The average distance moved by a molecule between collisions is called

Step 1: Analyze the question The question asks for the name of the average distance a molecule travels between one collision and the next. This comes from the kinetic theory of gases.

Step 2: Explain the concept In a gas, molecules move randomly at high speeds and keep colliding with each other and with the walls of their container. Between collisions, a molecule travels in a straight line. The average length of these straight-line paths is called the mean free path.

• "Mean" means average.
• "Free" means the molecule moves freely, with no collision.
• "Path" means the distance travelled.

The mean free path depends on:

• Pressure/density: at higher pressure or density, molecules are closer together, so the mean free path is shorter.
• Molecular size: larger molecules collide more often, so the mean free path is shorter.
• Temperature: at constant pressure, a higher temperature makes the gas less dense, so the mean free path is longer.

Step 3: Check each option

• A. Molecular distance: not a standard physics term.
• B. Intermolecular distance: this is the separation between neighbouring molecules, not the distance travelled between collisions.
• C. Mean distance: a vague term that is not used in kinetic theory.
• D. Mean free path: this is the correct term, as defined above.

Step 4: Answer The correct option is D. mean free path.

Exam tip: Break the phrase into its words. "Average" gives mean, "between collisions" gives free, and "distance moved" gives path. Together they make mean free path.