Continue from chp 9

Evaporation

Difference between boiling and evaporation

Evaporation
- Occurs at any temperature
- Slow process
- Takes place only at liquid surface
- No bubbles formed in liquid
- Temperature may change
- Thermal energy supplied by surroundings

boiling
- Occurs at fixed temperature.
- Quick process
- Takes place throughout liquid
- Bubbles formed in liquid
- Temperature remains constant
- Thermal energy supplied by energy source

Evaporation causes cooling

Evaporation requires thermal energy from the surrounding. Thermal energy from our body helps the water on the skin to evaporate. When it evaporates, it takes away the thermal energy from the surface of the skin, thus causing the body to feel cold.

How does evaporation occur?

Molecules of liquid are always moving randomly at different speeds. At the liquid surface, the more energetic molecules are able to overcome the downward attractive forces of other molecules and escape into the atmosphere. The liquid left behind are the less energetic molecules. The liquid with slower moving molecules is cooler.

Applications of evaporation

1) Experiencing cooling effect on skin when perspire evaporates, or when you apply perfume.
2) Wet clothes or puddles of water dry up after awhile.
3) A person with fever, sponged with water. When water evaporates, the temperature of the person will go down.
4) Refrigerator uses principle of condensation and evaporation, to keep the contents cold.

Factors affecting the rate of evaporation

1) Temperature
2) Humidity of the surrounding air
3) Surface area of the liquid
4) Movement of air
5) Pressure
6) Boiling point of the liquid