A chemical salt is an ionic compound created when an acid and a base react in a neutralization process. This reaction forms a compound consisting of positively charged ions (cations) and negatively charged ions (anions). While the solid form of any salt is electrically neutral, a solution of that salt dissolved in water can be acidic, basic, or neutral. Whether common table salt, sodium chloride (NaCl), is a neutral salt depends entirely on the chemical strengths of the acid and base used to form it. Determining the nature of NaCl’s parent compounds is the key to understanding its properties in a water solution.
Understanding What Makes a Salt Neutral
Chemical neutrality is defined on the pH scale, which specifies the acidity or basicity of an aqueous solution. A neutral solution has a pH value of 7.0, meaning the concentration of hydronium ions (\(H_3O^+\)) and hydroxide ions (\(OH^-\)) are balanced. A salt is classified as neutral if, when dissolved in water, it maintains a pH of 7.0.
The formation of a neutral salt must be the product of a reaction between a strong acid and a strong base. When such a salt dissolves, its ions separate completely in the water. Neither the cation nor the anion has a significant tendency to react with water molecules, a process known as hydrolysis.
Since the ions do not participate in hydrolysis, they are referred to as spectator ions. These ions remain dissolved without generating excess \(H_3O^+\) or \(OH^-\) ions, which shift the pH. The solution’s neutrality is maintained because the salt ions are chemically stable and do not alter the water’s equilibrium.
The Chemical Components of Sodium Chloride
Sodium chloride is a neutral salt because it is formed from the reaction of hydrochloric acid (HCl) and sodium hydroxide (NaOH). Hydrochloric acid is a strong acid, meaning it ionizes completely in water, while sodium hydroxide is a strong base that also dissociates fully. The reaction between these two strong components results in the complete neutralization of the acid and base properties, yielding sodium chloride and water.
When solid NaCl dissolves in water, it separates into the sodium cation (\(Na^+\)) and the chloride anion (\(Cl^-\)). The \(Na^+\) ion originates from the strong base NaOH, and the \(Cl^-\) ion comes from the strong acid HCl. Because both parent compounds are strong, their resulting ions are weak conjugate species.
This weakness means the \(Na^+\) and \(Cl^-\) ions lack the chemical drive to react with the water. The \(Na^+\) ion does not attract the hydroxide ion, and the \(Cl^-\) ion does not attract a hydrogen ion. This lack of hydrolysis ensures that the concentrations of \(H_3O^+\) and \(OH^-\) ions in the solution remain equal to those in pure water, fixing the pH at 7.0.
How Neutral Salts Differ from Acidic and Basic Salts
The neutrality of sodium chloride contrasts with the behavior of acidic and basic salts, which result from reactions involving at least one weak parent component.
An acidic salt forms when a strong acid reacts with a weak base, such as ammonium chloride (\(NH_4Cl\)), made from hydrochloric acid and ammonia. The cation from the weak base, like the ammonium ion (\(NH_4^+\)), is a strong conjugate acid that readily reacts with water. This hydrolysis involves the \(NH_4^+\) ion donating a proton to a water molecule, which generates excess hydronium ions (\(H_3O^+\)). The resulting solution becomes acidic, with a pH below 7.0.
Conversely, a basic salt forms from a weak acid and a strong base, such as sodium acetate. In this solution, the anion from the weak acid, like the acetate ion (\(CH_3COO^-\)), hydrolyzes by accepting a proton from a water molecule. This process releases excess hydroxide ions (\(OH^-\)) into the solution. The accumulation of these hydroxide ions shifts the solution’s pH above 7.0, making it basic. The strength of the parent acid or base dictates the final chemical nature of the salt solution.