Cooling by Evaporation

• Physics
• Secondary Education
• Age 12,13

15 March 2023

3.6 Cooling by Evaporation

Content objectives

• Explain the difference between boiling and evaporation
• Explain how evaporation can lower the average energy (heat) of the particles in a liquid
• Explain how evaporation can cause cooling effects

Which of these describes evaporation? 

1. when a solid changes to liquid
2. when a liquid changes to solid
3. when a liquid changes to gas
4. when a gas changes to liquid

Which particles cause evaporation?

The particles with the most energy.

The particles with the least energy.

Study these ideas.

The boiling point of water is 100oC.

Who is correct, Zara or Sofia?

Who is correct, Zara or Sofia? Explain.

Sweat is produced in the skin, which explains how sweat can cool (lower the temperature of) skin.

Sweat evaporates, removing thermal energy from skin.

Sweat evaporates, adding thermal energy to the skin.

Sweat is produced in a lower temperature than the skin.

Sweat is produced at a higher temperature than the skin.

Group discussion

Q1. Explain what happens to the average kinetic energy of the particles in the liquid.

Q2. Explain what happens to the temperature of the liquid.

A liquid is evaporating. The liquid is NOT being heated.

Group discussion

Arun has sweat on his face. The air temperature is 25oC. Arun uses a fan to blow air onto his face. The fan does not change the temperature of the air. 

Q3. Explain why the fan helps Arun to cool (lose heat) faster. 

Plenary (Sharing)

What's next?

End of Unit 3 Assessment