The collision between ideal gas molecules are considered to be perfectly elastic because

Analyze the question: It asks why collisions between ideal gas molecules are described as perfectly elastic.

Concept: In the kinetic theory of gases, an elastic collision is one in which no kinetic energy is lost. Total kinetic energy before the collision equals total kinetic energy after it. The molecules may exchange energy with one another, but none is converted to heat, sound, or deformation. This is why a gas at constant temperature does not "run down" and the molecules keep moving indefinitely.

Evaluating the options:

• A. They collide without losing energy. This is the definition of a perfectly elastic collision. ✔
• B. They move randomly in a straight line. This is a separate postulate (random, straight-line motion between collisions). It describes how they move, not why collisions are elastic.
• C. Their average kinetic energy is variable. This is wrong. The average kinetic energy is proportional to the absolute temperature, so it is constant at a fixed temperature. Variable energy would not explain elasticity.
• D. The distance between them is large compared to their size. This is also a postulate. It explains why intermolecular forces and molecular volume are negligible, but not why collisions are elastic.

Answer: Option A.

Exam tip: Whenever you see "perfectly elastic", think "no loss of kinetic energy". Link each kinetic theory postulate to its purpose:

• Elastic collisions → no energy loss
• Large distances → negligible volume and forces
• Random straight-line motion → gas fills its container
• Average KE ∝ T → energy depends on temperature