Acids, Bases and the pH Scale

The pH scale (0–14)

• 0–6 acidic, 7 neutral, 8–14 alkaline.
• Measured with universal indicator (colour) or a pH probe (more accurate, numerical).
• Each step of 1 pH = a ×10 change in hydrogen-ion concentration.

Neutralisation reactions

acid + base       → salt + water
acid + metal      → salt + hydrogen
acid + metal carbonate → salt + water + carbon dioxide

Naming the salt (from the acid)

Acid	Salt ending
Hydrochloric (HCl)	chloride
Sulfuric (H₂SO₄)	sulfate
Nitric (HNO₃)	nitrate

Example: HCl + NaOH → NaCl + H₂O.

Strong vs weak acids

• Strong (HCl, H₂SO₄, HNO₃) – fully ionise in water.
• Weak (e.g. ethanoic, citric) – only partially ionise.
• This is different from concentrated/dilute (which is about amount of acid per volume).

Tests for the gases

• Hydrogen – lit splint gives a squeaky pop.
• Carbon dioxide – turns limewater cloudy.

Exam tip

Strong/weak = degree of ionisation; concentrated/dilute = how much acid per volume. Don't mix these up.