Put an ice cube in a warm hand and it melts. Leave water in the freezer and it freezes. Heat a pot of water and it boils away. Matter can change state when it is heated or cooled.
Key idea
Heating: solid → liquid is melting; liquid → gas is evaporating (or boiling, when it bubbles). Cooling: gas → liquid is condensing; liquid → solid is freezing.
| Water | °C | °F |
|---|---|---|
| Freezes / melts | 0 °C | 32 °F |
| Body temperature | about 37 °C | about 98.6 °F |
| Boils (at sea level) | 100 °C | 212 °F |
Why it works
Heat makes particles move faster. When they move fast enough, they break out of their tight solid pattern (melting) or escape into the air (evaporating). Cooling slows them down so they stick together again.
Worked example: What state?
A puddle is at −5 °C on a winter morning. Is it ice or water?
- Water freezes at 0 °C.
- −5 °C is colder than 0 °C.
Answer: It is frozen: ice.
Common mistake
Thinking the white "steam" over a kettle is the gas. Water vapor is invisible. The white mist is tiny drops of liquid water made when the vapor cools in the air.
Melting does not change the mass. If 100 grams of ice melt, you have 100 grams of water.
Try it at home
- Put an ice cube on a plate in the sun and one on a plate in the shade. Which melts first?
- Breathe on a cold mirror or window. The fog is water vapor from your breath condensing into tiny drops.