The idea of a pure substance

In everyday life "pure" can mean "clean" or "natural" — like "pure orange juice". In science it means something very exact:

A pure substance is made of only one type of particle — a single element (like copper) or a single compound (like water). Nothing else is mixed in.

• Pure gold — every particle is a gold atom.
• Pure water — every particle is an H₂O molecule.
• "Pure" orange juice from a carton is not pure to a scientist — it is water, sugars, acids, vitamins and pulp all mixed together.

If it contains more than one substance, it is impure — a mixture.

Which one is pure?

Impure substances are mixtures

A mixture contains two or more substances that are not chemically joined together. Because they are only mixed, not bonded, they keep their own properties and can be separated again using physical methods.

• Air — mostly nitrogen and oxygen (plus argon, carbon dioxide…).
• Sea water — water with salt and other things dissolved in it.
• Muddy water — water with tiny bits of insoluble soil floating in it.

Key idea: a mixture is not a pure substance, even a mixture that looks completely clear. A cup of salty water looks like plain water, but it is a mixture of two substances.

Dissolving: making a solution

When a solid seems to "disappear" into a liquid, it has dissolved. The particles spread out evenly through the liquid — they are still there, just too small to see. This makes a solution. Learn these four words:

Watch out — dissolving is NOT melting! Melting means heating one solid until it turns to liquid (ice → water). Dissolving means a solid spreads out into a liquid that is already there (salt into water). No melting is happening.

Name the parts

Solubility: soluble or insoluble?

Solubility tells you how much of a solute will dissolve in a solvent.

• A solute is soluble if it dissolves (e.g. salt and sugar in water).
• A solute is insoluble if it does not dissolve (e.g. sand and chalk in water) — it just sinks or floats as bits.

Two useful facts about solubility:

• Most solids dissolve faster and in larger amounts when the solvent is hotter (hot tea dissolves sugar quickly).
• If you keep adding solute, eventually no more will dissolve — the solution is saturated.

Remember: "dissolved" is not the same as "disappeared". The solute is still in there — that is why salty water still tastes salty and weighs more than the water alone.

Soluble in water?

Tap whether each solid dissolves in water or not.

Splitting mixtures back up

Because a mixture is not chemically joined, we can separate it again using physical methods — no new substances are made. Which method you choose depends on the mixture:

• Filtration — separates an insoluble solid from a liquid.
• Evaporation — gets a dissolved solid back out of a solution.
• Distillation — gets the pure liquid (solvent) back from a solution.
• Chromatography — separates a mixture of coloured dyes.

Let's look at each one.

Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. Small liquid particles slip through the tiny holes; the bigger, insoluble solid bits are trapped.

Watch out: filtration only works for insoluble solids. A dissolved solid (like salt in water) slips straight through the paper with the liquid — you can't filter salt out of salty water.

Can you filter it?

Evaporation

Evaporation gets a dissolved solid back out of a solution. Heat the solution gently: the liquid (solvent) turns to vapour and escapes into the air, leaving the solid behind as crystals.

Note: evaporation keeps the solid but loses the liquid to the air. If you want the pure liquid back instead, you need distillation (next screen).

Distillation

Distillation gets the pure liquid back from a solution. It is really evaporation + condensation joined together: the solvent boils into vapour, then a cool condenser turns the vapour back into pure liquid, which drips into a clean container. The dissolved solid stays behind in the flask.

Filtration vs distillation: filtration cannot separate salt from water because salt is dissolved. Distillation can, because it collects the boiled-off water separately.

Getting pure water

Chromatography

Chromatography separates a mixture of coloured substances, such as the dyes in an ink or a food colouring. A spot of the mixture is put on paper, and a solvent soaks up the paper. Each dye travels a different distance, so they spread out into separate spots — this pattern is called a chromatogram.

Why pencil? The start line is drawn in pencil, not pen — pencil (graphite) doesn't dissolve in the solvent, so it won't run and spoil the result.

Match the technique

Tap a technique on the left, then tap the mixture it is best for on the right.

Is it pure? Test its melting/boiling point

How can you tell if a substance is really pure? Use its melting point or boiling point:

• A pure substance melts and boils at one exact temperature (pure water always boils at 100 °C at normal pressure).
• An impure substance (a mixture) melts and boils over a range of temperatures — and usually at a different temperature from the pure one (salty water boils above 100 °C).

Reading a boiling point

Short answer: why not filtration?

Pure or mixture?

Tap a substance, then tap the box it belongs in.

Three mistakes to avoid

• Dissolving is not melting. Melting needs heat and turns a solid into its own liquid. Dissolving spreads a solid through a different liquid.
• A solution is a mixture, not a pure substance. Salty water looks clear but contains two substances, so it is impure.
• Filtration only removes insoluble solids. A dissolved solid passes straight through the filter paper — use evaporation or distillation instead.

Also remember: separating a mixture makes no new substances — it is a physical change, so the substances are just sorted back out unchanged.

Choosing the method

The key ideas to know

You've covered the whole KS3 topic — pure substances, mixtures, dissolving and all four separation techniques. Press Finish to see your score.