We continue to review some of the most important materials in heat treatment and metallurgy.
Mercury (chemical symbol: Hg)
Mercury is a dense, shiny, silvery-colored metal whose most unique characteristic is that it is a liquid at room temperature (Fig. 1). The only other element in the periodic table that shares this attribute is bromine (Br). Mercury’s symbol (Hg) is derived from its Greek name hydrargyrum, which translates to “silver water” or “liquid silver.” Compared to other metals, mercury is an ineffective conductor of heat but a mild conductor of electricity.
Interestingly, mercury dissolves gold, silver and most other metal in a property known as amalgamation, which makes mercury an important tool in the mining of both precious metals. In amalgamation, the ore containing silver or gold is ground up and then mercury is mixed with it. The mercury combines with the silver or gold, forming an amalgam (Fig. 2), which is then separated from the waste ore. Water is sometimes used to promote the process. The amalgam is then heated to 816°C (1500°F) to make the mercury boil off, leaving only the silver or gold behind. The mercury vapor is captured by distillation. The process is controversial because of the extreme toxicity of mercury to humans, but it has been used for centuries.
Mercury has been known since ancient times, and it has been found in Egyptian tombs dated from 1500 BC. The ancient Greeks used cinnabar (mercury sulfide) in skin ointments. The ancient Egyptians and Romans used it as a cosmetic for coloring the cheeks and lips. Indeed, for most of history it has been considered therapeutic. Mercury is named after the planet Mercury, the fastest moving planet in the solar system. Fluorescent lights use mercury vapor contained in a glass tube. They work by running a current through the mercury, ionizing it. This causes electrons in the gas to emit photons at ultraviolet frequencies. The ultraviolet light is converted into visible light when it strikes a phosphor coating on the inside of the tube.
Mercury compounds are used in certain over-the-counter medicines, such as topical antiseptics, stimulant laxatives, diaper-rash ointment, eye drops, nasal sprays and some diuretics. A mercury amalgam of silver is used in dental fillings. In general, medical uses for mercury have been in decline as alternatives are discovered.
Here are a few important facts about mercury.
- Atomic number: 80
- Atomic weight: 200.592
- Melting point: 234.32 K (-38.83°C or -37.89°F)
- Boiling point: 629.88 K (356.73°C or 674.11°F)
- Density: 13.5336 grams per cubic centimeter
- Phase at room temperature: Liquid
- Element classification: Metal
- Period number: 6
- Group number: 12
- Group name: none