Mixtures and separation · Classify
Pure or mixture?
A carton of juice, a gold ring and a bottle of lab water all claim to be pure. Only one of them is — so what is the chemist actually asking?
Start here
One of these two labels means nothing to a chemist.
A carton says 100% pure orange juice. A bottle in the lab says distilled water. Both are honest labels. Only one of them describes something a chemist would call pure.
Which one is pure, and what is the word doing in the other?
The water is pure. The juice contains water, three different sugars, citric acid, pulp, vitamin C and more — dozens of substances, and no two cartons contain quite the same amounts. On the carton, pure means nothing was added to it. To a chemist it means something far stricter: one substance, and nothing else.
A pure substance contains one substance and nothing else — every particle in it is the same kind. A mixture contains two or more substances that are not chemically joined, and its recipe can be varied: you can make sea water saltier, or brass with more copper in it, and it is still sea water and still brass.
That second half is the useful test. A pure substance has a fixed composition you cannot change without turning it into something else. A mixture does not.
Your turn · eight samples on the bench
Pure, or mixture? Decide on all eight.
0 of 8 decided
Commit before you read the answer. Three of these are labelled or sold as pure and are not; one of them you cannot even see.
Sample 1
Distilled water
A colourless liquid from the bottle on the prep-room shelf.
Water and nothing else.
Pure.
One substance. Every particle in the bottle is a water particle, and you cannot make it more or less watery.
Sample 2
Orange juice, "100% pure"
Cloudy orange, straight from the carton in the fridge.
Water, three sugars, citric acid, pulp, vitamin C, and more.
Mixture.
Dozens of substances, and the amounts differ between oranges. Nothing was added — that is what the label claims — but plenty is in there.
Sample 3
Sea water
Clear, colourless, and it tastes of salt.
Water with sodium chloride and several other salts dissolved in it.
Mixture.
The Dead Sea is nine times saltier than the Atlantic and both are sea water. A composition you can vary is the signature of a mixture.
Sample 4
Oxygen from a cylinder
Invisible. You cannot see it, smell it or taste it.
Oxygen and nothing else.
Pure.
Being invisible tells you nothing about purity. This cylinder holds one substance; the air outside it holds several.
Sample 5
Granulated sugar
White crystals from a paper bag.
Sucrose and nothing else.
Pure.
A bag of sugar is one substance, sucrose, refined until nothing else is left. Being made in a factory does not make it impure.
Sample 6
Air
Invisible, exactly like the oxygen.
Nitrogen, oxygen, argon, carbon dioxide, water vapour.
Mixture.
Several gases, and the proportions change with where you are standing. Two invisible samples, two different answers.
Sample 7
A 9-carat gold ring
Uniform, shiny, gold-coloured all the way through.
37.5% gold, the rest copper and silver.
Mixture.
Under a third of it by count is gold. It is uniform, hard-wearing and worth having — and it is a mixture, on purpose.
Sample 8
Milk
Uniform white, with no bits you can pick out.
Water, fats, proteins, sugars, minerals.
Mixture.
Leave it to stand and the cream rises, which no pure substance ever does. Looking uniform is not the same as being one substance.
Three pure, five mixtures — and not one of the eight was settled by looking at it. The gold ring, the milk and the juice all look completely uniform and all three are mixtures; the oxygen is invisible and is pure. Every verdict came down to the same question: is there one substance in there, or more than one?
Key fact
A pure substance contains only one substance, in a fixed composition. A mixture contains more than one, in any proportion you like — and looking uniform is no evidence either way.
Four words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“It says 100% pure on the carton, and there is nothing bad in it, so it is pure.”
Everything in that sentence is true about the juice. Commit before you read on.
The carton is not lying. In food labelling, pure means nothing was added — no water, no sugar, no preservative. It is a promise about what was put in. A chemist's pure is a claim about what is there: one substance only. Orange juice fails that on its own, with nothing added at all.
The same word is used for clean, safe, natural and untouched, and none of those are chemistry. Distilled water is pure and has nothing in it you could live on. Pure sodium is pure and will set fire to your hand. Pure says how many substances, not how good.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What does a chemist mean by a pure substance?
Rung 2 · The one that catches people
Which one of these is a pure substance?
Rung 3 · Explain
A student filters sea water until it is completely clear and says "it is pure now — look at it". Explain why the sample is not pure, and what the clearness does tell you.
Rung 4 · Take it somewhere new
A bottle of mineral water lists fourteen dissolved minerals on the label and calls itself pure. A ring is sold as 9-carat gold and is only 37.5% gold. Neither seller is breaking the law. Explain what "pure" is doing in the first case, why the ring is deliberately not pure, and what a chemist would say about both.
Key note
A pure substance contains one substance only, and its composition is fixed. A mixture contains two or more substances, not chemically joined, in proportions you can vary. Neither can be identified by looking: air and oxygen are both invisible, and only one of them is pure.
Going further
Chemists and food labels are both allowed their meanings, and the law is on the label's side: in the UK, calling a juice "100% pure" is a claim that nothing was added, not a claim about composition. The chemist's version is the one that matters when it has to work — paracetamol is sold at 99.9% purity, and the missing 0.1% is checked, named and limited, because a tablet is swallowed by someone who cannot inspect it.
Some things are deliberately made impure. Pure gold is soft enough to mark with a fingernail, so a ring is an alloy — 9-carat gold is only 37.5% gold, and it lasts. Pure iron is soft too; adding a little carbon makes steel. In the same way, road salt is not pure sodium chloride, and that is on purpose. Mixtures designed for a job have their own name at GCSE: formulations.
Before this lesson
Next in this unit
At GCSE this becomes
- Pure substances have sharp melting points, and mixtures designed for a job — paints, fuels, alloys, medicines — are called formulations.
Where to next
- Next: Dissolving and solutions
- Previous: Conservation of mass
Atoms, elements and compounds
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