Most materials expand when their temperature goes up, most of the time. The best known exception is liquid water in the temperature interval 0^o C --4^o C (just above the melting temperature), where it is contracting with increasing temperature. The change in size caused by thermal expansion is never very large: near room temperature, gases increase in volume by less than 0.5% per degree Celsius, and the rate of thermal expansion of liquids and solids is 100 times smaller. For example, a steel post that is exactly 1 meter long at 0^o C is 1.2 millimeters longer at 100^o C. This seems negligible until we discover that it would take a 15 ton force to keep the post from expanding! (this assumes the post is a rod 1 inch in diameter). So it is very important to design bridges and buildings and car engines so that thermal expansion can take place.

When the temperature change is uniform throughout the object, the change in dimensions is also uniform: all distances get larger by the same fractional amount. So a washer gets larger and the hole in the washer also gets larger.

Many common kinds of thermometers make use of thermal expansion, as we have learned in the activities of this section.
At a change in phase, (where liquid turns into gas, or solid turns into liquid), we can get much larger changes in volume. This is different from thermal expansion as described here:

• It involves a complete rearrangement of the structure of the material.
• The change in density is not uniform and gradual throughout the material. As a block of ice melts, some parts are solid while others are liquid.
• A change of phase in a pure substance (like water) takes place over a very small temperature range, whereas thermal expansion takes place in almost every temperature range. We couldn't make a thermometer based on the change of density of water as it freezes (but we could use the freezing temperature to calibrate a thermometer).

Atoms