In this topic you will learn about how heat transfer causes changes in matter.

The expansion of matter when its temperature is raised is called thermal expansion. As the temperature of a material is raised, its particles move about faster. Each particle moves over a larger region. As a result, the material that is made up of the particles increases in volume.

Gases are made of tiny particles in rapid motion. The molecules hit each other and the wall of their container. All of the countless tiny hits of particles add up to create a push called pressure. Pressure is the force on each unit of area of surface.

When heat is applied to a solid, the particles begin to vibrate faster and faster as their temperature rises. Eventually the particles move fast enough to break free of the forces holding them in place. This process is called melting. Melting is the change of a solid into a liquid.

The particles in a liquid remain close together because of the forces of attraction. If heat is supplied to a liquid, the particles move faster. At some point they will move enough to escape the liquid and form a gas. The change of a liquid to a gas as molecules break free from each other is called vaporization.

If energy is removed from a gas, the particles slow down. The change of a gas into a liquid as molecules attract each other is called condensation. If energy is removed from a liquid the particles will eventually be locked back into a fixed position. The change of a liquid to a solid is called freezing.

The temperature of a substance does not change while a change of state occurs. The energy flowing into a substance breaks the attraction between the particles. It does not also speed them up.

The temperature of a vaporizing liquid stays unchanged while a liquid is boiling, even though the liquid is being heated. In a similar way, the energy it takes to break the attraction in melting or vaporization must be removed when particles bond back together during condensation or freezing.

The formation of bubbles of vapor that escape from a liquid that is being heated is called boiling. Liquids may also turn to gas at lower temperatures when particles vaporize at the surface. The vaporization of molecules from the surface of a liquid is called evaporation.