Concept:The kinetic model of an ideal gas assumes that gas molecules are tiny identical particles that move randomly, have negligible volume, and exert no forces on one another except during elastic collisions.
Explanation:Options A, B and C all agree with the standard assumptions of an ideal gas.
In an ideal gas at a fixed temperature, only the average kinetic energy of the molecules remains constant.
When molecules collide, they can exchange energy, so the speed of any individual molecule can change.
Therefore, a molecule does not keep a single fixed speed throughout its motion.
This makes the statement in option D an invalid ideal-gas assumption.
Answer:D. molecules move at constant speed between collisions