Aqueous Solution Definition

Aqueous Definition

Aqueous is a term used to describe a system which involves water. The word aqueous is also applied to describe a solution or mixture in which water is the solvent. When a chemical species has been dissolved in water, this is denoted by writing (aq) after the chemical name.

Hydrophilic (water-loving) substances and many ionic compounds dissolve or dissociate in water. For example, when table salt or sodium chloride is dissolved in water, it dissociates into its ions to form Na⁺(aq) and Cl^-(aq). Hydrophobic (water-fearing) substances generally do not dissolve in water or form into aqueous solutions. For example, mixing oil and water does not result in dissolving or dissociation. Many organic compounds are hydrophobic. Nonelectrolytes may dissolve in water, but they do not dissociate into ions and they maintain their integrity as molecules. Examples of nonelectrolytes include sugar, glycerol, urea, and methylsulfonylmethane (MSM).

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Reactions in Water or Aqueous Solution

By Anne Marie Helmenstine, Ph.D.

Properties of Aqueous Solutions

Aqueous solutions often conduct electricity. Solutions that contain strong electrolytes tend to be good electrical conductors (e.g., seawater), while solutions that contain weak electrolytes tend to be poor conductors (e.g., tap water). The reason is that strong electrolytes completely dissociate into ions in water, while weak electrolytes incompletely dissociate.

When chemical reactions occur between species in an aqueous solution, the reactions are usually double displacement (also called metathesis or double replacement) reactions. In this type of reaction, the cation from one reactant takes the place for the cation in the other reactant, typically forming an ionic bond. Another way to think of it is that the reactant ions "switch partners".

Reactions in aqueous solution may result in products that are soluble in water or they may produce a precipitate. A precipitate is a compound with a low solubility that often falls out of solution as a solid.

The terms acid, base, and pH only apply to aqueous solutions. For example, you can measure the pH of lemon juice or vinegar (two aqueous solutions) and they are weak acids, but you can't obtain any meaningful information from testing vegetable oil with pH paper.

Will It Dissolve?

Whether or not a substance forms an aqueous solution depends on the nature of its chemical bonds and how attracted the parts of the molecule are to the hydrogen or oxygen atoms in water. Most organic molecules won't dissolve, but there are solubility rules that can help identify whether or not an inorganic compound will produce an aqueous solution. In order for a compound to dissolve, the attractive force between a part of the molecule and hydrogen or oxygen has to be greater than the attractive force between water molecules. In other words, dissolution requires forces greater than those of hydrogen bonding.

By applying the solubility rules, it's possible to write a chemical equation for a reaction in aqueous solution. Soluble compounds are denoted using the (aq), while insoluble compounds form precipitates. Precipitates are indicated using (s) for solid. Remember, a precipitate does not always form! Also, keep in mind precipitation is not 100%. Small amounts of compounds with low solubility (considered insoluble) actually do dissolve in water.