Charles's Law: Volume decreases with temperature at constant pressure

Steps:

• Convert temperatures to Kelvin: 30°C is 303 K, 7°C is 280 K, showing a decrease.
• Helium is an ideal gas; lower temperature reduces average kinetic energy of particles.
• Slower-moving particles collide less forcefully with balloon walls, lowering internal pressure.
• Balloon contracts to reduce volume until internal pressure matches external pressure.

Why D is correct:

• Kinetic molecular theory states gas pressure arises from particle collisions; slower particles at lower temperature reduce collision frequency and force, decreasing pressure and thus volume (PV = nRT, constant P implies V ∝ T).

Why the others are wrong:

• A: Helium's boiling point is -269°C, so it remains gaseous at 7°C and does not condense.
• B: Diffusion is minimal for helium in latex balloons over short times; volume change is due to temperature, not escape.
• C: Diffusion rate decreases with temperature, but this does not explain volume reduction; pressure change is key.

Final answer: D