Chapter 20 - Solids, Liquids and Gases

Question 1

To avoid overheating, a runner sweats. As the sweat evaporates, it cools the body Use ideas about particles to explain why evaporation leads to cooling. [3 marks]

Answer 1

• evaporation as liquid into gas/vapour; - higher (kinetic) energy/faster particles/molecules leave/ evaporate; - reducing (average) energy of particles left /heat remaining; - reducing temperature;

Question 2

Suggest why a runner might cool down too quickly if he does not wear a foil sheet [2 marks]

Answer 2

• runner still covered in sweat /evaporation - not generating as much ‘new’ heat

Question 3

Explain how the foil sheet reduces heat loss [2 marks]

Answer 3

• barrier to reduce particle movement - reducing convection / evaporation

Question 4

A student blows up two balloons to the same size. She puts one balloon into a freezer. After a while, the student compares the two balloons. The balloon that has been cooled is smaller. Use ideas about particles to explain why the cooled balloon is smaller [4 marks]

Answer 4

• At lower temp; particles move at lower kinetic energy on average - So hit sides less often, with less energy; - Reducing force / pressure; - Tension in rubber pulls balloon material into smaller size

Question 5

Answer 5

Fault 1 - ‘different time in freezer’ does not give range of temps. Improvement 1 - Use water baths, use freezers set to different temps

Fault 2 - Hard to ‘measure temp of balloon with thermometer’. Improvement 2 - Measure temperature of surroundings

Fault 3 - Plotting ‘size’ is imprecise. Improvement 3 - plot a more precise quantity; e.g. volume / length