Liquid mercury grows and shrinks with changing temperature.
Liquid mercury grows and shrinks with changing temperature.

Mercury is a metallic element that is liquid at room temperature and normal pressure. As with most liquid substances, mercury reacts to temperature increases by expanding by a predictable amount; this and other properties made mercury useful in thermometers until they were phased out in favor of less toxic alternatives.

Thermal Expansion and Contraction

The volume of most materials increases as the temperature increases; scientists call this effect thermal expansion. The expansion occurs because the atoms in a material vibrate more strongly as you add heat energy to them; as the vibrations increase, the average distance between atoms also increases. For the same reason, materials contract as temperatures decrease. Each substance expands and contracts according to a characteristic quantity known as the coefficient of thermal expansion; this is a number that indicates the percentage of size change per degree Celsius. The expansion can be for an object’s volume, area or length; tables for each type of expansion list the substances and coefficients.

Mercury Expansion

Mercury’s volume coefficient of expansion is 0.00018, so it expands by .018 percent in volume for every degree of temperature increase. To make the difference easier to see, a mercury thermometer contains a reservoir of the metal and a thin glass capillary into which it can expand. The mercury in the reservoir grows in volume as temperatures rise, but the walls of the reservoir constrain the mercury; it has nowhere to go but into the thin column, which converts the slight volume change into a more noticeable linear change.

Freezing Point

In addition to its expanding and contracting with temperature, liquid mercury freezes solid at -38.83 °C (-37.89 °F). Because of this, mercury thermometers have limited utility in very cold places where temperatures dip to below mercury’s freezing point. Alcohol thermometers, which freeze at much lower temperatures, are used instead.

Boiling Point

Liquid mercury responds to very high temperatures by boiling into a vapor; this occurs at 356.73 °C (674.11 °F). As with most liquids, reduced pressures lower mercury’s boiling point. To circumvent the limitations of mercury, high-temperature thermometers use thermocouples, infrared light sensors and other ways of measuring hot objects.