What is a salt? (O level chemistry)

What is a salt?

A salt is a substance formed when the hydrogen of an acid is replaced by a metal.

Three common ways of making a salt:

• When an acid neutralise a base or an alkalis.
  For example, sulphuric acid neutralises sodium hydroxide (an alkali) to produce sodium sulphate (a salt) and water.

  H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

• When an acid reacts with a metal.
  For example, sulphuric acid reacts with magnesium to produce magnesium sulphate (a salt) and hydrogen.

  H₂SO₄(aq) + Mg(s) → MgSO₄(aq) + 2H₂(g)

• When an acids reacts with a metal carbonates.
  For example, sulphuric acid reacts with copper carbonate to produce copper sulphate (a salt), carbon dioxide and water.

  H₂SO₄(aq) + CuCO₃(s) → CuSO₄(aq) + CO₂(g) + H₂O(l)

However, sometimes a salt is produced without a metal being involved in the reaction. For example, ammonium solution (a non-metallic solution) neutralises sulphuric acid to produce ammonium sulphate (a salt).

Basicity of acids in salt formation

Different acids have different a different number of hydrogen atoms that can be replaced during the formation of salts. The number of replaceable hydrogen atoms in an acid is called its basicity.

• A monobasic acid has one replaceable hydrogen atom. Examples of monobasic acids are nitric acid (HNO₃) and hydrochloric acid (HCl). They react and form salts as follows:

  HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

  HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

• A dibasic acid has two replaceable hydrogen atoms. Examples of dibasic acids are sulphuric acid (H₂SO₄) and carbonic acid (H₂CO₃). They react and form salts as follows:

  H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

  H₂CO₃(aq) + 2NaOH(aq) → Na₂CO₃(aq) + 2H₂O(l)

• A tribasic acid has three replaceable hydrogen atoms. Examples of tribasic acids are phosphoric acid (H₃PO₄) and citric acid [C₃H₅O(COOH)₃]. They react and form salts as follows:

  H₃PO₄(aq) + 3NaOH(aq) → Na₃PO₄(aq) + 3H₂O(l)

  C₃H₅O(COOH)₃(aq) + 3NaOH(aq) → C₃H₅O(COONa)₃(aq) + 3H₂0(l)

Naming salts

A salts inherits its name from the names of the metal involved and the acid involved. For example:

Acid salts

A normal salt is formed by replacing all the replaceable hydrogen in an acid. If some of the hydrogen is replaced and some is not, then the salt formed is call an acid salt. Formation of acid salts is only possible in basic and tribasic acids.

Examples of acid salts are:

• Sodium Hydrogen Sulphate – NaHSO₄
• Calcium Hydrogen Carbonate – Ca(HCO₃)₂
• Sodium Dihydrogen Phosphate – NaH₂PO₄
• Disodium Hydrogen Phosphate – Na₂HPO₄

Unlike normal salts, acid salts still contain some replaceable hydrogen which means they can react as acids.

Hydrated salts

Hydrated salts are salts that have water molecules (called water of crystallisation) inside their crystal structures. This water of crystallisation determines their shape and colour.

Examples of hydrated salts are:

• Calcium Sulphate – CaSO₄.2H₂O
• Copper (II)Sulphate – CuSO₄.5H₂O
• Cobalt (II) Chloride – CoCl₂.6H₂O
• Magnesium Sulphate – MgSO₄.7H₂O
• Iron (II) Sulphate – FeSO₄.7H₂O
• Sodium Carbonate – Na₂CO₃.10H₂O

The water of crystallisation is not chemically combined to the atoms of the salt, hence the dot notation in the formulae. When a hydrated salt is heated, it loses the water of crystallisation as steam. This results in the crystal losing its shape and becoming a powder.

As an example, let us look at hydrated copper (II) sulphate. Hydrated copper (II) sulphate is blue and crystalline, but on heating, it turns into a white powder as it loses the water of crystallisation.

CuSO₄.5H₂O (s) → CuSO₄ (s) + 5H₂O (g)

White anhydrous copper (II) sulphate is formed. When a little water is added, it changes backs to blue crystals of hydrated copper (II) sulphate.