Isothermal Expansion of an Ideal Gas

Steps:

• Recognize the process as isothermal since temperature is constant during slow expansion.
• Note that expansion increases volume, so gas particles move further apart on average.
• Recall that average kinetic energy of particles is proportional to temperature (KE = 3/2 kT).
• Conclude that constant temperature keeps average speed unchanged, as speed relates to sqrt(KE).

Why B is correct:

• Boyle's law and kinetic theory show that in isothermal expansion, pressure decreases as volume increases, but particle speed stays constant due to unchanged kinetic energy.

Why the others are wrong:

• A: Average speed does not decrease; kinetic energy remains constant.
• C: Average speed does not increase; temperature is unchanged.
• D: Particles do not stay the same distance apart; expansion increases volume and separations.

Final answer: B