Part 2: The gas particles travel randomly in straight-line motion where their movement can be described by the fundamental laws of mechanics.

   Gas particles are always in motion except for the very specific condition known as Absolute Zero. As they travel, they will travel in straight lines. The only way that they will deviate from that straight-line path is if they collide with other particles or the walls of any container that they might be in.

   This helps to explain how gases diffuse. If a gas is in a confined space or a closed container, then it will expand outward to fit the larger volume or leave the container. The straight-line motion will explain how this can happen. The only way that they will return into the container is if they collide with another object and bounce back inside.

 In theory, a container of gas will diffuse to the point at which the gas will be evenly distributed throughout the Universe. In reality, that will not happen because of a wide range of environmental factors that have impact on the diffusion process.