Chapter 4 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

S2.4.4 Chapter 4

Evaporation and drying

Where does the water go when a wet cloth dries?

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

  • Explain that water evaporates into the air as a gas
  • Test which place dries a wet cloth fastest

Hang a wet cloth in the sun. Later it is dry. Where did the water go?

A wet dotted cloth hangs on a line, labelled wet cloth. Three arrows rise from it, labelled water vapour. An arrow labelled later points to a dry white cloth on the same line, labelled dry cloth. Labels: water vapour, later, wet cloth, dry cloth.water vapourlaterwet clothdry cloth
The wet cloth dries. Its water goes up into the air.

The water did not vanish. It went into the air. It turned into a gas. Water vapour is water as a gas. Evaporation is water turning into water vapour.

You cannot see water vapour. It mixes with the air.

Draw it first

Draw a dish drying out

  1. A dish with dotted water in it, labelled water. Then the water level is lower, labelled less water, and three arrows go up from it into the air. A label above them reads water vapour. Last, the dish is empty, labelled empty dish. The arrows still point up. Labels: water, less water, water vapour, empty dish.water

    1 Draw a dish. Fill it with dotted water.

  2. A dish with dotted water in it, labelled water. Then the water level is lower, labelled less water, and three arrows go up from it into the air. A label above them reads water vapour. Last, the dish is empty, labelled empty dish. The arrows still point up. Labels: water, less water, water vapour, empty dish.water vapourless water

    2 Draw less water. Draw arrows going up. Write water vapour.

  3. A dish with dotted water in it, labelled water. Then the water level is lower, labelled less water, and three arrows go up from it into the air. A label above them reads water vapour. Last, the dish is empty, labelled empty dish. The arrows still point up. Labels: water, less water, water vapour, empty dish.water vapourempty dish

    3 Draw an empty dish. The water is now in the air.

Evaporation goes on all the time, in sun or in shade. It is faster when it is warm. It is faster when wind blows. It is faster when the water is spread out.

Water in a wide dish dries sooner than in a cup. A cold cloth dries too. It just takes longer.

A puddle lies flat on the ground, labelled puddle. The Sun, labelled Sun, shines at the top right. Two arrows on the left, labelled wind, point toward the puddle. Three arrows rise from the puddle, labelled water vapour. Labels: Sun, wind, water vapour, puddle.Sunwindwater vapourpuddle
Sun and wind dry a puddle faster. The water goes up as vapour.

Evaporation is not boiling. Boiling is when water bubbles hard all the way through. Pure water boils at 100 °C at sea level. High up a mountain, it boils at a lower temperature.

Two pictures side by side. On the left, a pot of dotted water with round bubbles all through it, labelled boiling, with the words bubbles, all through. On the right, a shallow dish of water with three arrows rising from the top of the water, labelled evaporating, with the words from the top. Labels: boiling, bubbles, all through, evaporating, from the top.boilingbubblesall throughevaporatingfrom the top
Boiling is hot and bubbly. Evaporation is quiet, at the top.

Boiling water makes steam. Steam is water vapour. You cannot see it. The white cloud over a kettle is not steam. It is tiny drops of water. A later chapter shows how they form.

A kettle on the left, labelled kettle, with a spout. A dashed line leaves the spout. It shows the steam, which you cannot see, labelled steam. Farther along is a dotted cloud labelled tiny drops. Labels: kettle, steam, tiny drops.tiny dropssteamkettle
Steam is the clear part. The white cloud is tiny drops of water.

See it three ways

As a picture

Two boxes with an arrow between them. The first box, labelled water, holds touching dots that fill the bottom. The arrow points to the second box, labelled water vapour. In it, a few dots are far apart all through a closed box. Labels: water, water vapour.waterwater vapour

In words

Water at the top of a puddle slowly turns into water vapour. Water vapour is a gas. It goes into the air.

In symbols

Evaporation: at any temperature. Boiling: pure water at sea level, 100 °C.

Try it

Which place dries a cloth fastest?

You need: three small cloths the same size, a bowl of water, a sunny place, a shady place, a clock or a watch.

Be safe: Do not look at the Sun. Do not drink the water.

  1. Wet all three cloths in the bowl. Wring each one the same.
  2. Spread one cloth flat in the sun. Spread one flat in the shade.
  3. Fold the third cloth into a heap. Put it in the shade.
  4. Touch each cloth every ten minutes. Which one is dry first?

What you will see: The cloth in the sun dries first. The spread cloth in the shade dries next. The folded cloth dries last.

If you cannot try it, this is what happens.
Three black bars show how long each cloth takes to dry. A taller bar means a longer time. The bar labelled sun is the shortest. The bar labelled shade is taller. The bar labelled folded is the tallest. A label above the bars reads time to dry. Labels: time to dry, sun, shade, folded.time to drysunshadefolded

A wrong idea many people have

When a wet cloth dries, the water is gone for good.

Why it feels right: The cloth is dry. You cannot see the water any more.

The test that breaks it: Put a wet cloth in a clear plastic bag. Close it tight. Leave it in the sun for an hour. Look inside the bag. Tiny drops of water cover the inside of the bag. Keep bags away from small children.

What is true: The water went into the air as water vapour. The bag held it in, so some turned back into drops.

Key words

evaporation
water turning into water vapour.
water vapour
water as a gas. You cannot see it.
boiling
water bubbling hard all the way through. It happens only when water is very hot.

Back to your guess

The answer is C: The water goes into the air.

