A thermometer is a device used to measure temperature. Thermometers are thin hollow tubes of glass partially filled with a liquid.
The application of heat to a liquid (or any other type of matter) causes that liquid to expand. When a thermometer is heated, the liquid expands and takes up more volume inside the tube. Thus the height of the liquid in the tube rises. The temperature is read by comparing the height of the liquid to calibrated markings on the glass.
A desirable property of a liquid used in a thermometer is a high coefficient of thermal expansion. This means even a small change in temperature results in a significant change in volume.
Mercury was the original liquid of choice for thermometers. However, mercury is very toxic. Colored ethanol solutions are now preferred, both because they are not dangerous when accidentally broken and much easier to dispose of without harming the environment. Thus, non-toxicity is another desirable property of a thermometer liquid.
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