Chapter 2 of 8

All 8 chapters
  1. 1Elements and compounds, readunderstood
  2. 2Pure substances and mixtures, readunderstood
  3. 3Separating mixtures, readunderstood
  4. 4Physical and chemical changes, readunderstood
  5. 5Rusting and other reactions, readunderstood
  6. 6Burning and fire safety, readunderstood
  7. 7Acids and alkalis, readunderstood
  8. 8Neutralisation, readunderstood

S3.6.2 Chapter 2

Pure substances and mixtures

A bottle says pure spring water, and the water is clear. Would a scientist call it pure?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Tell a pure substance from a mixture
  • Explain why sea water and air are mixtures

In shops, pure often means clean or natural. A scientist uses the word in a different way. This chapter shows you how.

Draw it first

Draw a pure substance and a mixture

  1. Two boxes of groups side by side, with a key under them. In the box labelled pure water, every group is one large circle with the letter O joined to two small circles with the letter H. In the box labelled salty water, there are the same water groups, and also single circles with the letters Na and Cl that are not joined to the water or to each other. The key names the circles: hydrogen, oxygen, sodium and chlorine. Each circle has its letter inside.OOOOHHHHHHHHpure waterHhydrogenOoxygenNasodiumClchlorine

    1 Draw water groups, all the same. Nothing else is in the box. This is a pure substance.

  2. Two boxes of groups side by side, with a key under them. In the box labelled pure water, every group is one large circle with the letter O joined to two small circles with the letter H. In the box labelled salty water, there are the same water groups, and also single circles with the letters Na and Cl that are not joined to the water or to each other. The key names the circles: hydrogen, oxygen, sodium and chlorine. Each circle has its letter inside.OOOOHHHHHHHHpure waterNaClNaClOOOOHHHHHHHHsalty waterHhydrogenOoxygenNasodiumClchlorine

    2 Draw water groups, and add single circles, Na and Cl, for the dissolved salt. They are not joined to the water. This is a mixture.

A pure substance is only one substance. It is either an element, such as gold, or a compound, such as water. All its groups of atoms are the same.

A mixture has two or more substances that are not joined together. The substances in a mixture keep their own properties.

Two boxes side by side, with a key under them. In the box labelled gold, every circle has the letters Au, and the circles touch in neat rows. In the box labelled pure water, every group is one large circle with the letter O joined to two small circles with the letter H, and the groups touch and are jumbled under a flat top. The key names the circles: gold, oxygen and hydrogen. Each circle has its letter inside.AuAuAuAuAuAuAuAuAugoldOOOOHHHHHHHHpure waterAugoldOoxygenHhydrogen
Two pure substances. Gold is an element. Water is a compound. Each box has only one kind of group.

Pure does not mean clean. Pure also does not mean natural or safe. A glass of clear tap water is a mixture. It has dissolved salts in it that you cannot see.

Air is a mixture of gases. About 78 of every 100 parts are nitrogen. About 21 parts are oxygen. The rest, about 1 part, is argon, carbon dioxide and a few other gases.

A box of ten pairs of circles, labelled air. 8 pairs are circles with the letter N, and 2 pairs are circles with the letter O. The pairs are far apart in a closed box, the two kinds are mixed together, and no pair is joined to another. A key names the circles: nitrogen and oxygen. Each circle has its letter inside.OOOONNNNNNNNNNNNNNNNairNnitrogenOoxygen
A small sample of air: 8 pairs of nitrogen and 2 pairs of oxygen. The gas particles are far apart, and the two gases are mixed, not joined.

Each gas in the air keeps its own properties. The oxygen in the air still helps things burn. The nitrogen does not. The gases are side by side, so air is a mixture.

Sea water is a mixture too. It is mostly water, with salt dissolved in it, and a little of other things. When salt dissolves, it splits into separate particles among the water groups. Its parts are not joined.

A box labelled sea water. It holds water groups, each one large circle with the letter O joined to two small circles with the letter H. Mixed among them are single circles with the letters Na and Cl, which are the dissolved salt, not joined to the water or to each other. The key names the circles: hydrogen, oxygen, sodium and chlorine. Each circle has its letter inside.ClNaClNaOOOOOOHHHHHHHHHHHHsea waterHhydrogenOoxygenNasodiumClchlorine
Sea water. The dissolved salt is split into separate Na and Cl particles among the water groups. This is a small drawing: real sea water has many more water groups for each Na and Cl.

