Chapter 5 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

S2.4.5 Chapter 5

Condensation

Why is the outside of a cold cup wet?

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 why drops form on a cold cup or a pot lid
  • Explain where the dew on grass comes from

A cup of cold water stands in a warm room. Soon the outside of the cup is wet. Where does the water come from?

The air holds water vapour. Water vapour is water as a gas. You cannot see it. Evaporation put it there.

Draw it first

Draw why a cold cup gets wet

  1. A cup of cold water, labelled cold water. Small dots in the air round the cup are water vapour, labelled water vapour. Two arrows point from the dots to the cup. Drops of water sit on the outside of the cup, labelled drops.cold water

    1 Draw a cup of cold water. Write cold water.

  2. A cup of cold water, labelled cold water. Small dots in the air round the cup are water vapour, labelled water vapour. Two arrows point from the dots to the cup. Drops of water sit on the outside of the cup, labelled drops.cold waterwater vapour

    2 Draw dots in the air for water vapour. Write water vapour.

  3. A cup of cold water, labelled cold water. Small dots in the air round the cup are water vapour, labelled water vapour. Two arrows point from the dots to the cup. Drops of water sit on the outside of the cup, labelled drops.cold waterwater vapourdrops

    3 Draw arrows from the dots to the cup. Draw drops on the outside. Write drops.

Water vapour in the air touches the cold cup. The vapour cools. It turns back into liquid water.

Condensation is water vapour turning back into liquid. It happens when the vapour touches something cold.

The drops form on the outside of the cup. That is where the air touches the cold cup.

See it three ways

As a picture

Two boxes joined by an arrow. The box on the left holds a few particles spread far apart and is labelled vapour. Above the arrow is the word cools. The box on the right holds particles packed together at the bottom and is labelled liquid.vapourcoolsliquid

In words

Water vapour touches something cold. It cools. It turns into drops of liquid water.

A pot lid does the same. A grown up boils water in a pot. Hot water vapour rises and touches the cooler lid. It turns into drops. The drops run down the lid.

A pot with a lid, labelled pot and lid. Dotted water fills the bottom of the pot, labelled water. Three arrows rise from the water toward the lid, labelled water vapour. Drops hang under the lid, labelled drops. Labels: lid, drops, water vapour, pot, water.liddropswater vapourpotwater
Water vapour touches the cooler lid. It turns into drops.

Grass gets cool at night. Water vapour in the air touches the cool grass. It turns into tiny drops. These drops are dew. Most dew forms on clear, still nights.

Dew is not rain. It does not fall from the sky. It forms right on the grass. It forms on leaves and stones too.

A night scene. A moon, labelled night, is at the top right. Dots in the air, labelled water vapour, drift down toward blades of grass, labelled grass. Small drops, labelled dew, sit on the tips of the grass.water vapournightgrassdew
Dew forms on cool grass at night.

Later the Sun warms the grass. The dew evaporates into the air again.

Try it

Where do the drops come from?

You need: a cup, metal if you have one, the coldest water you can get, with ice if you have it, a little ink or paint to colour the water, a plate, a white cloth, a warm room.

Be safe: Never drink or taste the water in this activity. Ink and paint are not safe.

  1. Fill the cup with the coldest water you can get. Water from a clay pot or a deep well is good.
  2. Stir the ink or paint into the cold water.
  3. Wipe the outside of the cup dry. Stand it on the plate.
  4. Wait ten minutes. Look at the outside of the cup.
  5. Touch a drop with the white cloth. Is it coloured?

What you will see: The outside of the cup gets wet with drops. The drops are clear, not coloured. The coloured water did not come through the cup. The drops came from the air. If no drops form, the water is not cold enough. Try on a damp day, or add ice if you can.

If you cannot try it, this is what happens.
A cup of cold water with ink in it, labelled coloured water. Drops sit on the outside of the cup, labelled clear drops. The drops have no colour.clear dropscoloured water

A wrong idea many people have

The water on a cold cup comes through the cup.

Why it feels right: The cup holds water. So the drops on the outside must have leaked out.

The test that breaks it: If you have a fridge, cool a dry, empty cup in it. Bring it into a warm room. Its outside still gets wet. Or fill a cup with cold coloured water. The drops on the outside are clear. Water that came through the cup would be coloured.

What is true: The drops come from the air. Water vapour touches the cold cup and turns into liquid.

Key words

water vapour
water as a gas. You cannot see it.
condensation
water vapour turning back into liquid on something cold.
dew
tiny drops of water on cool grass and leaves at night.

Back to your guess

The answer is B: The water comes from the air.

The water comes from the air. Water vapour in the air touches the cold cup. It turns into drops.

Check yourself

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

  1. Where do the drops on a cold cup come from?

    Tap your answer.

    1. Yes, that is right. Water vapour in the air touches the cold cup. It cools and turns into drops.

    2. Not this one. A dry cold cup with nothing inside still gets wet. And coloured water gives clear drops. Try another.

    3. Not this one. Cold does not make water. The water was in the air as vapour. Try another.

    See the answer

    From the air

    Water vapour in the air touches the cold cup. It cools and turns into drops.

    • If you chose B: A dry cold cup with nothing inside still gets wet. And coloured water gives clear drops.
    • If you chose C: Cold does not make water. The water was in the air as vapour.

