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Thermal expansion


Matter is made up of atoms or molecules. These particles experience the effects of temperature in the form of agitation.

When temperature increases, most materials undergo an increase in their voume. This expansion, at the microscopic level, corresponds to a greater distance between the molecules. It is this increase in volume that we call expansion. Inversely, a decrease in temperature results in contraction. This property can be observed for all of the states of matter but it often isn’t visible.

Click on the button labelled "Molecules" to zoom in.

  • To illustrate a case of thermal expansion for each of the states of matter.
  • To approach the kinetic theory of heat (molecular thermal agitation).

Guide - Download

The coefficients of expansion are small, regardless of the materials chosen.  This subject is therefore difficult to use in classroom experimentation because the phenomena will be difficult to observe.

It is preferable that the idea of states of matter (solid, liquid, gas) is dealt with in advance.

By way of introduction, the animation is projected without the molecular model. Ask the class to describe what was just shown in their own words.

How might one explain this phenomenon?

The teacher  projects  the animation again, this time showing the microscopic level. The explanations lie at the molecular level.

This phenomenon exists all around us, but at low amplitudes, and it often proceeds very slowly.

To illustrate this phenomenon, here are some other examples that couild be used, in the form of questions:

  • Gaps are left between rails on a railroad to account for  the effects of thermal expansion.
  • The electrical wires between pylons sag so that they can adapt to shortening in the winter.
  • In the desert, the great temperature variations between day and night can crack rocks.  This is called thermoclasty.
  • An over-inflated car tire that is heated can burst.


Mention that when a material expands, its volume changes, but not its mass.