Condensation and evaporation

Question 1

What is condensation?

Answer 1

When gas turns to liquid.

Question 3

What happens when a gas cools?

Answer 3

The particles in the gas slow down and lose kinetic energy, then the attractive forces between the particles pull them closer together.

Question 4

How does a gas become a liquid?

Answer 4

If the temperature gets cold enough and the gas particles get close enough together that condensation takes place.

Question 5

When does water vapour in the air condense?

Answer 5

When it comes into contact with cold surfaces (e.g. Drinks glasses).

Question 6

What is evaporation?

Answer 6

When liquid turns to gas.

Question 7

What happens during evaporation?

Answer 7

Particles escape from a liquid.

Question 8

At what temperature can particles evaporate from a liquid?

Answer 8

At temperatures that are much lower than the liquids boiling point.

Question 9

When can liquid particles near the surface escape and become a gas particle?

Answer 9

• If the particles are travelling in the right direction to escape the liquid.
• If the particles are travelling fast enough (they have enough kinetic energy) to overcome the attractive forces of the particles in the liquid.

Question 10

What particles are the most likely to evaporate from the liquid?

Answer 10

The fastest particles (with the most kinetic energy).

Question 11

Why can the cooling effect be really useful?

Answer 11

E.g. When you’re hot or exercise you sweat, as the water from the sweat on your skin evaporates this cools you down.

Question 12

What happens when the fastest particles evaporate from the liquid?

Answer 12

The average speed and kinetic energy of the remaining particles decreases.

Question 13

What does the decrease in average particle energy mean?

Answer 13

That the temperature of the remaining liquid falls, and the liquid cools.

Question 14

When will the rate of evaporation be faster?

Answer 14

• When the temperature is higher.
• When the density is lower.
• When the surface area is larger.
• When the airflow over the liquid is greater.

Question 15

Why will the rate of evaporation be faster when the temperature is higher?

Answer 15

The average particle energy will be higher, so more particles will have enough energy to escape.

Question 16

Why will the rate of evaporation be faster when the density is lower?

Answer 16

The forces between the particles will usually be weaker, so more particles will have enough energy to overcome these forces and escape the liquid.

Question 17

When will the rate of condensation be faster?

Answer 17

• When the temperature of the gas is lower.
• When the temperature of the surface the gas touches is lower.
• When the density is higher.
• When the airflow is less.

Question 18

Why will the rate of condensation be faster when the temperature of the gas is lower?

Answer 18

The average particle energy in the gas is lower, so more particles will slow down enough to clump together to form liquid droplets.

Question 19

Why will the rate of condensation be faster when the density is higher?

Answer 19

The forces between the particles will be stronger, so fewer particles will have enough energy to overcome these forces and will instead clump together and form a liquid.

Question 20

Why will the rate of condensation be faster when the airflow is less?

Answer 20

The concentration of the substance in the air will be higher, so the rate of condensation will be greater.

Question 21

Why will the rate of evaporation be faster if the airflow over the liquid is greater?

Answer 21

The air above the liquid is replaced more quickly, so the concentration of the evaporating substance in the air it’s evaporating into is lower.

Question 22

What is the steam rising from a boiling kettle actually?

Answer 22

Invisible water vapour condensing to form tiny water droplets as it spreads into cooler air.