Evaporation and condensation
Chemistry · Key Stage 2 · The School
Two changes, opposite ways
EVAPORATION is liquid becoming gas: water leaves a puddle as invisible water vapour. CONDENSATION is the reverse — gas becoming liquid when it cools, which is why droplets appear on a cold window or a cold glass. Both are physical changes: the water is the same water throughout, only its state has changed.
What speeds evaporation up
Three things. HEAT — warmer water evaporates faster, which is why washing dries quicker in the sun. SURFACE AREA — a spread-out puddle goes faster than a deep one, which is why washing is hung out rather than left in a heap. And MOVING AIR — a breeze carries away vapour so more can leave, which is why a windy day dries washing even when it is cold.
Where you meet them
Steam appearing above a kettle is water vapour that has condensed back into tiny droplets in cooler air — the truly gaseous part just above the spout is invisible. A mirror mists because warm damp air meets cold glass. Clouds form when rising air cools enough for vapour to condense. Dew appears overnight for the same reason.
Pour identical puddles of water onto two identical trays. Put one on a warm sunny windowsill and one in a cool shaded cupboard. Check every hour. The warm one shrinks visibly faster. Same water, same tray, same amount — only the temperature differed, so temperature must be what changed the rate. Now hold a cold spoon over a mug of hot water and watch the reverse happen in seconds.