Chapter 7 of 7

All 7 chapters
  1. 1Solids, liquids and gases, readunderstood
  2. 2Air is all around us, readunderstood
  3. 3Melting and freezing, readunderstood
  4. 4Evaporation and drying, readunderstood
  5. 5Condensation, readunderstood
  6. 6The water cycle, readunderstood
  7. 7Clean water, readunderstood

S2.4.7 Chapter 7

Clean water

If water looks clear, is it safe to drink?

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.

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

By the end you can

  • Separate sand from water with a cloth filter
  • Explain why salt that dissolves passes through a filter
  • Explain why filtered water still needs boiling before drinking

Mei has a jar of muddy water. She wants to clean it. Can a cloth do the job?

Draw it first

Draw a cloth filter

  1. A cup with a cloth tied over its top, labelled cloth. An arrow brings muddy water, labelled muddy water, down onto the cloth. Bits of sand, labelled sand, stay on top of the cloth. Drops of water drip through into the cup. The water in the cup is labelled filtered water.cloth

    1 Draw a cup. Tie a cloth over its top. Write cloth.

  2. A cup with a cloth tied over its top, labelled cloth. An arrow brings muddy water, labelled muddy water, down onto the cloth. Bits of sand, labelled sand, stay on top of the cloth. Drops of water drip through into the cup. The water in the cup is labelled filtered water.clothmuddy watersand

    2 Pour muddy water on the cloth. The sand stays on top. Write muddy water and sand.

  3. A cup with a cloth tied over its top, labelled cloth. An arrow brings muddy water, labelled muddy water, down onto the cloth. Bits of sand, labelled sand, stay on top of the cloth. Drops of water drip through into the cup. The water in the cup is labelled filtered water.clothmuddy watersandfiltered water

    3 Water drips into the cup. Write filtered water.

Muddy water has bits in it. The bits are sand and soil. You can see them. They do not dissolve. To dissolve is to mix into water. The pieces get too small to see.

Salt and sugar dissolve. Sand does not.

A filter has tiny holes. Water goes through the holes. Bigger bits stay behind. A cloth is a filter.

The holes in the cloth are too small for sand. They are big enough for water. So the sand stays on the cloth.

Now stir salt into water. The salt seems to vanish. It has dissolved. It broke into pieces too small to see. They are mixed all through the water.

Pour the salty water through a cloth. The salt goes through with the water. Its pieces are far smaller than the holes. Sugar does the same.

See it three ways

As a picture

Two pictures of a cloth, labelled cloth. On the left, labelled sand, bits of sand sit on top of the cloth and stay there. Only water goes through, shown by an arrow. The words under it are stays on top. On the right, labelled salt, dots of salt are above and below the cloth, and arrows go to the cloth and through it. The words under it are goes through.sandsaltclothstays on topgoes through

In words

Sand is bits you can see. The cloth holds them back. Salt dissolves into pieces too small to see. They go through the cloth with the water.

Water can hold germs too. Germs are living things so small that you cannot see them. Some germs make people ill. They are small enough to go through a cloth. So clear water can still have germs in it.

On the left, a glass of clear water, labelled clear water. Lines lead from it to a big circle on the right, a close up of the water. Inside the circle are a few small circles, which are germs, and a few small black squares, which are salt dissolved in the water. A key under the circle names them: germs and salt. Labels: clear water, germs, salt.clear watergermssalt
Clear water can hold germs and salt. You cannot see them.

Try it

Filter sand, then test salt

You need: two cups, a clean cloth and string, a spoon of sand, a spoon of salt, water, a saucer, a sunny place.

Be safe: Never drink or taste the water in this activity.

  1. Stir a spoon of sand into a cup of water.
  2. Tie a clean cloth over the mouth of the second cup.
  3. Pour the sandy water slowly onto the cloth.
  4. Look at the cloth. Look at the water in the cup.
  5. Wash both cups and the cloth. Stir a spoon of salt into a cup of clean water. Wait until the salt is out of sight.
  6. Tie the cloth over the other cup. Pour the salty water through it.
  7. Pour a little of it onto a saucer. Put the saucer in the sun.
  8. When the saucer is dry, look at it.

