Kinetic theory of matter: All matter is made up of atoms and molecules that are constantly moving. Temperature is a measure of the average kinetic energy (energy of motion) of all the molecules or atoms in a material. When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases. The motion and spacing of the particles determines the state of matter of the substance. The end result of increased molecular motion is that the object expands and takes up more space.

In the gaseous state, molecules have so much kinetic energy that they fly off in all directions but repeatedly collide and bounce off of other molecules. --- boiling temperature - condensation temperature --- In the liquid state, atoms or molecules have sufficient kinetic energy to overcome the chemical bonds that held them in their crystal lattice and move independently, yet they don't have enough energy to separate completely from other atoms. --- melting temperature - crystallization temperature --- In the solid state, chemical bonds are stronger than the kinetic energy of the atoms. The atoms are locked into their crystal lattice positions

Solids, liquids and gases all expand when heat is added. When heat leaves all substances, the molecules vibrate slower. The atoms can get closer which results in the matter contracting

In the picture to the left, some workers are trying to reconnect two rails that have separated due to the extreme cold. To fix the problem, the workers have lit an oil-soaked rope that lies next to the track. The heat of the fire will cause the tracks to expand so that they can be reconnected once again.

Liquids expand when heated and they contract when cooled. Think about mercury in a thermometer! Ice however, expands when it freezes. As the water molecules lose heat, they vibrate less, and are able to form geometric patterns that take up more space. Gases: Gases expand when heated and contract when cooled. For instance, a balloon will expand if it is left in a hot car. It will get smaller if it is put in the refrigerator. Liquids: