Fig. 1 shows a glass jar used to store dry biscuits. A student screws on the lid while the jar is warm, then leaves it in a cold room. A flexible centre pan...
Question 1 Report
Fig. 1 shows a glass jar used to store dry biscuits. A student screws on the lid while the jar is warm, then leaves it in a cold room. A flexible centre panel in the lid moves inward after several hours. The jar contains trapped air but no water enters it.
(a) State whether the pressure of the trapped air decreases or increases as the jar cools. [1]
(b) Explain why the lid panel moves inward. [3]
(c) State which pressure pushes mainly on the outside of the lid. [1]
(d) Describe the change in the average kinetic energy of the air molecules. [2]
(e) When the lid is opened, state what happens to the pressures inside and outside the jar. [2]
(f) Explain why a small amount of water vapour in the jar could make the inward movement larger when it cools. [2]
Answer Details
(a) The pressure of the trapped air decreases as the jar cools. [1]
(b) On cooling, molecules move more slowly. They collide with the lid less often and with less force, reducing the pressure inside. Atmospheric pressure outside is then greater than the internal pressure, so it pushes the flexible panel inward. [3]
(c) The external pressure is atmospheric pressure. [1]
(d) The average kinetic energy of the air molecules decreases because their temperature decreases. [2]
(e) When opened, air moves into the jar until the internal and external pressures become equal. [2]
(f) Water vapour can condense to liquid water as the jar cools. This removes molecules from the gas, lowering the internal pressure further and causing a larger inward movement. [2]
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