What you will see: The cloth holds back the sand. The water in the cup has no sand. The salty water goes through the cloth too. When the saucer dries, a white crust is left. It is salt. The salt went through the cloth with the water. Only the water rose into the air.

If you cannot try it, this is what happens.
Two saucers. The saucer on the left holds salt water, labelled salt water. An arrow points to the saucer on the right, with the word dries above the arrow. The saucer on the right has white crumbs in it, labelled salt is left.salt waterdriessalt is left

Only the water rises. The salt stays behind. So rain is not salty, even when it came from the sea.

A pot of dotted water with open round bubbles all through it, labelled pot and bubbles. Three arrows rise from the water, labelled water vapour. Small black squares of salt are spread all through the water, labelled salt stays in the water. Labels: water vapour, pot, salt stays, in the water, bubbles.water vapourbubblespotsalt staysin the water
Boiling kills germs. The water rises, and the salt stays.
Four boxes joined by arrows, in order: muddy water, cloth, boil, cool. Each arrow points to the next box. Labels: muddy water, cloth, boil, cool.muddy waterclothboilcool
A cloth takes out sand. A grown up boils the water. Then it cools.

A wrong idea many people have

If water is clear, it is safe to drink.

Why it feels right: Dirty water looks muddy. So water that looks clear must be clean.

The test that breaks it: Stir salt into water until it looks clear. Pour it through a cloth. It stays clear. Dry a little in a saucer in the sun. White salt is left. Clear water can hold things you cannot see. Do not taste it.

What is true: Clear water can hold salt, sugar and germs. A cloth does not take them out. A grown up must boil water before anyone drinks it.

Key words

filter
something with tiny holes. Water goes through. Bigger bits stay behind.
dissolve
to mix into water in pieces too small to see.
germs
living things too small to see. Some make people ill.

Back to your guess

The answer is D: No. Clear water can still hold germs.

Clear water is not always safe. A cloth holds back sand and mud. It does not stop germs. A grown up must boil water before anyone drinks it.

Check yourself

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

  1. Which of these does a cloth filter hold back?

    Tap your answer.

    1. Yes, that is right. The holes are too small for sand. They are big enough for water, dissolved salt and germs.

    2. Not this one. Dissolved salt is in pieces too small to see. They pass through the cloth. Try another.

    3. Not this one. Germs are tiny. They go through the holes in a cloth. Try another.

    See the answer

    Sand

    The holes are too small for sand. They are big enough for water, dissolved salt and germs.

    • If you chose B: Dissolved salt is in pieces too small to see. They pass through the cloth.
    • If you chose C: Germs are tiny. They go through the holes in a cloth.

  2. Ravi pours salty water through a cloth. Is the salt still in the water?

    Tap your answer.

    1. Not this one. A cloth holds back only bits you can see. Dissolved salt is too small. Try another.

    2. Not this one. Dissolved salt is still salt. Dry a little of the water and white salt is left. Try another.

    3. Yes, that is right. The salt broke into tiny pieces. They go through the holes with the water. A dried saucer shows the salt.

    See the answer

    Yes. Dissolved salt goes through the cloth.

    The salt broke into tiny pieces. They go through the holes with the water. A dried saucer shows the salt.

    • If you chose A: A cloth holds back only bits you can see. Dissolved salt is too small.
    • If you chose B: Dissolved salt is still salt. Dry a little of the water and white salt is left.

  3. Tariq filters muddy water through a cloth. It looks clear. What must a grown up do before anyone drinks it?

    Tap your answer.

