Acidic, basic and neutral salts

Salts

Salts are formed when acids react with bases.

acid + base → salt + water

When strong acids react with strong bases they neutralise each other, because the salts formed are neutral.

The salts of weak acids or weak bases are not neutral though…

Pure water has a pH of 7. It turns universal indicator green.

Sodium chloride is the salt formed when hydrochloric acid (a strong acid) reacts with sodium hydroxide (a strong base).

Add salt to water…

…and stir. The solution remains neutral.

When NaCl dissolves in water the ionic crystal breaks up, the ions separate and become surrounded by water molecules:

NaCl(s) → Na+(aq) + Cl-(aq)

Since the pH of the water remains neutral, we know that neither Na+ nor Cl- react with water.

Sodium ethanoate (sodium acetate, CH3COONa), is formed when ethanoic acid (a weak acid) reacts with sodium hydroxide (a strong base).

Sodium ethanoate is added to water.

As it dissolves the colour of the universal indicator changes.

The final solution has a pH of about 8.5.

Water Sodium ethanoate

Sodium ethanoate is an ionic compound. When it dissolves in water the ions separate and become surrounded by water molecules:

CH3COONa(s) → CH3COO-(aq) + Na+(aq)

One of these ions reacts with water to form OH-. Since we know Na+ does not react with water, it must be CH3COO-:

CH3COO-(aq) + H2O(l) OH-(aq) + CH3COOH(aq)

The salt of a weak acid and a strong base is basic.

Another basic salt you should remember from Year 12 is sodium carbonate:

Na2CO3(s) → 2Na+(aq) + CO32–(aq)

CO32–(aq) + H2O(l) HCO3

–(aq) + OH–(aq)

Ammonium chloride is formed when HCl (a strong acid) reacts with ammonia, NH3, which is a weak base.

When solid ammonium chloride is added to water the pH drops to about 5.

The ammonium ion reacts with water to form H3O+:

NH4Cl(s) → NH4+(aq) + Cl-(aq)

The salt of a strong acid and a weak base is acidic.

NH4+(aq) + H2O(aq) H3O+(aq) + NH3(aq)

NH4Cl

pH = 5

NaCl

pH = 7

CH3COONa

pH = 8.5

Learn these compounds and these pHs.