The temperature increases when matter gains heat. It becomes bigger or expands. When it loses heat, the temperature decreases. It becomes smaller or contracts. Solid, liquid and gas expand when hot and contract when cooled. Water is an exception. When water solidifies to form ice, it expands and the size increase. Since the mass remains the same, the density of ice decreases. This explains why ice floats on water. When a matter is heated, the particles inside it gain more energy. The particles move faster and collide with each other. The particles become further apart and take more space. As a result, the matter expands. On the other hand, when the matter is cooled, the particles lose energy. Their movements slow down and less collision happen. The particles get closer together and take less space. The matter contracts as a result.
Expand and Contract8 04 2012