    1. Not this one. Clear water can hold germs. A cloth does not take them out. Try another.

    2. Not this one. A cloth cannot stop germs, so a second go does not help. Boiling kills them. Try another.

    3. Not this one. Standing lets mud sink. The germs stay in the water. Try another.

    4. Yes, that is right. A cloth holds back sand. It does not stop germs. Boiling kills germs.

    See the answer

    Boil it hard, then let it cool.

    A cloth holds back sand. It does not stop germs. Boiling kills germs.

    • If you chose A: Clear water can hold germs. A cloth does not take them out.
    • If you chose B: A cloth cannot stop germs, so a second go does not help. Boiling kills them.
    • If you chose C: Standing lets mud sink. The germs stay in the water.

  4. Salty water goes through a cloth. A grown up then boils it. Is the salt gone?

    Tap your answer.

    1. Not this one. Only the water rises as vapour. The salt stays in the pot. Try another.

    2. Yes, that is right. The water becomes vapour. The salt stays behind in the pot.

    3. Not this one. A cloth cannot take out dissolved salt. It goes through with the water. Try another.

    See the answer

    No. Boiling kills germs but leaves the salt.

    The water becomes vapour. The salt stays behind in the pot.

    • If you chose A: Only the water rises as vapour. The salt stays in the pot.
    • If you chose C: A cloth cannot take out dissolved salt. It goes through with the water.

  5. From an earlier chapter

    Sand, water and steam. Which one is a liquid?

    Tap your answer.

    1. Yes, that is right. A liquid takes the shape of its cup. Its amount stays the same. Water is a liquid.

    2. Not this one. Sand is made of hard grains. It is a solid. Go to S2.4.1. Try another.

    3. Not this one. Steam is water vapour. That is water as a gas. Go to S2.4.4. Try another.

    See the answer

    Water

    A liquid takes the shape of its cup. Its amount stays the same. Water is a liquid.

    • If you chose B: Sand is made of hard grains. It is a solid. Go to S2.4.1.
    • If you chose C: Steam is water vapour. That is water as a gas. Go to S2.4.4.

  6. Draw a cloth filter. Show where the sand stays and where the water goes.

    See the answer

    A cup with a cloth over it. The sand sits on top of the cloth. Water drips through into the cup.

    The holes in the cloth are too small for sand. They let the water through.

  7. Water from a stream is clear and cold. Can you tell by looking if it is safe?

    Tap your answer.

    1. Not this one. Clear water can hold germs and salt. You cannot see them. Try another.

    2. Yes, that is right. You cannot see germs or dissolved salt. A grown up must boil the water before anyone drinks it.

    3. Not this one. Never taste water to test it. You cannot taste or smell most germs. Try another.

    See the answer

    No. Germs are too small to see.

    You cannot see germs or dissolved salt. A grown up must boil the water before anyone drinks it.

    • If you chose A: Clear water can hold germs and salt. You cannot see them.
    • If you chose C: Never taste water to test it. You cannot taste or smell most germs.

  8. Ravi stirs sugar into water until it vanishes. Where is the sugar?

    Tap your answer.

    1. Not this one. Dissolved sugar is still sugar. Dry some of the water and the sugar comes back. Try another.

    2. Not this one. Nothing is lost. The sugar is in the water, in pieces too small to see. Try another.

    3. Yes, that is right. Sugar dissolves into tiny pieces that mix all through the water. They are still there.

    See the answer

    It is in the water, in pieces too small to see.

    Sugar dissolves into tiny pieces that mix all through the water. They are still there.

    • If you chose A: Dissolved sugar is still sugar. Dry some of the water and the sugar comes back.
    • If you chose B: Nothing is lost. The sugar is in the water, in pieces too small to see.

Keep it

Copy this into your exercise book.

A cloth filter holds back sand and mud. Salt and sugar dissolve, so they go through. A cloth does not stop germs. Clear water is not always safe. A grown up must boil water before anyone drinks it.

Draw: A cup with a cloth on top. Sand on the cloth. Drops in the cup.

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 (712 KB). It opens on any phone.