Which one of the following is the wrong assumption of kinetic theory of gases?

Let us recall the basic assumptions of the kinetic theory of gases:

1. A gas consists of a large number of tiny particles (atoms or molecules) that are in constant, random motion.
2. The volume occupied by the gas molecules themselves is negligible compared to the total volume of the gas. This means the molecules are point masses and are separated by large distances relative to their sizes.
3. The collisions between gas molecules and between molecules and the container walls are perfectly elastic. This implies that both momentum and kinetic energy are conserved during collisions.
4. There are no intermolecular forces (attractive or repulsive) between the molecules except during collisions.
5. The average kinetic energy of the gas molecules is directly proportional to the absolute temperature of the gas.

Now, let us examine each option:

Option A: "Momentum and energy always remain conserved."
This is a correct assumption. According to the kinetic theory, collisions are elastic, so both momentum and kinetic energy are conserved. Therefore, this is not a wrong assumption.

Option B: "Pressure is the result of elastic collision of molecules with the container's wall."
This is also correct. The pressure exerted by the gas is due to the continuous bombardment of molecules on the walls of the container. Since the collisions are elastic, the molecules rebound and exert a force, leading to pressure. Hence, this is not a wrong assumption.

Option C: "Molecules are separated by great distances compared to their sizes."
This is a standard assumption. The kinetic theory considers molecules as point masses with negligible volume compared to the container's volume, so the separation between molecules is much larger than their sizes. This is correct.

Option D: "All the molecules move in straight line between collision and with same velocity."
The first part, "all the molecules move in straight line between collision," is correct because, in the absence of forces (as per the assumptions), molecules travel in straight lines until they collide. However, the second part, "with same velocity," is incorrect. The kinetic theory does not assume that all molecules have the same velocity. Instead, molecules have a distribution of velocities (as described by the Maxwell-Boltzmann distribution) due to random motion and collisions. Velocities vary among molecules and change over time. Therefore, this option contains a wrong assumption.

Hence, the correct answer is Option D.