Evaporation

This lesson covers:

  1. The difference between evaporation and boiling
  2. The effect of temperature, surface area, and air flow on the rate of evaporation
  3. The cooling effect of evaporation

Evaporation vs Boiling


People often think 'boiling' and 'evaporating' are the same thing. However, in science they refer to different processes.

Evaporation


Illustration showing the evaporation process with molecules escaping from the surface of a liquid.
  1. Evaporation occurs when liquid molecules escape from the surface of a liquid and become gas.
  2. Evaporation occurs at any temperature.

Boiling

Illustration showing the difference between evaporation and boiling with molecules escaping from the liquid surface and bubbles forming within the liquid.
  1. Boiling is when bubbles of gas form within the liquid.
  2. Boiling only happens once the boiling point has been reached. 

Factors affecting evaporation rate


The rate of evaporation is affected by several factors including temperature, surface area, and air flow.

Temperature

  1. As temperature increases, the kinetic energy of the particles in the liquid increases.
  2. This means more particles have enough energy to escape the surface of the liquid.
  3. This results in a higher rate of evaporation.

Surface area

  1. With a larger surface area, more particles are close to the surface.
  2. Therefore more particles are able to escape the surface in a given time. 
  3. This results in a higher rate of evaporation.

Air flow

  1. Moving air carries away particles that have escaped the surface of the liquid. 
  2. This prevents the escaped particles dropping back into the liquid.
  3. This increases the rate of evaporation.

Evaporation lowers the temperature of a liquid


  1. Molecules that escape the surface of a liquid are usually those with the most kinetic energy.
  2. This leaves the remainder of the liquid with a lower kinetic energy on average. 
  3. As the average kinetic energy decreases, the temperature of the liquid decreases.

Evaporation occurs when liquid molecules escape from the surface of a liquid and become gas. At what temperature does evaporation occur?

At any temperature

Above the boiling point

At the boiling point

Below the boiling point

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Which of the following are true about boiling?

Boiling only happens once the boiling point has been reached.

Boiling is when bubbles of gas form within the liquid.

Boiling occurs when liquid molecules escape from the surface of a liquid and become gas.

Boiling occurs at any temperature.

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Which of the following factors affect the rate of evaporation?

Surface area

Air flow

All of the above

Temperature

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How does temperature affect the rate of evaporation?

As temperature increases, the rate of evaporation increases.

As temperature increases, the rate of evaporation stays the same.

Temperature has no effect on the rate of evaporation.

As temperature increases, the rate of evaporation decreases.

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How does surface area affect the rate of evaporation?

Surface area has no effect on the rate of evaporation.

A larger surface area decreases the rate of evaporation.

A larger surface area has no effect on the rate of evaporation.

A larger surface area increases the rate of evaporation.

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How does air flow affect the rate of evaporation?

Moving air causes boiling instead of evaporation.

Air flow has no effect on the rate of evaporation.

Moving air increases the rate of evaporation.

Moving air decreases the rate of evaporation.

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