How atoms and molecules are affected by Heat:

Back to Energy

Kinetic theory of matter: All matter is made up of atoms and molecules that are constantly moving.

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.

Solid at Different Temperatures





Mass of the object remains the same, however. 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. Again, the mass is not changed.

Liquid at Different Temperatures



Solids: Sidewalks, bridges, telephone lines, railroad tracks, and other countless objects all expand on a hot summer day. They contract when they lose their heat.

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: 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. Remember that Density = Mass/Volume. As the volume of ice increases, the density of ice decreases and the ice floats.


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.


Common misconceptions: Students have difficulty understanding that molecules are constantly moving in all states of matter. Many students accept the fact that molecules are moving in liquid water because they can see water flow. However, these students have difficulty believing that the molecules in ice are also moving. Nevertheless, molecules are always moving, even in substances such as ice where no motion of the substance is visible.

Back to Energy