There are two ways that I can think of in which gases behave. The molecules can move around faster as the temperature increases. Also, they will diffuse from an area of higher concentration to an area of lower concentration until there is roughly equal parts on both sides.
A model would simply be a picture or diagram showing what I have explained. For example, showing air molecules as various dots in different boxes with different temperatures, having the air molecules increase in speed with greater temperature. Or with the diffusion example mentioned, a picture of a gas starting off in a room and slowly spreading out into the other room. Like showing in a series of pictures the gas migrating until it is equal parts.