This grade 6 science lesson on changes of state notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on changes of state and check yourself with the quiz.
Quick answer: Water changes state when it gains or loses heat. Also, it melts and freezes at 0 °C, and it boils and condenses at 100 °C. During each change the temperature stays constant, because the heat rearranges the particles rather than making them move faster.
Level: Grade 6 science (Year 7 · Class 6) · Curriculum: MS-PS1-4
Changes of state notes: the key ideas
What are the changes of state?
When a substance gains or loses heat, it can change from one state to another. Water is the best example because we see all three states every day: ice, liquid water and water vapour. In addition, a change of state is a physical change. The substance is still water afterwards, and its mass has not changed.
A change of state happens when a substance turns from solid, liquid or gas into a different one of these states because it has gained or lost heat.
| Change | From → to | Heat | Everyday example |
|---|---|---|---|
| Melting | solid → liquid | taken in | an ice lolly dripping |
| Freezing | liquid → solid | given out | water turning to ice in a tray |
| Evaporation | liquid → gas | taken in | a puddle drying after rain |
| Boiling | liquid → gas | taken in | water bubbling in a kettle |
| Condensation | gas → liquid | given out | droplets on a cold bottle |
At which temperatures does water change state?
Pure water melts and freezes at 0 °C. Moreover, it boils, and steam condenses, at 100 °C when the air pressure is normal at sea level. Below 0 °C water is ice, between 0 °C and 100 °C it is liquid, and above 100 °C it is a gas. Similarly, the graph shows a sample of ice heated at a steady rate.
Why does the temperature stay the same while ice melts?
We call the flat sections of the graph plateaus. During melting, the heat you add pulls particles out of their fixed places in the ice. It does not make them move faster, so the thermometer does not move. Also, the same happens at 100 °C: the heat separates the particles, and the temperature stays at 100 °C until all the liquid has gone.
While a pure substance is melting, freezing, boiling or condensing, its temperature stays constant.
What do the particles do in each change?
When a solid melts, vibrating particles gain energy and break free from their fixed positions. Then, during boiling, they gain enough energy to leave the liquid entirely and spread far apart. Freezing and condensation run in reverse: particles lose energy and move into closer, more orderly arrangements.
Kwame in Accra heats water at 20 °C. In addition, the temperature rises by 4 °C every minute. How long does it take to reach 100 °C?
Rise needed = 100 − 20 = 80 °C.
Time = 80 ÷ 4 = 20 min.
The water reaches 100 °C after 20 min.
A block of ice starts to melt after 3 min and is all melted at 11 min. Then the water warms from 0 °C to 100 °C in 25 min.
Melting time = 11 − 3 = 8 min, at 0 °C.
From the start of melting to the start of boiling = 8 + 25 = 33 min.
Melting lasts 8 min at a steady 0 °C, and boiling begins 33 min after melting began.
How is evaporation different from boiling?
Evaporation happens only at the surface of a liquid and can occur at any temperature. Moreover, the fastest particles at the surface escape. Boiling happens throughout the liquid, forms bubbles of vapour, and occurs only at the boiling point. That is why a puddle disappears on a warm day while it stays far below 100 °C.
Steam is invisible. Similarly, the white cloud above a kettle is not steam; it is tiny droplets of liquid water that formed when the hot vapour met cooler air and condensed.
Heat goes IN for melting, evaporation and boiling. Heat comes OUT for freezing and condensation. Also, if the substance gets more spread out, heat went in.
Why do wet clothes dry on a warm, windy day?
Warmer water has more fast particles able to escape, and wind carries escaped vapour away before it can return. A bigger exposed surface also helps. In addition, all three factors raise the rate of evaporation.
- Melting and freezing point of pure water: 0 °C.
- Boiling and condensation point of pure water: 100 °C at sea level.
- Melting, evaporation and boiling take in heat; freezing and condensation give out heat.
- The temperature stays constant (a plateau) during a change of state.
- Evaporation happens at the surface at any temperature; boiling happens throughout at 100 °C.
How water melts, boils and freezes: frequently asked questions
At what temperature does water freeze and boil?
Pure water freezes at 0 °C and boils at 100 °C at normal air pressure. Moreover, the same temperatures apply to melting and condensation.
Why does the temperature stay constant while water boils?
The heat separates the particles from one another and turns liquid into gas. It does not raise the average speed of the particles, so the temperature stays at 100 °C.
What is the difference between evaporation and boiling?
Evaporation occurs only at the surface and at any temperature below the boiling point. Boiling occurs throughout the liquid, with bubbles, at the boiling point.
Is the white cloud above a kettle steam?
No. Steam is an invisible gas. Similarly, the cloud you see consists of tiny droplets of liquid water that condensed when the vapour cooled.
What is condensation?
Condensation is the change from gas to liquid. Particles of water vapour lose energy when they meet a cold surface and come together as droplets.
