Thermal Expansion

Matter normally expands when heated and contracts when cooled. In a solid, the atoms vibrate about fixed mean positions; heating makes them vibrate more vigorously, pushing neighbouring atoms slightly farther apart on average, which shows up as an overall increase in the solid's dimensions. In a liquid or a gas, where the particles are free to move about, heating similarly increases their speed and the average separation between them. Because liquids are less tightly bound than solids, and gases far less so than liquids, the amount of expansion for a given temperature rise generally increases in the order solid < liquid < gas. …