Questions on Pressure
Q11. A pen cap floats in a plastic lemonade bottle three-quarters full of water .
If you squeeze the bottle the pen cap sinks to the bottom. If you then let go of the bottle, the pen cap floats to the surface
(a) When the bottle is squeezed what, if anything, happens to:
(i) the distance between the air molecules inside the bottle?
The distance gets smaller as the air molecules get closer to each other. 
(1 mark)
(ii) the distance between the water molecules inside the bottle?
The distance between the water molecules does not change.
(1 mark)
(iii) the pressure of the air trapped inside the pen cap?
The pressure of the trapped air increases.
The pressure within a liquid at a certain depth is due to the liquid above it and the air pressure pressing on the liquid surface. That air pressure has incresed due to the squeezing of the bottle - so the pressure within the pen cap will increase also.
(1 mark)
(iv) the volume of the air trapped inside the pen cap?
The volume of the air trapped inside the pen cap decreases
(1 mark)
(b) Explain why the pen cap sinks when you squeeze the bottle.
Water flows into the cap as the trapped air shrinks in volume because it is under pressure
The pen cap is now less buoyant because the density of the contents has increased producing less upthrust on it.
(2 marks)
Maximum 6 marks
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