Constant pressure due to movable piston

Steps:

• Air is trapped by the piston, so mass remains constant as no air escapes.
• Piston is free to move and exposed to atmosphere, maintaining constant external pressure.
• Initially in hot water, air is heated; upon cooling, temperature decreases.
• Piston contracts volume to compensate, keeping internal pressure equal to atmospheric pressure.

Why A is correct:

• Ideal gas law (PV = nRT) with constant n (mass) and P (atmospheric) requires V to decrease proportionally with T during cooling.

Why the others are wrong:

• B: Pressure stays constant as piston adjusts volume to match external pressure.
• C: Mass cannot change; air is sealed in the cylinder.
• D: Neither mass nor pressure changes under these conditions.

Final answer: A