Aqueous solution

An aqueous solution is a chemical mixture where water acts as the solvent. The term "aqueous" is derived from the word "aqua," meaning it relates to or is dissolved in water. In chemistry, scientists often denote these mixtures by adding the symbol (aq) to a chemical formula. For example, sodium chloride (NaCl) in water is written as NaCl(aq). Because water is naturally abundant and acts as an excellent solvent, it is considered a ubiquitous solvent. This means it is found everywhere in chemical science. In many laboratory settings, the word "solution" refers specifically to an aqueous solution unless a different solvent is named.

Whether a substance dissolves in water depends on its chemical nature. Substances that interact well with water are described as hydrophilic, which means "water-loving." A common example of a hydrophilic substance is sodium chloride. In contrast, substances that do not dissolve well are called hydrophobic, or "water-fearing." To dissolve, a substance must be able to match or exceed the strong attractive forces that water molecules exert on one another. If a substance cannot overcome these internal water forces, it will not dissolve. Instead, the molecules may form an insoluble solid known as a precipitate.

Aqueous solutions can be categorized by how they conduct electricity. This ability depends on whether the dissolved substance becomes ionized. When a substance is ionized, it breaks into charged particles called ions. Solutions that conduct electric current efficiently are made of strong electrolytes. These substances are completely or substantially ionized when they enter the water. In these solutions, ions move freely through the solvent. Weak electrolytes are substances that undergo only limited ionization in water. While they do contain some ions, they do not conduct electricity as well as strong electrolytes.

Some substances behave differently when they enter water. These are known as non-electrolytes. While non-electrolytes do dissolve in water, they maintain their molecular integrity. This means they do not dissociate, or break apart, into ions. Common examples of non-electrolytes include sugars, urea, glycerol, and ethanol. Because they remain as whole molecules rather than charged ions, they cannot carry an electric current. This distinction is vital for understanding how different chemicals behave in a liquid environment.

Water also plays a central role in defining acids and bases through the Arrhenius definition. An Arrhenius acid is a substance that dissociates to release hydrogen ions (H+) when dissolved in water. Hydrogen chloride (HCl) is a primary example of this process. An Arrhenius base is a substance that dissociates to release hydroxide ions (OH-) in water. Sodium hydroxide (NaOH) serves as a classic example of a base. In any aqueous solution, these hydrogen and hydroxide ions exist in a specific equilibrium. At a temperature of 298 K, this balance is expressed by the constant Kw, which equals 1 x 10−14.

Chemicals in water often undergo metathesis reactions, which are also called double-displacement reactions. During a metathesis reaction, a cation (a positively charged ion) displaces another part of a molecule to form a new ionic bond with an anion (a negatively charged ion). This process can lead to a precipitation reaction. A precipitation reaction occurs when two aqueous strong electrolyte solutions are mixed together. If the reaction produces an insoluble solid, that solid is called the precipitate. Scientists use solubility charts to determine which compounds will remain aqueous and which will become precipitates.

To accurately track these chemical changes, scientists use specific mathematical tools. They utilize complete ionic equations and net ionic equations to show how ions dissociate and interact. When performing complex calculations regarding these reactions, it is necessary to know the molarity of the solutions. Molarity refers to the concentration of the aqueous solution. By understanding the concentration and the way ions swap during metathesis, chemists can predict the results of complex aqueous interactions. This knowledge connects to broader fields like drug permeability and inorganic chemistry.