. As the temperature of a solid increases, the vibrations of the individual molecules become larger. When these vibrations become larger, the average distance between the molecules increases to accommodate these larger oscillations, and the solid expands. In a liquid or a gas, the individual molecules move faster as the temperature increases, and the collisions between individual molecules become more violent. Since the molecules are moving faster, they move farther apart as they travel a larger distance in the time between collisions. . A This explains why most materials become less dense as their temperature is increased. . B This explains why most materials become more dense as their temperature is increased