pH, Acids and Bases
What the pH scale measures, how acids and bases react, and how to avoid common errors.
The pH scale
The pH scale measures how acidic or alkaline a solution is, commonly on a scale from 0 to 14.
- pH below 7: acidic.
- pH 7: neutral (pure water at about 25°C).
- pH above 7: alkaline.
Each step on the scale represents a tenfold change in hydrogen ion concentration. A solution of pH 3 has ten times the hydrogen ion concentration of one with pH 4.
Acids and bases
An acid produces hydrogen ions (H⁺) in water. A base can neutralise acids; a base that dissolves in water is called an alkali and produces hydroxide ions (OH⁻).
Common examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH).
Indicators
Indicators change colour with pH. Litmus is red in acid and blue in alkali. Universal indicator gives a range of colours that correspond to approximate pH values.
Neutralisation
Neutralisation is the reaction of an acid with a base to form a salt and water.
acid + base → salt + water
Example: HCl + NaOH → NaCl + H₂O
Acid and a metal carbonate: acid + carbonate → salt + water + carbon dioxide.
Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
Strong and weak acids
A strong acid ionises completely in water. A weak acid ionises only partly. Strength is not the same as concentration. Concentration is how much acid is dissolved per unit volume.
Safety
Wear eye protection, handle acids and alkalis carefully, and follow your teacher's instructions. Concentrated solutions are hazardous.
Common mistakes
- Treating strength and concentration as the same thing.
- Saying pH 5 is "slightly" more acidic than pH 6 without noting the factor of ten.
- Forgetting that neutralisation produces a salt as well as water.
Quick recap
pH below 7 is acidic, 7 neutral, above 7 alkaline. Acid + base gives salt + water. Strength differs from concentration.