Q1.
Observe your surroundings. What are the conditions that affect how fast water evaporates? What differences do you see in evaporation on a cold day versus a hot day? Discuss with your friends. The following words may help in your discussion—fan, drying cloth, sweating, windy day, hot day, rainy day.
Answer
Four conditions decide how fast water evaporates. Each of them can be seen in the words the book suggests:
| Condition | Evaporation is faster when… | What you see around you |
|---|---|---|
| Temperature | it is hotter | On a hot day clothes dry by noon; in winter the same clothes stay damp till evening |
| Movement of air | air moves faster | A fan dries a mopped floor quickly; on a windy day washing dries fastest |
| Exposed area | the water is spread over a larger area | A drying cloth spread wide on the line dries much sooner than one left bunched up |
| Humidity | the air is drier | On a rainy day the air is already full of vapour, so clothes and floors dry very slowly |
Hot day versus cold day: on a hot day the water particles already have more energy, so many more of them can escape from the surface each second. The puddle dries in an hour, sweat disappears from the skin at once and leaves us cooled, and the water in an open bucket goes down noticeably. On a cold day, all of this happens, but far more slowly — evaporation never stops, it only slows down.
Why all four conditions work in the same direction: evaporation is the escape of the fastest particles from the surface. Heat gives them more speed; a large surface gives more of them a door to escape from; moving air carries the escaped vapour away so the door does not get blocked; and dry air has more room to receive them.
Tip: When you design an experiment on this, change only one of the four and keep the other three the same. That is exactly what Activities 8.7 and 8.8 do.