There and back: melting, freezing, boiling, condensing
Melting and boiling need heat: the substance gains energy. Freezing and condensation lose heat: the substance gives energy to its surroundings. These changes are reversible. Water can melt and freeze again, and it can evaporate and condense again. A change of state is a physical change: no new substance forms, and water stays H₂O molecules in all three states; only the spacing and the attractions between the molecules change. In a chemical change, new substances form, and it is usually not easy to reverse: frying an egg cannot be undone. Dew, fog and clouds form when water vapour in the air loses heat and condenses. Look at it from the particles: when a gas loses energy its particles move slower and come closer, and the attractions pull them into a liquid. The four changes compared: melting (gain heat, attractions weaker, spaces larger), boiling (gain, weaker, much larger), freezing (lose, stronger, smaller), condensation (lose, stronger, much smaller).
| Change | From → to | Heat | Attractions | Spaces |
|---|---|---|---|---|
| melting | solid → liquid | gained | weaker | larger |
| boiling | liquid → gas | gained | weaker | much larger |
| freezing | liquid → solid | lost | stronger | smaller |
| condensation | gas → liquid | lost | stronger | much smaller |
Droplet detective, with a grown-up. Fill a glass with cold tap water and a second glass with room-temperature water, and leave both on a table. After ten minutes look at the outside of each glass and describe what you see. Breathe onto a cold spoon or mirror and describe the mist. Write where the water on the outside came from.