See it three ways

With things

Think of a bowl of rice and beans. You can see two things, and each is still rice or beans. A compound is different. Its parts are joined, and it has new properties.

As a picture

A table with the columns idea, compound and mixture. In the row atoms, a compound is joined and a mixture is separate. In the row amounts, a compound is fixed and a mixture can vary. In the row example, a compound is water and a mixture is air.ideacompoundmixtureatomsjoinedseparateamountsfixedcan varyexamplewaterair

In words

In a compound, the atoms are joined and the amounts are always the same. In a mixture, the substances are not joined, and you can have more or less of each.

Try it

Mix and take apart

You need: a bowl, a handful each of dried rice, dried beans and small stones, a spoon.

Be safe: Wash the stones and your hands afterwards. Do not eat anything from the bowl.

  1. Pour the rice, the beans and the stones into the bowl. Stir with the spoon.
  2. Look at the bowl. Can you still tell the three things apart?
  3. Pick out the stones by hand. Then pick out the beans. Rice is left in the bowl. Wash your hands afterwards.

What you will see: Each thing is still itself. The rice is still rice. The beans and stones are unchanged. You can take them apart again, because a mixture has no joined parts.

If you cannot try it, this is what happens.
Three boxes in a row under the words sorted piles. The first box is dotted and labelled rice. The second box is hatched and labelled beans. The third box is cross hatched and labelled stones.sorted pilesricebeansstones

A wrong idea many people have

Clear water must be pure.

Why it feels right: You can see through it, and nothing in it shows. So it seems there is nothing else in it.

The test that breaks it: Sea water is clear, yet it is salty. Leave a dish of it in the sun until the water has gone. A white crust of salt is left. The salt was in the water all the time.

What is true: Pure means only one substance. Many clear liquids are mixtures, because dissolved things cannot be seen.

Key words

pure substance
a substance that is only one thing, an element or a compound.
mixture
two or more substances together that are not joined.

Back to your guess

The answer is A: Not always. In science, pure means only one substance, and clear water can hold others.

Pure means only one substance. Clear water can still have salts dissolved in it, so it is a mixture. Air and sea water are mixtures too.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these is a pure substance?

    Tap your answer.

    1. Not this one. Air looks clear and clean, but it holds nitrogen, oxygen and other gases side by side. That is a mixture. Go to S3.6.2. Try another.

    2. Not this one. Sea water is clear, but it has salt dissolved in the water. That is a mixture. Go to S3.6.2. Try another.

    3. Yes, that is right. Oxygen gas has only one kind of atom, so it is one substance. Air and sea water each have several substances in them.

    See the answer

    oxygen gas

    Oxygen gas has only one kind of atom, so it is one substance. Air and sea water each have several substances in them.

    • If you chose A: Air looks clear and clean, but it holds nitrogen, oxygen and other gases side by side. That is a mixture. Go to S3.6.2.
    • If you chose B: Sea water is clear, but it has salt dissolved in the water. That is a mixture. Go to S3.6.2.

  2. Box A holds pure water. Box B holds salty water. Which box is a mixture, and why?

    Two boxes side by side, with a key under them. The box labelled box A has only water groups, each one large circle with the letter O joined to two small circles with the letter H. The box labelled box B has water groups and also single circles with the letters Na and Cl. The key names the circles: hydrogen, oxygen, sodium and chlorine. Each circle has its letter inside.OOOOHHHHHHHHbox AClNaClNaOOOOHHHHHHHHbox BHhydrogenOoxygenNasodiumClchlorine
    Tap your answer.

    1. Not this one. Joined atoms in groups that are all the same make a pure compound. A mixture has different groups, not joined. Go to S3.6.1. Try another.

    2. Yes, that is right. Box B holds water and salt side by side. Box A holds only water groups, all alike, so it is pure.

    3. Not this one. Box A has only water. A mixture needs two or more substances. Go to S3.6.2. Try another.

    See the answer

    Box B. It has two substances, water and salt, that are not joined together.

    Box B holds water and salt side by side. Box A holds only water groups, all alike, so it is pure.

    • If you chose A: Joined atoms in groups that are all the same make a pure compound. A mixture has different groups, not joined. Go to S3.6.1.
    • If you chose C: Box A has only water. A mixture needs two or more substances. Go to S3.6.2.

  3. Why is air a mixture?

    Tap your answer.

    1. Not this one. Oxygen is only about one fifth of the air. Most of it is nitrogen. Go to S3.6.2. Try another.

    2. Yes, that is right. The gases in air are mixed but not joined, and each keeps its own properties. That is what makes air a mixture.

