# 2.1–2.2 Water and the Water Cycle

F.1 Integrated Science · Chapter 2 · Lesson 5

Changes of state, energy, clouds and faster evaporation.

## Key points

- **Three states**: ice is solid, water is liquid, and water vapour is gas. Water vapour itself is invisible.
- **Changes of state**: follow the direction in the table, then decide whether energy is absorbed or released.
- **Fixed temperatures**: pure water melts/freezes at $0\,{}^{\circ}\mathrm{C}$ and boils at $100\,{}^{\circ}\mathrm{C}$ at normal atmospheric pressure.
- **Heating plateaus**: during melting and boiling of pure water at constant pressure, energy changes its state without raising its temperature.
- **Evaporation (蒸發)** occurs at the liquid surface, can occur below the boiling point and can cool the water left behind.
- **Faster evaporation**: higher temperature, lower **humidity (濕度)**, faster air flow or a larger exposed surface area.
- **Water cycle**: sunlight warms water → evaporation → warm air carries vapour upwards → cooling to the **dew point (露點)** → condensation.
- **Clouds** contain tiny suspended water droplets and/or ice crystals. Droplets grow and fall as rain; ice crystals can grow and fall as snow.
- **Precipitation (降水)** returns water to the ground and water sources. The water cycle recycles water.

## Must know

| Change | Direction | Energy |
|---|---|---|
| Melting | Solid → liquid | Absorbed |
| Freezing | Liquid → solid | Released |
| Boiling / evaporation | Liquid → gas | Absorbed |
| Condensation | Gas → liquid | Released |
| Sublimation (Extension) | Solid → gas | Absorbed |
| Deposition (Extension) | Gas → solid | Released |

## Common mistakes

- ✗ “The white mist is water vapour.” ✓ Water vapour is invisible; visible mist consists of tiny liquid droplets.
- ✗ “Melting ice must get hotter.” ✓ Pure ice absorbs energy while melting at constant pressure, without a temperature rise.
- ✗ “Evaporation needs boiling.” ✓ Evaporation occurs at the surface and can occur below the boiling point.
- ✗ “Clouds are water in the gas state.” ✓ Clouds contain tiny liquid droplets and/or ice crystals.
- ✗ “Higher humidity dries clothes faster.” ✓ Lower humidity speeds evaporation; keep other relevant conditions the same.

## Quick check

1. What change turns gas directly into solid, and is energy absorbed or released?
2. At normal atmospheric pressure, what is the boiling point of pure water? Does continued boiling raise its temperature?
3. Name four changes that speed up evaporation.
4. As moist air rises, what must happen before water vapour condenses? What forms a cloud?
5. Why can evaporation cool the water left behind?

## Answers

1. Deposition (Extension); energy is released.
2. $100\,{}^{\circ}\mathrm{C}$; no, at constant pressure while it boils.
3. Higher temperature, lower humidity, faster air flow, larger exposed surface area.
4. It cools to its dew point; tiny suspended droplets and/or ice crystals form clouds.
5. Evaporation absorbs energy from the water and its surroundings.

## Did you know?

Droplets on the outside of a cold bottle come from water vapour in the surrounding air, not from water leaking through the bottle.