In chemistry, the term “salt” extends far beyond table salt, or sodium chloride (NaCl). A chemical salt is a vast category of compounds fundamental to countless chemical and biological processes. These substances are defined by their unique structural components and the specific way they form, setting them apart from acids, bases, and pure elements. A salt is held together by subatomic forces that bind its constituent parts into a stable, electrically neutral whole.
Defining Chemical Salts: Ionic Bonds and Cations/Anions
A salt is formally classified as an ionic compound composed of positively and negatively charged particles called ions. These ions are held together by a strong electrostatic attraction known as an ionic bond. This attraction results from the transfer of electrons between atoms, typically from a metal atom to a non-metal atom.
The positive ion is termed a cation, originating from a base or a metal, such as the sodium ion (\(\text{Na}^{+}\)). Conversely, the negative ion is called an anion, usually derived from an acid or a non-metal, like the chloride ion (\(\text{Cl}^{-}\)). In table salt, the sodium cation and the chloride anion combine in a precise ratio to ensure the overall compound has no net electrical charge.
When solid, the ions arrange themselves in a continuous, repeating three-dimensional pattern known as a crystal lattice. This crystalline structure is a hallmark of ionic compounds, distinguishing them from molecular compounds where atoms share electrons in covalent bonds.
How Salts Form: The Neutralization Reaction
The primary chemical process for creating a salt is the neutralization reaction between an acid and a base. An acid donates a hydrogen ion (\(\text{H}^{+}\)), and a base accepts a hydrogen ion, often containing a hydroxide ion (\(\text{OH}^{-}\)). In this reaction, the acid and base effectively neutralize each other.
The reaction’s products are always a salt and water. For example, when hydrochloric acid (\(\text{HCl}\)) reacts with sodium hydroxide (\(\text{NaOH}\)), the hydrogen ion combines with the hydroxide ion to form water (\(\text{H}_2\text{O}\)). Simultaneously, the remaining cation (\(\text{Na}^{+}\)) joins with the remaining anion (\(\text{Cl}^{-}\)) to form the salt, sodium chloride (\(\text{NaCl}\)).
Salts can also be formed through other reactions, including the direct reaction of a metal with an acid, the reaction between a metal and a non-metal, or the reaction between two different salt solutions.
Key Physical and Chemical Properties
Salts exhibit a characteristic set of physical properties arising directly from the strength of their ionic bonds and crystal lattice structure. Due to the powerful electrostatic forces holding the ions together, most inorganic salts have high melting and boiling points. For example, sodium chloride requires approximately \(801^\circ\text{C}\) to melt, demonstrating the energy needed to break the lattice.
A defining chemical property is their electrical conductivity. In their solid, crystalline state, salts do not conduct electricity because the charged ions are locked rigidly in place. However, when dissolved in a polar solvent like water or melted, the ions become mobile. This freedom of movement allows the solution or molten salt to conduct an electrical current, making them excellent electrolytes.
Solubility in water is another property, as many salts readily dissolve in this polar solvent. Polar water molecules are effective at pulling the individual cations and anions out of the crystal lattice. While many common salts are highly soluble, there are exceptions, such as silver chloride (\(\text{AgCl}\)) and barium sulfate (\(\text{BaSO}_4\)), which have very low solubility.
Classifying Salts by Solution pH
Not all salts dissolve in water to produce a neutral solution with a \(\text{pH}\) of 7. The \(\text{pH}\) of a salt solution depends on hydrolysis, which is the reaction of the salt’s ions with water. Salts are classified into three main groups based on the \(\text{pH}\) of their aqueous solutions.
Neutral salts are formed from the reaction of a strong acid and a strong base, such as sodium chloride (\(\text{NaCl}\)). Neither the cation nor the anion reacts significantly with water. This ensures the concentrations of hydrogen and hydroxide ions remain equal, resulting in a neutral \(\text{pH}\).
Acidic salts result from the reaction between a strong acid and a weak base, like ammonium chloride (\(\text{NH}_4\text{Cl}\)). The cation from the weak base reacts with water in a cationic hydrolysis process, generating excess hydrogen ions (\(\text{H}^{+}\)) and making the solution acidic (\(\text{pH}\) below 7). Conversely, basic salts are formed from a weak acid and a strong base, such as sodium acetate (\(\text{CH}_3\text{COONa}\)). The anion undergoes anionic hydrolysis, producing hydroxide ions (\(\text{OH}^{-}\)) that make the solution basic (\(\text{pH}\) greater than 7).