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Year 7 · Science · Chemical sciences

Separating mixtures

Students distinguish pure substances from mixtures and use filtration, evaporation, decanting, magnetic separation and paper chromatography to separate them.

Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.

Learning goals

Learners will

  • Distinguish between pure substances and mixtures.
  • Choose a suitable technique to separate a given mixture based on the properties of its parts.
  • Separate a sand, salt and water mixture.

Success looks like

  • I can explain the difference between a pure substance and a mixture.
  • I can match filtration, evaporation, decanting and magnetic separation to suitable mixtures.
  • I can describe the steps to separate sand and salt.

The big idea

A pure substance contains only one kind of particle. A mixture contains two or more substances that are not chemically joined, so each keeps its own properties. Sea water, air and soil are mixtures.

Because the parts of a mixture keep their own properties, we can use those properties to separate them.

  • Filtration separates an insoluble solid (like sand) from a liquid. The liquid passes through as filtrate; the solid stays behind as residue.
  • Evaporation separates a dissolved solid (like salt) from a liquid by letting the liquid turn into a gas. Salt is harvested this way from sea water in evaporation ponds in parts of Australia.
  • Decanting is carefully pouring off a liquid and leaving heavier solids that have settled at the bottom. Magnetic separation pulls out magnetic materials like iron and steel.
  • Paper chromatography separates the different dyes in an ink. Dyes that dissolve well and stick less to the paper travel further.

Water treatment plants that supply Australian cities use similar steps: letting particles settle, then filtering, then disinfecting the water.

Worked examples

Separating sand, salt and water

You have a mixture of sand and salt. How can you get both back separately?

  1. Add warm tap water and stir. The salt dissolves; the sand does not.
  2. Filter the mixture. The sand is the residue in the filter paper.
  3. Pour the salty filtrate into a shallow dish and leave it in a warm, sunny spot.
  4. The water evaporates over a few days, leaving salt crystals.
  5. Let the sand dry in the filter paper.

Answer: Dissolve, filter (sand stays as residue), then evaporate the filtrate (salt remains).

Choosing a method

Which technique would separate steel paper clips from sand?

  1. Look for a property that differs: steel is magnetic, sand is not.
  2. Run a magnet over the mixture.

Answer: Magnetic separation.

Reading a chromatogram

A black felt-tip ink separates into blue, yellow and purple bands. What does this show?

  1. Each band is a different dye.
  2. The ink is made of at least three dyes, so it is a mixture, not a pure substance.

Answer: The black ink is a mixture of at least three dyes.

Practice check

Have a go, then check your answers. Each answer comes with an explanation.

  1. 1.Which technique separates sand from water?
  2. 2.How is salt obtained from sea water in salt ponds?
  3. 3.Which of these is a mixture?
  4. 4.In filtration, what is the solid left in the filter paper called?
  5. 5.Which technique would separate the dyes in a felt-tip ink?
  6. 6.What is the liquid that passes through a filter called?
  7. 7.Carefully pouring a liquid off a settled solid is called d________.
  8. 8.Can filtration remove dissolved salt from water? Answer yes or no.
Answer guide for parents

  1. Which technique separates sand from water?

    Filtration — Sand is insoluble, so filtering traps it while water passes through.

  2. How is salt obtained from sea water in salt ponds?

    Evaporation — The Sun evaporates the water and leaves salt behind.

  3. Which of these is a mixture?

    Sea water — Sea water contains water, salts and other dissolved substances.

  4. In filtration, what is the solid left in the filter paper called?

    Residue — The solid caught by the filter is the residue.

  5. Which technique would separate the dyes in a felt-tip ink?

    Paper chromatography — Chromatography separates dissolved dyes as they travel up paper.

  6. What is the liquid that passes through a filter called?

    filtrate — The liquid that passes through is the filtrate.

  7. Carefully pouring a liquid off a settled solid is called d________.

    decanting — This technique is decanting.

  8. Can filtration remove dissolved salt from water? Answer yes or no.

    no — Dissolved particles pass straight through filter paper; evaporation is needed.

Watch out for

  • Students often think filtering removes salt. Dissolved substances pass through filter paper.
  • Some think 'pure' means 'natural' or 'clean'. In science, pure means one substance only; fresh spring water is a mixture.
  • Some believe dissolved substances are destroyed. Evaporation shows they were there all along.

Tips for parents

  • Cone a coffee filter in a funnel or the cut-off top of a plastic bottle; it filters better than a single paper towel.
  • Use the sun or a warm windowsill for evaporation rather than a stove.
  • Safety: wear eye protection, take care with the cut edge of a plastic bottle (adult to cut), and do not taste any mixtures.

Go further

  • Run chromatography on several black or brown felt-tips and compare their dye patterns.
  • Design a water filter using gravel, sand and cotton wool, and compare cloudy water before and after. (Filtered water is still not safe to drink.)
  • Research how desalination plants in Perth, Sydney or Adelaide produce drinking water.