Why is mercury liquid at room temperature?

All metals turn liquid at some temperature. This one happens to be useful…

By Sarah Jensen

When we call someone “mercurial,” we’re invoking the Roman god Mercury whose swift movements from place to place gives us the adjective meaning “erratic or volatile.” He also lent his name to the metal mercury, a quick-silvery liquid we’re most familiar with through its role in thermometers.

Though we know most metals in their solid state, all of them melt too if they get hot enough, says Craig Carter, a professor of materials science and engineering. Some metals melt at lower temps than others. Tin becomes liquid at 231.8˚C (447.8˚F); at the other end of the spectrum, tungsten has the highest melting point of any metal: 3,422˚C (6,192˚F). Somewhere in the middle is mercury, which stays in a liquid state until its temperature drops to -40°.

“A metal is full of bonds between the atoms that make it up,” explains Carter. “What determines a material’s melting point has everything to do with the energy associated with the bonds. The formation of the bond transforms some of the kinetic energy into bond energy. The energy of a bond in mercury is very low, so it tends to disorganize at lower temperatures.

“Think of the kinetic energy in a thrown baseball,” he suggests. “The work the pitcher does to toss the ball is imparted into velocity, part of the formula used to measure kinetic energy. Now imagine many microscopic balls. They don’t move together with a single velocity, but in random directions, each with its own kinetic energy — and the higher the temperature, the faster and more randomly they move.”

The atoms that make up a metal behave in the same way: as temperatures rise, they’re activated by kinetic energy. That was the principle behind the old mercury thermometer that hung beside the back door: On a hot summer’s day, atoms in the mercury moved faster and faster, colliding with one another and increasing the distances between them, causing the mercury to rise in the thermometer’s tube. Conversely, when mercury cools, the distances between atoms lessen, they stop moving quite so violently, and the mercury becomes a solid. “Liquids are more disorganized than solids, so at low temperatures, solids tend to be favored,” explains Carter. “At higher temperatures, disorganization tends to be favored, and the system goes to a liquid state.” (Turning up the heat, even more, results in a gaseous state that’s even more disorganized and lower in bonding energy.)

Material scientists, metallurgists, and mechanical engineers pay close attention to the melting points of various metals, as do electrical engineers as they melt silicon into crystals to create semiconductors. “If you want to process materials, you need to know how metals behave at various temperatures,” advises Carter.

In general, adding another substance to a metal causes its melting point to go down and its behavior to change, he continues. Anyone with an amalgam filling in one of their back molars is a walking example of that concept. “Before the advent of resin composite fillings, dentists filled cavities with a substance that had a very low melting point,” explains Carter. While mercury alone would quickly liquefy in a patient’s mouth, mixing it with silver, tin, copper, and other trace metals resulted in a restorative material that would remain solid even during a meal of hot coffee and potatoes straight from the oven.

Thanks to Rashid of Pakistan for this question.

Posted: November 6, 2012