    3. Not this one. The gases in air are not joined to each other. Each gas keeps its own properties, so air is a mixture. Go to S3.6.2. Try another.

    See the answer

    It has different gases, such as nitrogen and oxygen, side by side and not joined.

    The gases in air are mixed but not joined, and each keeps its own properties. That is what makes air a mixture.

    • If you chose A: Oxygen is only about one fifth of the air. Most of it is nitrogen. Go to S3.6.2.
    • If you chose C: The gases in air are not joined to each other. Each gas keeps its own properties, so air is a mixture. Go to S3.6.2.

  4. Sea water is clear, so you can see through it. Why is it still a mixture?

    Tap your answer.

    1. Not this one. Dissolved salt is far too small to see. Clear does not mean pure. Go to S3.6.2. Try another.

    2. Not this one. The salt is not joined to the water. It is spread through it, and you can get it back. Go to S3.5.7. Try another.

    3. Yes, that is right. Two substances, water and salt, are mixed together. Neither is joined to the other, so it is a mixture.

    See the answer

    It has water and dissolved salt together, and the salt is not joined to the water.

    Two substances, water and salt, are mixed together. Neither is joined to the other, so it is a mixture.

    • If you chose A: Dissolved salt is far too small to see. Clear does not mean pure. Go to S3.6.2.
    • If you chose B: The salt is not joined to the water. It is spread through it, and you can get it back. Go to S3.5.7.

  5. Air holds nitrogen and oxygen side by side. The oxygen in it still helps things burn. What does this show?

    Tap your answer.

    1. Not this one. The gases in air are not joined to each other. If they were joined, the oxygen would not behave like oxygen any more. Go to S3.6.1. Try another.

    2. Yes, that is right. The gases in air are side by side and not joined. So each still behaves as it does on its own. That is what a mixture is.

    3. Not this one. A pure substance is only one thing. Air holds several gases, so it is a mixture, even though we need only the oxygen. Go to S3.6.2. Try another.

    See the answer

    Air is a mixture, because each gas keeps its own properties.

    The gases in air are side by side and not joined. So each still behaves as it does on its own. That is what a mixture is.

    • If you chose A: The gases in air are not joined to each other. If they were joined, the oxygen would not behave like oxygen any more. Go to S3.6.1.
    • If you chose C: A pure substance is only one thing. Air holds several gases, so it is a mixture, even though we need only the oxygen. Go to S3.6.2.

  6. The picture of air shows 10 pairs of gas particles in all. 2 pairs are oxygen. How many pairs are not oxygen?

    A box of ten pairs of circles, labelled air. 8 pairs are circles with the letter N, and 2 pairs are circles with the letter O. The pairs are far apart in a closed box. A key names the circles: nitrogen and oxygen. Each circle has its letter inside.OOOONNNNNNNNNNNNNNNNairNnitrogenOoxygen
    See the answer

    8 pairs

    There are 10 pairs in all, and 2 are oxygen. 10 − 2 = 8. These are nitrogen pairs.

  7. From an earlier chapter

    Which of these is a compound?

    Tap your answer.

    1. Yes, that is right. Water has hydrogen and oxygen atoms joined together, so it is a compound. Gold and oxygen gas are elements.

    2. Not this one. Gold has only one kind of atom, so it is an element. Go to S3.6.1. Try another.

    3. Not this one. Oxygen atoms are joined in pairs, but all are oxygen. One kind of atom is an element. Go to S3.6.1. Try another.

    See the answer

    water

    Water has hydrogen and oxygen atoms joined together, so it is a compound. Gold and oxygen gas are elements.

    • If you chose B: Gold has only one kind of atom, so it is an element. Go to S3.6.1.
    • If you chose C: Oxygen atoms are joined in pairs, but all are oxygen. One kind of atom is an element. Go to S3.6.1.

  8. About 21 cm³ of every 100 cm³ of air is oxygen. A sample of air is 200 cm³. About how much oxygen is in it?

    See the answer

    42 cm³

    200 cm³ is 2 lots of 100 cm³. 2 × 21 = 42.

Keep it

Copy this into your exercise book.

A pure substance is only one thing. A mixture has two or more things not joined. Clear does not mean pure. Air and sea water are mixtures.

Draw: Draw a box of water groups, and a second box with water groups and salt pairs. Label them pure water and mixture.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (936 KB). It opens on any phone.