According to the kinetic theory of gases, there are several key principles that help us understand the behavior of gases at the molecular level. Let's break down each statement and explain why it is true.

Collisions are always elastic:
In the kinetic theory of gases, it is assumed that gas molecules are point masses that are in constant motion. When these molecules collide with each other or with the walls of the container, they do so without any loss of energy. This means that the total kinetic energy of the system remains constant before and after the collision. Consequently, the collisions are considered to be elastic, where kinetic energy is conserved. This assumption helps explain various gas properties, such as pressure and temperature.

Heavier molecules transfer more momentum to the wall of the container:
The momentum of an object is the product of its mass and velocity. According to the kinetic theory, gas molecules are in constant random motion, and their average kinetic energy is directly proportional to their temperature. Heavier molecules have greater mass, and therefore, they possess higher momentum. When these molecules collide with the walls of the container, they transfer momentum to the wall, exerting a force that contributes to the overall pressure of the gas. Hence, heavier molecules transfer more momentum to the wall of the container.

Only a small number of molecules have very high velocity:
In a gas sample, molecules possess a range of velocities. According to the Maxwell-Boltzmann distribution, the majority of molecules have velocities close to the average velocity, while only a small fraction of molecules have very high or very low velocities. This distribution follows a bell-shaped curve, with the peak corresponding to the most probable velocity. Therefore, it is true that only a small number of molecules have very high velocities.

Between collisions, the molecules move in straight lines with constant velocities:
According to the kinetic theory, gas molecules move in a straight line with constant velocity in the absence of any external forces. However, the random collisions with other molecules or with the container walls cause their direction and speed to change. These collisions lead to a continuous change in velocity and direction, resulting in the overall random motion of gas molecules. Therefore, it is not accurate to say that the molecules move in straight lines with constant velocities between collisions.

In conclusion, the kinetic theory of gases provides a framework for understanding the behavior of gases based on the motion of their individual molecules. It explains that collisions are elastic, heavier molecules transfer more momentum, only a small number of molecules have very high velocities, and molecules move in random directions with changing velocities due to collisions.