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Evaporation is a liquid changing into a gas at its surface without boiling. It happens at all temperatures, faster when hotter. Some surface particles have enough energy to escape into the air as vapour.
The molecules that leave are the higher-energy ones, so the liquid left behind is cooler. Sweat cools us this way.
Refrigerators use this cooling. A refrigerant evaporates in the cooling coils and absorbs heat from inside. The vapour is compressed and condensed outside, giving the heat away. Modern coolants avoid ozone-damaging CFCs.
1. Temperature: hotter liquid, faster evaporation.
2. Surface area: a wide, shallow dish dries faster than a tall, narrow one.
3. Air movement: wind carries vapour away, so more can escape.
4. Humidity: dry air takes up vapour quickly; humid air slows evaporation.
5. Nature of the liquid: alcohol evaporates faster than water.
6. Pressure: lower air pressure makes it easier for molecules to escape.
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Evaporation is a liquid changing into a gas at its surface without boiling. It happens at all temperatures, faster when hotter. Some surface particles have enough energy to escape into the air as vapour.
The molecules that leave are the higher-energy ones, so the liquid left behind is cooler. Sweat cools us this way.
Refrigerators use this cooling. A refrigerant evaporates in the cooling coils and absorbs heat from inside. The vapour is compressed and condensed outside, giving the heat away. Modern coolants avoid ozone-damaging CFCs.
1. Temperature: hotter liquid, faster evaporation.
2. Surface area: a wide, shallow dish dries faster than a tall, narrow one.
3. Air movement: wind carries vapour away, so more can escape.
4. Humidity: dry air takes up vapour quickly; humid air slows evaporation.
5. Nature of the liquid: alcohol evaporates faster than water.
6. Pressure: lower air pressure makes it easier for molecules to escape.
Anything that helps surface molecules escape, or clears the vapour away, speeds up evaporation, and with it the cooling.
Q1. Equal water is put in a wide, shallow dish and a tall, narrow container at the same temperature. Which dries first? (Comprehensive 10.5)
The wide, shallow dish: more surface, more molecules escaping each second.
Q2. Why does a liquid evaporate faster when the air is dry? (Constructed Response 10.3)
Dry air takes up the vapour readily, so more molecules escape than return.
✗ Water must be heated to 100 °C to evaporate.
✓ Evaporation happens at all temperatures, even at room temperature.
✗ Clothes dry faster on a humid day.
✓ High humidity slows evaporation; dry air speeds it up.
✗ A fridge needs CFCs to cool.
✓ Any refrigerant that evaporates at low temperature works; modern ones are CFC-free.
1. What does a liquid turn into once it is evaporated?
(d) MCQ 10.3 (p. 17): the vapour is a gas, so (d). For a liquid other than water, "gas" alone also fits; ask your teacher.
2. Evaporation causes cooling because:
(b) p. 11, Fig. 10.10: cooler, lower-energy molecules are left behind.
3. Which makes wet clothes dry more slowly?
(d) p. 11: when the air is full of moisture, evaporation slows down.
4. In a refrigerator, heat is absorbed from inside when the refrigerant:
(a) p. 12: as it evaporates, it absorbs a large amount of heat.
5. Alcohol evaporates faster than water because its molecules:
(c) p. 11, factor 5.
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