KS3 · Chemistry

Pure substances

A carton says '100% pure orange juice'. Nothing added, straight from oranges. So it's pure, right? A chemist would say no, and a thermometer can prove it.

Chemistry · Pure substances

Is it pure? Depends who's asking

Pick a material, then pick the group you think it belongs in.

Everyday rule: call it 'pure' if it comes from one source with nothing added.

Still to sort

'Pure' (everyday sense) (0)

From one source, nothing added.

Not 'pure' (everyday sense) (0)

Several things mixed together on purpose.

8 of 8 still to sort.

Sort all eight with the everyday rule first. Then flip to the science rule and sort the same eight again. Watch which ones jump.

Watch out: A label that says 'pure' is using the everyday meaning. It tells you nothing about how many chemicals are inside.

What do you really think?

Just a tiny bit of something else

A sample is almost entirely one chemical. Mixed in with it is a very small amount of a second chemical.

Which is closest to what you think right now?

It's basically pure. An amount that small doesn't count.It's a mixture. The small amount is an impurity, and it can change how the material behaves.It depends where it came from. If it all came from one place, it's pure.It's a mixture, and that impurity must be ruining it. Impurities are always bad.

How sure are you?

I'm sureI think soI'm guessing

It's basically pure. An amount that small doesn't count.

This is a common misconception

Tempting, but in science amount doesn't come into it. If a second substance is there at all, the material is a mixture, whether it's half the sample or a tiny speck.

It's a mixture. The small amount is an impurity, and it can change how the material behaves.

This matches the evidence

Spot on. A chemical found in a material in very small amounts is called an impurity. Small doesn't mean harmless: an impurity can change the material's properties, including its melting point and boiling point.

It depends where it came from. If it all came from one place, it's pure.

This is a common misconception

That's the everyday meaning of 'pure' sneaking back in. Science doesn't ask where a material came from. It asks how many different chemicals are in it, and here there are two.

It's a mixture, and that impurity must be ruining it. Impurities are always bad.

This is a common misconception

You're right that it's a mixture, but impurities aren't always bad news. They change a material's properties, and that change can be useful or unhelpful depending on what you need the material for.

One chemical means pure. Two or more, in any amounts, means a mixture. A small amount of a second chemical is an impurity, and it can change the material's properties.

Predict, then check

Here's the clue scientists use to test for purity. First, a prediction.

You heat a small beaker of pure water and a big pan full of pure water until each one boils. What temperature does each one boil at?

The purity test

Let the thermometer decide

Choose what the thermometer shows as the sample melts.

What the thermometer shows

Melting points and boiling points are the properties scientists use most often to check purity.

2 possible results.

On Heat the sample and measure the temperature as it melts. 2 branches to choose from.

Labels can't be trusted, but melting points can. Heat a sample, measure the temperature as it melts, and follow what you see.

Watch out: The test is not 'does it melt at 100 °C?'. That number belongs to boiling water. The test is whether it melts at ONE temperature or over a RANGE.

Your turn to explain

Say it in your own words

Describe the difference between a pure substance and a mixture. [3 marks]

Your answer

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

The mark scheme

A pure substance contains only one chemical (one type of substance); a mixture contains two or more different substances. (1 mark)
The substances in a mixture can be physically separated. (1 mark)
A pure substance melts or boils at one fixed temperature; a mixture melts or boils over a range of temperatures. (1 mark)

Model answer

A pure substance is made of only one chemical, but a mixture has two or more different substances in it, and they can be physically separated. You can tell them apart by heating: a pure substance melts and boils at one fixed temperature, while a mixture melts or boils over a range of temperatures.

Number of chemicals, whether it can be separated, and one temperature versus a range: those three ideas are the whole difference.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What 'pure' really means to a scientist, and how to test for it.

What you need to know

In science, a pure substance contains only one chemical (one type of substance).
Everyday 'pure' means from one source with nothing added. That's not the science meaning.
A pure substance can be an element (pure oxygen, pure carbon) or have particles with more than one kind of atom (pure water, pure carbon dioxide).
A mixture has two or more different substances that can be physically separated. Any amount of a second substance makes a mixture.
Many natural materials, including rocks, sea water and air, are mixtures.
An impurity is a chemical present in very small amounts. It can change properties such as melting and boiling point, and can be useful or unhelpful.
Pure substances melt and boil at one fixed temperature (pure water boils at 100 °C, whatever the amount). Mixtures melt and boil over a range.

The big picture

In science, a pure substance contains only one chemical. It can be an element, like pure oxygen, or made of particles with more than one kind of atom, like pure water. A mixture contains two or more different substances, in any amounts, and can be physically separated. Many natural materials, such as rocks, sea water and air, are mixtures. A small amount of a second chemical is an impurity, which can change properties such as melting and boiling point. Pure substances melt and boil at one fixed temperature; mixtures do so over a range, which is how we test for purity.

Key points

1Pure in science = only one chemical present.

2Fresh orange juice and orange squash are mixtures; pure water is a pure substance.

3Elements and single compounds can both be pure substances.

4Mixture = two or more substances, physically separable, in any amounts.

5Impurity = a small amount of another chemical that can change melting and boiling points.

6Fixed melting or boiling point = pure; a range of temperatures = mixture.

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Worked example

Problem

A bottle of spring water says 'Pure — nothing added'. A student tests a sample and finds it boils over a range of temperatures instead of at one temperature. Is the label wrong? Explain using both meanings of 'pure'.

⚠ Watch out

Deciding purity from the label or from where a material came from. 'Natural', 'nothing added' and 'from one source' are everyday ideas. In science the only question is how many different chemicals are present, and even a tiny amount of a second one makes a mixture.

🧠

Memory hook

One chemical, one temperature. More than one chemical, a range. The label can say what it likes; the thermometer tells the truth.

✓

Check yourself

A solid melts over a range of temperatures, not at one. Is it a pure substance or a mixture? And what do we call the small amount of extra chemical in it?

Flashcards

(14)

What does 'pure' mean in science?

Only one chemical (one type of substance) is present.

What does 'pure' usually mean in everyday life?

From one source, with nothing added, like 'pure orange juice'.

Is fresh orange juice a pure substance?

No. It contains more than one chemical, so it's a mixture, even with nothing added.

Does a pure substance have to be an element?

No. Pure oxygen is an element, but pure water and pure carbon dioxide have particles with more than one kind of atom. Both are pure substances.

What is a mixture?

Two or more different substances together, which can be physically separated.

Does it matter how much of the second substance is in a mixture?

No. Any amount of a second substance, however small, makes it a mixture.

Name three materials found in nature that are mixtures.

Rocks, sea water and air.

What is an impurity?

A chemical found in a material in very small amounts.

What can an impurity do to a material?

Change its properties, including its melting point and boiling point. The change can be useful or unhelpful.

How does a pure substance melt or boil?

At one fixed, definite temperature.

At what temperature does pure water boil?

100 °C.

Does a bigger sample of a pure substance boil at a higher temperature?

No. The size of the sample doesn't change the boiling point.

How does a mixture melt or boil?

Over a range of temperatures, not at one single temperature.

Which properties are most often used to tell a pure substance from a mixture?

Melting points and boiling points.

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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Is it pure? Depends who's asking

Just a tiny bit of something else

Let the thermometer decide

Say it in your own words

More KS3 Chemistry topics