The water goes into the air as a gas. Water does not vanish.

Check yourself

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

  1. A wet cloth dries in the sun. Where does the water go?

    Tap your answer.

    1. Not this one. Water does not vanish. It goes into the air as water vapour. Try another.

    2. Yes, that is right. Water evaporates. It turns into a gas, water vapour, and mixes with the air.

    3. Not this one. The cloth is dry, so the water has left it. It is in the air. Try another.

    See the answer

    It goes into the air as water vapour.

    Water evaporates. It turns into a gas, water vapour, and mixes with the air.

    • If you chose A: Water does not vanish. It goes into the air as water vapour.
    • If you chose C: The cloth is dry, so the water has left it. It is in the air.

  2. Which cloth dries fastest?

    Tap your answer.

    1. Yes, that is right. Water evaporates faster when it is warm and spread out.

    2. Not this one. A folded cloth in the shade dries slowly. Warm and spread out is faster. Try another.

    3. Not this one. The bag keeps the vapour in, so the cloth stays wet. The water must reach the open air. Try another.

    See the answer

    Spread out in the warm sun

    Water evaporates faster when it is warm and spread out.

    • If you chose B: A folded cloth in the shade dries slowly. Warm and spread out is faster.
    • If you chose C: The bag keeps the vapour in, so the cloth stays wet. The water must reach the open air.

  3. Is evaporation the same as boiling?

    Tap your answer.

    1. Not this one. Boiling needs a lot of heat. Water also evaporates when it is cool. Try another.

    2. Not this one. Water evaporates in the shade too. It is just slower. Try another.

    3. Yes, that is right. Evaporation happens at any temperature, but slowly when it is cold. Boiling needs water to be very hot.

    See the answer

    No. Water evaporates even when it is not hot.

    Evaporation happens at any temperature, but slowly when it is cold. Boiling needs water to be very hot.

    • If you chose A: Boiling needs a lot of heat. Water also evaporates when it is cool.
    • If you chose B: Water evaporates in the shade too. It is just slower.

  4. What is the white cloud above a kettle?

    Tap your answer.

    1. Not this one. Steam is water vapour, and you cannot see it. The cloud is something else. Try another.

    2. Not this one. There is no fire in the water. The cloud is made of water. Try another.

    3. Not this one. Air is clear, and you cannot see it. The white cloud is made of water. Try another.

    4. Yes, that is right. Steam is clear. The white cloud is tiny drops of water.

    See the answer

    Tiny drops of water

    Steam is clear. The white cloud is tiny drops of water.

    • If you chose A: Steam is water vapour, and you cannot see it. The cloud is something else.
    • If you chose B: There is no fire in the water. The cloud is made of water.
    • If you chose C: Air is clear, and you cannot see it. The white cloud is made of water.

  5. From an earlier chapter

    A puddle of ice sits in the morning sun. Later it is a dry patch. What happened, in order?

    Tap your answer.

    1. Yes, that is right. Heat melts the ice into water. Then the water evaporates into the air.

    2. Not this one. It was already ice. The sun heated it, so it melted first. Go to S2.4.3. Try another.

    3. Not this one. The ice melts first into water. Then the water evaporates. Go to S2.4.3. Try another.

    See the answer

    Melting, then evaporation

    Heat melts the ice into water. Then the water evaporates into the air.

    • If you chose B: It was already ice. The sun heated it, so it melted first. Go to S2.4.3.
    • If you chose C: The ice melts first into water. Then the water evaporates. Go to S2.4.3.

  6. You want wet washing to dry fast. Give two things you can do.

    See the answer

    Spread it out. Hang it in the sun and the wind.

    Water evaporates faster when it is warm, windy and spread out.

  7. Kofi seals a wet cloth in a clear bag. He leaves it in the sun. Drops form inside the bag. Where do they come from?

    Tap your answer.

    1. Not this one. A bag cannot make water. The water was in the wet cloth. Try another.

    2. Yes, that is right. The cloth dried. Its water went into the air in the bag as vapour. Some turned back into drops.

    3. Not this one. The bag is closed plastic. Water does not come through it. The drops came from the cloth. Try another.

    See the answer

    Water left the cloth as vapour and became drops.

    The cloth dried. Its water went into the air in the bag as vapour. Some turned back into drops.

    • If you chose A: A bag cannot make water. The water was in the wet cloth.
    • If you chose C: The bag is closed plastic. Water does not come through it. The drops came from the cloth.

  8. Mei tests where a wet cloth dries fastest. What must be the same for every cloth?

    Tap your answer.

    1. Not this one. The place is the one thing she changes. It is different for each cloth. Try another.

    2. Not this one. The time is what she measures. It will be different. Try another.

    3. Not this one. Then she could not tell what made the difference. A fair test changes one thing. Try another.

    4. Yes, that is right. In a fair test only the place changes. The cloths are the same size and equally wet.

    See the answer

    The size of each cloth and how wet it is

    In a fair test only the place changes. The cloths are the same size and equally wet.

    • If you chose A: The place is the one thing she changes. It is different for each cloth.
    • If you chose B: The time is what she measures. It will be different.
    • If you chose C: Then she could not tell what made the difference. A fair test changes one thing.

Keep it

Copy this into your exercise book.

Water evaporates into the air as a gas called water vapour. It does not vanish. It happens at any temperature. It is faster when warm, windy or spread out. Boiling is different. Steam cannot be seen.

Draw: Draw a dish with a little water. Draw three arrows going up from it. Write water vapour above them.

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.