GCSE · Chemistry · AQA · Spec 8462

Pure substances

'Pure milk': nothing added, so it's pure, right? Not to a chemist. Here's why, and how a thermometer can prove whether a sample is pure.

Heat a solid and watch what happens while it melts

Another substance mixed in 0 %

Another substance mixed in: 0 %. Temperature rise while it melts: 0 °C

Start at 0 %: one substance on its own. From 4 to 10 minutes the solid is melting, and the line goes flat: it melts at one temperature, 80 °C. Now drag towards 10 %, mixing in more of a second substance. The flat stretch tilts into a slope: at 10 % the temperature creeps up from 72 °C to 78 °C while it melts. That's a range. (An invented solid, so the numbers are only there to show the pattern.)

Watch out: The flat line doesn't mean the heating has stopped. The sample is still being heated the whole time; while a pure solid is melting, its temperature just doesn't rise.

The chemist's meaning

Pure substance or mixture?

Sort each sample. One substance, or more than one?

Still to sort

Pure substance (0)

One element or one compound, on its own.

Where the line is: A compound can contain several elements and still be pure. Count substances, not elements.

Mixture (0)

Two or more substances mixed together.

Where the line is: A mixture can look completely even, like a clear solution. Looks don't tell you.

6 of 6 still to sort.

In chemistry, a pure substance is a single element or a single compound, not mixed with any other substance. So there's only one question to ask: is there just one substance here, or more than one?

Everyday word vs chemistry word

Is 'pure milk' pure?

A milk carton says PURE MILK. In everyday language that means nothing has been added to it: it's unadulterated, in its natural state.

Is this milk a pure substance in chemistry? Pick the idea closest to what you think right now.

Yes. Nothing has been added to it, so it's pure.Yes. It's natural, so it's pure.No. In chemistry, pure means one substance only, and milk has several substances in it.No. Milk contains more than one element, so it can't be pure.

How sure are you?

I'm sureI think soI'm guessing

Yes. Nothing has been added to it, so it's pure.

This is a common misconception

That's exactly what the label means: nothing added. But a chemist asks a different question: how many substances are in it? Milk contains water, fats, proteins and sugars, all mixed together. Nothing added, yes. One substance, no. So in chemistry, milk is a mixture.

Yes. It's natural, so it's pure.

This is a common misconception

Natural and pure aren't the same thing in chemistry. Air is natural, and it's a mixture of gases. Where something comes from doesn't matter. What matters is whether it's one substance or several, and milk is several.

No. In chemistry, pure means one substance only, and milk has several substances in it.

This matches the evidence

Spot on. Milk is a mixture of water, fats, proteins and sugars. The carton uses the everyday meaning of pure (nothing added). Chemistry uses a stricter one: a single element or compound, not mixed with anything else.

No. Milk contains more than one element, so it can't be pure.

This is a common misconception

Right answer, wrong reason, and the reason matters. Water contains two elements, hydrogen and oxygen, and water on its own IS a pure substance. Counting elements doesn't work. Count substances: milk has several, so it's a mixture.

Same word, two meanings. Everyday pure: nothing added. Chemistry pure: one substance only. Pure milk passes the first test and fails the second.

Your turn

Use the data to decide

A student heats three white solids, A, B and C, and records when each one starts and finishes melting. Solid A: starts at 132 °C, finishes at 132 °C. Solid B: starts at 104 °C, finishes at 111 °C. Solid C: starts at 71 °C, finishes at 71 °C. Which of the solids are pure substances? Use the data to explain your answer. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

A and C are pure (both needed). (1 mark)
A and C each melt at one specific temperature: they start and finish melting at the same temperature. (1 mark)
B is not pure (it is a mixture) because it melts over a range, from 104 °C to 111 °C. (1 mark)

Model answer

A and C are pure substances. Each one starts and finishes melting at the same temperature, A at 132 °C and C at 71 °C, so each melts at a specific temperature. B is not pure. It melts over a range of temperatures, from 104 °C to 111 °C, which shows it is a mixture.

Say what the data shows, then what it means: one temperature means pure, a range means a mixture. Notice it's not about how high the temperature is. C melts lowest and it's still pure.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

To a chemist, 'pure milk' isn't pure. And a thermometer can prove whether something is.

What you need to know

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
In everyday language, pure means nothing has been added, so it is unadulterated and in its natural state, like 'pure milk'. Pure milk is still a mixture in chemistry.
Pure elements and compounds melt and boil at specific temperatures.
A mixture melts or boils over a range of temperatures, so melting point and boiling point data can tell pure substances apart from mixtures.

The big picture

The word 'pure' means two different things. In everyday language, pure means nothing has been added, like 'pure milk'. In chemistry, a pure substance is a single element or compound, not mixed with any other substance, so pure milk is actually a mixture. Pure elements and compounds melt and boil at specific temperatures, while a mixture melts or boils over a range of temperatures. That's why melting point and boiling point data can tell you whether a sample is pure.

Key points

1Chemistry pure = one substance only. A compound such as water is pure, even though it contains two elements.

2Everyday pure = nothing added. That's a different idea, and it doesn't make something a single substance.

3One specific melting or boiling temperature points to a pure substance. A range of temperatures points to a mixture.

4On a heating curve, a pure solid gives a flat section while it melts. A mixture's temperature keeps rising as it melts.

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Worked example

Problem

A clear, colourless liquid is sold as pure water. A student boils some of it. It starts to boil at 101 °C, and the temperature keeps rising to 104 °C while it boils. Pure water boils at 100 °C at normal atmospheric pressure, and the room is at normal pressure. Is the liquid pure water?

⚠ Watch out

Saying a compound isn't pure because it contains more than one element. Water (H₂O) contains two elements but is one substance, so it's pure. Count substances, not elements.

🧠

Memory hook

One substance, one temperature. More than one substance, a range of temperatures.

✓

Check yourself

Cover the page. Explain the difference between the everyday meaning and the chemistry meaning of 'pure'. Then describe what you'd see on a heating curve for a pure solid compared with a mixture.

Flashcards

(9)

What is a pure substance in chemistry?

A single element or compound, not mixed with any other substance.

What does 'pure' mean in everyday language?

Nothing has been added: unadulterated and in its natural state, like 'pure milk'.

Is 'pure milk' a pure substance in chemistry?

No. Milk is a mixture of several substances, including water, fats, proteins and sugars.

Can a compound be a pure substance, even though it contains more than one element?

Yes. Water contains hydrogen and oxygen, but it is one substance, so on its own it is pure.

How do pure elements and compounds melt and boil?

At specific temperatures.

How does a mixture melt or boil?

Over a range of temperatures.

What does the heating curve of a pure solid look like while it melts?

It goes flat: the temperature stays the same until all the solid has melted.

Iron and sulfur powders stirred together, or iron sulfide: which is pure?

Iron sulfide. It is one compound. The stirred powders are two substances mixed together.

Salt water looks clear and even. Is it pure?

No. It's water and sodium chloride mixed together, so it's a mixture. Looks don't tell you.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Heat a solid and watch what happens while it melts

Pure substance or mixture?

Is 'pure milk' pure?

Use the data to decide

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