Gas expansion at constant pressure

Steps:

• Heating air increases molecular kinetic energy, raising pressure if volume fixed.
• Movable piston allows expansion to maintain constant pressure.
• Volume increases proportionally with temperature per ideal gas law.
• Larger volume with fixed molecule count spreads them farther, reducing density (mass/volume).

Why A is correct:

• Ideal gas law (PV = nRT) at constant pressure means volume rises with temperature, increasing inter-molecular distance and decreasing density.

Why the others are wrong:

• B: Density decreases due to volume expansion, doesn't stay constant.
• C: Average distance increases as molecules spread in larger volume.
• D: Both distance and density change with expansion.

Final answer: A