### Correct Option: A. they collide without losing energy #### Detailed Explanation: 1. **Understanding Elastic Collisions**: - In physics and chemistry, an elastic collision is defined as a collision in which both momentum and kinetic energy are conserved. This means that when two ideal gas molecules collide, they do not lose any kinetic energy in the process. Instead, they simply exchange energy and momentum. 2. **Ideal Gas Behavior**: - Ideal gases are theoretical gases that perfectly follow the gas laws. One of the key assumptions of the kinetic molecular theory of gases is that the collisions between gas molecules are perfectly elastic. This assumption simplifies the calculations and helps in understanding the behavior of gases under various conditions. 3. **Why Option A is Correct**: - Option A states that gas molecules collide without losing energy. This is the fundamental characteristic of elastic collisions. During a collision, the total kinetic energy before the collision is equal to the total kinetic energy after the collision. Therefore, the energy is conserved, which is why we say the collisions are perfectly elastic. 4. **Examining Other Options**: - **Option B: they move randomly in a straight line**: - While it is true that gas molecules move randomly and can travel in straight lines between collisions, this does not explain why their collisions are elastic. The randomness of their motion does not relate to the conservation of energy during collisions. - **Option C: their average kinetic energy is variable**: - This statement is misleading. While the average kinetic energy of gas molecules can change with temperature (as temperature increases, so does the average kinetic energy), this does not pertain to the nature of the collisions themselves. The collisions can still be elastic regardless of the average kinetic energy being variable. - **Option D: the distance between them is large compared to their size**: - This statement refers to the ideal gas assumption that gas molecules are point particles with negligible volume compared to the distances between them. While this is a valid assumption for ideal gases, it does not explain the elastic nature of their collisions. The distance between molecules does not affect whether energy is lost during a collision. #### Summary of Key Points: - Elastic collisions conserve both momentum and kinetic energy. - Ideal gas molecules are assumed to collide elastically, meaning they do not lose energy during collisions. - The randomness of molecular motion (Option B) and the variability of average kinetic energy (Option C) do not explain the elastic nature of collisions. - The relative size of gas molecules compared to the distance between them (Option D) is an assumption of ideal gas behavior but does not relate to energy conservation during collisions. ### Revision Summary: - Elastic collisions conserve kinetic energy and momentum. - Ideal gas molecules collide elastically, meaning no energy is lost. - Random motion and variable kinetic energy do not define elastic collisions. - The size of gas molecules relative to their distance is an assumption, not a reason for elastic collisions.