, (b) the solid left on the filter paper, (c) the liquid that passes through.
Question 4 · (3 marks)
Explain how chromatography separates different dyes in a mixture of ink. In your answer, explain what causes the dyes to travel different distances up the paper.
Question 5 · (2 marks)
South Africa faces water shortages. Explain how distillation can be used to obtain fresh drinking water from seawater. In your answer, state the physical property that makes distillation work for this mixture.
Question 6 · (4 marks)
For each mixture below, state the most appropriate separation technique and explain the physical property difference that makes it work: (a) muddy water, (b) a mixture of ethanol and water, (c) copper coins mixed with iron nails, (d) a mixture of gravel and fine sand.
Question 7 · (5 marks)
The table below gives the approximate boiling point ranges of the fractions collected when crude oil is separated in a fractionating column using fractional distillation:
FractionApprox. boiling range (°C)Collected at column height
Refinery gasbelow 25Very top
Petrol35 – 75Near top
Paraffin (kerosene)150 – 250Middle
Diesel200 – 350Lower-middle
Bitumenabove 350Bottom
(a) Using the table, explain why refinery gas is collected at the very top of the column while bitumen is collected at the bottom. (b) A learner claims: "The fraction with the lowest boiling point must be the heaviest." Use the data in the table to explain why this is incorrect. (c) A sample of liquid has a boiling point of approximately 300°C. Using the table, predict which fraction it belongs to and roughly where in the column it would be collected.