  2. What is condensation?

    Tap your answer.

    1. Not this one. That is evaporation. Condensation goes the other way. Go to S2.4.4. Try another.

    2. Not this one. That is freezing. In condensation, a gas becomes a liquid. Go to S2.4.3. Try another.

    3. Yes, that is right. Condensation is water vapour turning back into liquid. It happens on something cold.

    See the answer

    Water vapour turning into liquid drops

    Condensation is water vapour turning back into liquid. It happens on something cold.

    • If you chose A: That is evaporation. Condensation goes the other way. Go to S2.4.4.
    • If you chose B: That is freezing. In condensation, a gas becomes a liquid. Go to S2.4.3.

  3. Amina sees dew on the grass in the morning. Where did it come from?

    Tap your answer.

    1. Not this one. Dew is not rain. It forms on the grass from the air. Try another.

    2. Not this one. Air does not turn into water. Only the water vapour in the air turns into drops. Try another.

    3. Not this one. Dew forms from vapour in the air. It does not rise as drops from the ground. Try another.

    4. Yes, that is right. The grass was cool. Vapour touched it and turned into drops of dew.

    See the answer

    Water vapour in the air cooled on the grass.

    The grass was cool. Vapour touched it and turned into drops of dew.

    • If you chose A: Dew is not rain. It forms on the grass from the air.
    • If you chose B: Air does not turn into water. Only the water vapour in the air turns into drops.
    • If you chose C: Dew forms from vapour in the air. It does not rise as drops from the ground.

  4. A grown up boils a pot of water with a lid on. Drops form under the lid. Why?

    Tap your answer.

    1. Not this one. No water is made. The water was in the pot, as vapour. Try another.

    2. Yes, that is right. The vapour cools on the lid and turns back into liquid. This is condensation.

    3. Not this one. The lid is not cold enough to freeze water. The vapour cools into liquid drops. Go to S2.4.3. Try another.

    See the answer

    Hot vapour touches the cooler lid and turns into drops.

    The vapour cools on the lid and turns back into liquid. This is condensation.

    • If you chose A: No water is made. The water was in the pot, as vapour.
    • If you chose C: The lid is not cold enough to freeze water. The vapour cools into liquid drops. Go to S2.4.3.

  5. From an earlier chapter

    Wet washing hangs in the wind. Where does its water go as it dries?

    Tap your answer.

    1. Not this one. Water does not vanish. It evaporates into the air as vapour. Go to S2.4.4. Try another.

    2. Not this one. The washing hangs in the air. Its water evaporates into the air. Go to S2.4.4. Try another.

    3. Yes, that is right. The water evaporates. It turns into water vapour that you cannot see.

    See the answer

    Into the air, as water vapour

    The water evaporates. It turns into water vapour that you cannot see.

    • If you chose A: Water does not vanish. It evaporates into the air as vapour. Go to S2.4.4.
    • If you chose B: The washing hangs in the air. Its water evaporates into the air. Go to S2.4.4.

  6. Draw a cold cup with drops on the outside. Add arrows to show where the water comes from.

    See the answer

    A cup with drops on the outside. Arrows point from the air to the cup.

    The drops come from water vapour in the air. They do not come from inside the cup.

  7. Two glasses stand in the same warm room. One holds cold water. One holds warm water. Which glass gets wet drops on the outside?

    Two glasses side by side. The first holds dotted water with two cubes of ice, labelled cold water. The second holds dotted water with no ice, labelled warm water. Labels: cold water, warm water.cold waterwarm water
    Tap your answer.

    1. Not this one. The outside of that glass is not cold. Vapour turns into drops only on something cold. Try another.

    2. Not this one. Only a cold glass cools the air that touches it. The warm glass stays warm. Try another.

    3. Not this one. The drops do not come from inside the glass. They come from the air, on a cold glass. Try another.

    4. Yes, that is right. Vapour in the room touches the cold glass and turns into drops. The warm glass is not cold enough.

    See the answer

    The glass of cold water

    Vapour in the room touches the cold glass and turns into drops. The warm glass is not cold enough.

    • If you chose A: The outside of that glass is not cold. Vapour turns into drops only on something cold.
    • If you chose B: Only a cold glass cools the air that touches it. The warm glass stays warm.
    • If you chose C: The drops do not come from inside the glass. They come from the air, on a cold glass.

  8. Mei puts a dry, empty cup in a fridge. Later she takes it into a warm room. Drops form on the outside. What does this show?

    Tap your answer.

    1. Yes, that is right. Vapour in the room touched the cold cup and turned into drops. The cup held no water.

    2. Not this one. The cup was empty and dry. It had no water to sweat. Try another.

    3. Not this one. The cup was dry when she took it out. The drops came later, from the air. Try another.

    See the answer

    The water comes from the air in the room.

    Vapour in the room touched the cold cup and turned into drops. The cup held no water.

    • If you chose B: The cup was empty and dry. It had no water to sweat.
    • If you chose C: The cup was dry when she took it out. The drops came later, from the air.

Keep it

Copy this into your exercise book.

Water vapour in the air touches something cold. It cools and turns into drops. This is condensation. It makes drops on a cold cup and dew on grass. The drops come from the air.

Draw: A cold cup. Dots in the air. Arrows to the cup. Drops outside.

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

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