What Are Amalgams & How Do They Differ from Solutions?

According to Princeton's WordNet Search, an amalgam in its literal sense is "an alloy of mercury with another metal..." In fact every known metallic element, excepting only platinum and iron, dissolves in mercury. Hence, at one time, mercury was stored in iron bottles. The most commonly known amalgam is the silver amalgam used in dentistry in fillings.¹ But "is that all there is?" - by no means. There are some pretty fascinating properties of amalgams. What is special about these alloys, sometimes redundantly termed "mercury amalgams," how do they behave, and to what uses may they be put?

An Amalgam is More Than...

An amalgam is more than a mere solution. A solution is comprised of one or more substances dissolved in one or more other substances called the solvent. Sometimes the dissolved substance comes apart, but at other times it does not. For example, when salt is dissolved in water to form a solution, the NaCl molecules break apart into Na and Cl ions. When sugar is dissolved in water, however, it is not broken apart into ions. Sugar is said to be non-ionic. Compared to either of these, an amalgam can react differently.

Example of Alkali and Other Reactive Metal Amalgams

Alkali metals are found along the far left-hand side of the periodic table of the elements. Alkali metals have an electron they readily give up to assume a charge of plus one. This means they are very chemically reactive. These metals readily amalgamate with mercury, giving off large amounts of heat in doing so. Compound-like formulas have been identified within the amalgam, demonstrating these substances are not mere solutions. For instance, Acta Crystallographica² describes the formation of the actual compounds, KHg and KHg₂, that form between potassium (K) and mercury (Hg) upon amalgamation.

In addition to alkali metals, alkaline earth metals, such as calcium, strontium, and barium also form compounds in the amalgamation process. Also, ammonia forms a very reactive amalgam.

Amalgam Properties and Uses

When mercury predominates, such as in the extraction of precious metals in mining, the amalgam remains a fluid. In other uses, where the alloyed metal predominates, the amalgam is solid, but readily fusible. It maintains a metallic luster.

Once used in a variety of ways, such as in mirror manufacture, amalgams are largely being phased out of use due to the hazardous nature of liquid mercury and mercury vapor, though dental amalgams are still used. In the laboratory, amalgams are sometimes used as reducing reagents in organic chemistry reactions. Even in that instance, other reagents such as lithium aluminum hydride (LiAlH₄) are being developed as substitutes. Mercury is still used to amalgamate and extract silver and gold in placer mining.

¹ Other metals are used in dental amalgams as well.

² Acta Cryst. (1955). 8, 705-710, "The crystal structures of KHg and KHg₂," by E.J. Duwell and N.C. Baenziger.

Penny cyclopaedia of the Society for the Diffusion of Useful Knowledge (Great Britain), Volumes 15 to 16, p. 102.

State of Arizona, Department of Mines and Mineral Resources - Treating Gold Ores by Amalgamation

Chest of Books - Henley's Twentieth Century Formulas, Recipes And Processes, "General Properties of Amalgams," by Norman W. Henley, Vol. 1, 1916.