evaporation

from liquid to gas without boiling

Evaporation explains why clothes dry on a clothesline. Evaporation is the process in which a liquid changes to a gas without becoming hot enough to boil. It occurs when individual liquid particles at the exposed surface of the liquid absorb just enough energy to overcome the force of attraction with other liquid particles. If the surface particles are moving in the right direction, they will pull away from the liquid and move into the air. This is illustrated in the Figure 1.1.

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factors that affect the rate of evaporation

Many factors influence how quickly a liquid evaporates. They include: temperature of the liquid. A cup of hot water will evaporate more quickly than a cup of cold water. exposed surface area of the liquid. The same amount of water will evaporate more quickly in a wide shallow bowl than in a tall narrow glass. presence or absence of other substances in the liquid. Pure water will evaporate more quickly than salt water. air movement. Clothes on a clothesline will dry more quickly on a windy day than on a still day. concentration of the evaporating substance in the air. Clothes will dry more quickly when air contains little water vapor.

evaporative cooling

Did you ever notice that moving air cools you down when youre hot and sweaty? For example, if you sit in front of a fan, you feel cooler. Thats because moving air helps to evaporate the sweat on your skin. But why does the evaporation of sweat cool you down? When a liquid such as sweat evaporates, energetic particles on the surface of the liquid escape into the air. After these particles leave, the remaining liquid has less energy, so it is cooler. This is called evaporative cooling. Q: On a hot day, high humidity makes you feel even hotter. Can you explain why? A: Humidity is a measure of the amount of water vapor in the air. When humidity is high, sweat evaporates more slowly because there is already a lot of water vapor in the air. The slower evaporation rate reduces the potential for evaporative cooling.

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instructional diagrams

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questions

which process causes a mud puddle to dry up in the sunshine?

a) boiling

b) vaporization

-->  c) evaporation

d) condensation

factors that affect the rate of evaporation include the

a) exposed surface area of the liquid.

b) temperature of the liquid.

c) air movement above the liquid.

-->  d) all of the above

clothes on a clothesline will dry more quickly if the air is dry.

-->  a. true

b. false

salt water evaporates more quickly than pure water.

a. true

-->  b. false

diagram questions

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