What Is a Universal Solvent in Chemistry?

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Technically, a solvent is a component of a solution present in the greater amount. In contrast, solutes are present in a smaller amount. In the common usage, a solvent is a liquid that dissolves chemicals, such as solids, gases, and other liquids.

Key Takeaways: Universal Solvent

  • A universal solvent theoretically dissolves any other chemical.
  • A true universal solvent does not exist.
  • Water is often called the universal solvent because it dissolves more chemicals than any other solvent. However, water only dissolves other polar molecules. It does not dissolve nonpolar molecules, including organic compounds such as fats and oils.

Universal Solvent Definition

A universal solvent is a substance that dissolves most chemicals. Water is called the universal solvent because it dissolves more substances than any other solvent. However, no solvent, including water, dissolves every chemical. Typically, "like dissolves like." This mean polar solvents dissolve polar molecules, such as salts. Nonpolar solvents dissolve nonpolar molecules such as fats and other organic compounds.

Why Water Is Called the Universal Solvent

Water dissolves more chemicals than any other solvent because its polar nature gives each molecule a hydophobic (water-fearing) and hydrophilic (water-loving) side. The side of the molecules with two hydrogen atoms has a slight positive electrical charge, while the oxygen atom carries a slight negative charge. The polarization lets water attract many different types of molecules. The strong attraction to ionic molecules, such as sodium chloride or salt, allows water to separate the compound into its ions. Other molecules, such as sucrose or sugar, aren't torn into ions, but disperse evenly in water.

Alkahest as the Universal Solvent

Alkahest (sometimes spelled alcahest) is a hypothetical true universal solvent, capable of dissolving any other substance. Alchemists sought the fabled solvent, as it could dissolve gold and have useful medicinal applications.

The word "alkahest" is believed to have been coined by Paracelsus, who based on the Arabic word "alkali". Paracelsus equated alkahest with the philosopher's stone. His recipe for alkahest included caustic lime, alcohol, and carbonate of potash (potassium carbonate). Paracelsus' recipe could not dissolve everything.

After Paracelsus, alchemist Franciscus van Helmont described the "liquor alkahest", which was a sort of dissolving water that could break any material into its most basic matter. Van Helmont also wrote of "sal alkali", which was a caustic potash solution in alcohol, capable of dissolving many substances. He described mixing sal alkali with olive oil to produce sweet oil, likely glycerol.

While alkahest is not a universal solvent, it still finds use in the chemistry lab. Scientists use Paracelsus' recipe, mixing potassium hydroxide with ethanol to clean lab glassware. The glassware is then rinsed with distilled water to leave it sparkling clean.

Other Important Solvents

Solvents fall into three broad categories. There are polar solvents, such as water; nonpolar solvents like acetone; and then there is mercury, a special solvent that forms an amalgam. Water is by far the most important polar solvent. There are several nonpolar organic solvents. For example, tetrachloroethylene for dry cleaning; acetors, methyl acetate, and ethyl acetate for glue and nail polish; ethanol for perfume; terpenes in detergents; ether and hexane for spot remover; and a host of other solvents specific for their purpose.

While pure compounds may be used as solvents, industrial solvents tend to consist of combinations of chemicals. These solvents are given alphanumeroc names. For example, Solvent 645 consists of 50% toluene, 18% butyl acetate, 12% ethyl acetate, 10% butanol, and 10% ethanol. Solvent P-14 consists of 85% xylene with 15% acetone. Solvent RFG is made with 75% ethanol and 25% butanol. Mixed solvents can affect miscibility of solutes and may improve solubility.

Why There Is No Universal Solvent

Alkahest, had it existed, would have posed practical problems. A substance that dissolves all others cannot be stored because the container would be dissolved. Some alchemists, including Philalethes, got around this argument by claiming alkahest would only dissolve material down to its elements. Of course, by this definition, alkahest would be unable to dissolve gold.