A salt is chemically defined as an ionic compound that results from the neutralization reaction between an acid and a base. These compounds are characterized by a lattice structure held together by ionic bonds between a positively charged cation and a negatively charged anion. When dissolved in water, salts dissociate into these constituent ions, creating a solution that can conduct electricity. While the most familiar example is sodium chloride, commonly known as table salt, the term encompasses a vast category of substances. The diverse chemical structures and behaviors of these compounds allow for several ways to classify them, depending on the specific property being examined.
Classification by Ionic Composition
One method of distinguishing salts is by the complexity of the ions composing the crystal structure. Simple salts contain only two types of ions, such as the sodium cation (\(\text{Na}^+\)) and the chloride anion (\(\text{Cl}^-\)) in ordinary table salt. These compounds fully dissociate into their two component ions when dissolved in water.
Double salts are formed by the co-crystallization of two different simple salts in a single crystal lattice. An example is Mohr’s salt, which has the formula \(\text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}\). Double salts maintain their distinct structure in the solid state. Like simple salts, they completely separate into all their individual ions when dissolved in an aqueous solution.
Complex salts, also called coordination compounds, feature a central metal ion bonded to neutral molecules or other ions, known as ligands, forming a complex ion. Unlike the other two types, a complex salt does not fully dissociate into all simple ions upon dissolving. Instead, it releases simple counter-ions, like potassium, but the central iron ion and its surrounding cyanide ligands remain intact as a single, complex ion, \(\text{[Fe}(\text{CN})_6]^{4-}\).
Classification by Reaction in Water
Salts can also be classified by the \(\text{pH}\) of their aqueous solution, which is determined by the relative strengths of the parent acid and parent base used to form them. This behavior is a result of a process called hydrolysis, where one or both of the salt’s ions react with water.
Salts formed from a strong acid and a strong base, such as sodium chloride (\(\text{NaCl}\)), are considered neutral. Neither ion reacts significantly with water, resulting in a solution with a \(\text{pH}\) of approximately 7.
Acidic salts are produced from the reaction of a strong acid and a weak base, such as ammonium chloride (\(\text{NH}_4\text{Cl}\)). In this case, the cation from the weak base reacts with water, generating excess hydrogen ions (\(\text{H}^+\)). This reaction causes the solution’s \(\text{pH}\) to fall below 7.
Conversely, basic salts are formed from a strong base and a weak acid, like sodium acetate (\(\text{CH}_3\text{COONa}\)). The anion from the weak acid hydrolyzes water, producing hydroxide ions (\(\text{OH}^-\)) that make the solution \(\text{pH}\) greater than 7.
The final category includes salts derived from both a weak acid and a weak base, such as ammonium carbonate. The resulting solution \(\text{pH}\) is dependent on the relative strengths of the parent acid and base components. If the weak acid is stronger than the weak base, the solution will be slightly acidic, but if the weak base is stronger, the solution will be slightly basic.
Classification by Water Content
A separate classification system distinguishes salts based on whether water molecules are physically incorporated into their crystal structure during crystallization. Anhydrous salts are those that contain no water molecules in their structure. The term “anhydrous” literally means “without water.”
In contrast, hydrated salts, or hydrates, have a specific number of water molecules bound within the crystal lattice. This incorporated water is known as the water of crystallization. The formula for a hydrate explicitly shows this water content, such as in copper(II) sulfate pentahydrate (\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)). The presence or absence of this water can dramatically alter the salt’s physical characteristics, most notably its color. For example, hydrated copper(II) sulfate is a bright blue crystalline solid, but when the water is removed, the resulting anhydrous copper(II) sulfate is a white powder.