8.0 SALTS

What do table salt, baking soda, and calcium carbonate (limestone) have in common? They are all salts. In chemistry, a salt is an ionic compound formed when the hydrogen ion (H⁺) in an acid is replaced by a metal ion (or ammonium ion, NH₄⁺). This chapter covers the different methods of preparing salts, the rules that determine whether a salt is soluble or insoluble, how to predict precipitation reactions, and how heat affects different types of salts. These are essential skills for any chemist, from the laboratory to industrial applications.

8.1 WHAT IS A SALT?

A salt is an ionic compound formed from the reaction between an acid and a base. The hydrogen ions (H⁺) of the acid are replaced by metal ions (or ammonium ions).

General formula: A salt consists of a positive ion (cation) from a base (metal or NH₄⁺) and a negative ion (anion) from an acid.

Examples of common salts and their parent acids:

• Sodium chloride (NaCl): From hydrochloric acid (HCl) and sodium hydroxide (NaOH).
• Potassium nitrate (KNO₃): From nitric acid (HNO₃) and potassium hydroxide (KOH).
• Calcium sulfate (CaSO₄): From sulfuric acid (H₂SO₄) and calcium hydroxide (Ca(OH)₂) or calcium carbonate.
• Ammonium chloride (NH₄Cl): From hydrochloric acid and ammonia solution (NH₃, a base).
• Copper(II) sulfate (CuSO₄): From sulfuric acid and copper(II) oxide (CuO, a base).

8.2 SOLUBILITY RULES FOR SALTS

Before preparing a salt, y