Changing pH (SQA National 5 Chemistry): Revision Note

Neutral water

• Water is neutral because it contains equal concentrations of hydrogen ions and hydroxide ions

Water ionising

• Even though water looks like a molecular liquid, a tiny number of water molecules split (or dissociate) into ions

H₂O (l) ⇌ H⁺ (aq) + OH^- (aq)

• The ⇌ symbol means the reaction is reversible, it happens in both directions:

  • Forward reaction: some water molecules dissociate into ions

  • Reverse reaction: ions recombine to form water molecules again

Neutral water

• In pure water, the rate of the forward reaction (forming ions) equals the rate of the reverse reaction (reforming water)

• This produces equal concentrations of H⁺ (aq) and OH^- (aq)

• Since the ion concentrations are equal:

  • pH = 7

  • So, water is neutral

Diluting solutions

Effect of dilution on acids & alkalis

• Diluting an acid with water decreases the concentration of hydrogen ions, H⁺ (aq)

  • This means the solution becomes less acidic and the pH moves towards 7

• Diluting an alkali with water decreases the concentration of hydroxide ions, OH^- (aq)

  • This means the solution becomes less alkaline and the pH moves towards 7

• In both cases, dilution moves the pH closer to neutral, because adding water reduces ion concentration

• Acid dilution example:

  • HCl (aq) → H⁺ (aq) + Cl^- (aq)

  • When water is added, [H⁺] decreases, so the pH increases

• Alkali dilution example:

  • NaOH (aq) → Na⁺  (aq) + OH^- (aq)

  • When water is added, [OH^-] decreases, so the pH decreases

Forming solutions

Acidic & alkaline oxides

• The solubility of metal and non-metal oxides determines the pH of the resulting solution

Soluble non-metal oxides

• Soluble non-metal oxides dissolve in water to form acidic solutions

• For example, carbon dioxide dissolves in water to form carbonic acid:

CO₂ (g) + H₂O (l) → H₂CO₃ (aq)

• The carbonic acid contains H⁺ (aq) ions, which makes the solution acidic:

H₂CO₃ (aq) ⇌ 2H⁺ (aq) + CO₃^2- (aq)

Soluble metal oxides

• Soluble metal oxides dissolve in water to form alkaline solutions

• For example, sodium oxide dissolves in water to form sodium hydroxide:

Na₂O (s) + H₂O (l) → 2NaOH (aq)

• The sodium hydroxide contains OH^- (aq) ions, which makes the solution alkaline:

NaOH (aq) → Na⁺ (aq) + OH^- (aq)

Bases & alkalis

• Bases are substances that neutralise acids to form a salt and water

• Common bases include:

  • Metal oxides (e.g. CuO)

  • Metal hydroxides (e.g. NaOH, Ca(OH)₂)

  • Metal carbonates (e.g. CaCO₃)

  • Ammonia (NH₃)

• Bases that are soluble in water are called alkalis

• Alkalis produce hydroxide ions (OH^-) in solution