For the same substance, at the same temperature, the average speed of molecules in a vapor is the same as the average speed of molecules in a liquid.

• A. True
• B. Cannot be determined
• C. Depends on pressure
• D. There is no difference

Answer: (B)

The idea here is that average speeds depend on how freely molecules can move in the phase they're in. In a gas, molecules are far apart and move largely without being held by neighbors, so their speeds are set by temperature according to a Maxwell-Boltzmann distribution. In a liquid, molecules are densely packed and strongly interact with each other, so their motion is much more constrained and dominated by local vibrations and brief jumps rather than free translational motion. Because the way we define and measure “average speed” is not the same across these two very different states—and because the statement doesn’t specify which phase is being considered or how speed is defined—there isn’t enough information to say whether the average speeds are the same. Therefore, the correct choice is that it cannot be determined from the given information.