Topic 1 of 7
Acids, alkalis and pH
Read acidity as an ion balance, not just a colour.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Acids, alkalis and pH
Read acidity as an ion balance, not just a colour.
An acid produces H+ ions in aqueous solution. An alkali is a soluble base that produces OH- ions in water. A base neutralises an acid; insoluble copper(II) oxide is a base but is not an alkali. Acidic and alkaline solutions both contain H+ and OH-; their relative concentrations differ.
| Solution at room temperature | Relative ions | pH and typical indicator colour |
|---|---|---|
| Acidic | More H+ than OH- | Below 7; red through orange/yellow as pH increases |
| Neutral | Equal H+ and OH- | 7; green |
| Alkaline | More OH- than H+ | Above 7; blue through purple |
Compare colours with the supplied indicator chart. Universal Indicator estimates a pH range; a suitable calibrated pH probe gives a numerical reading. We use qualitative comparisons here, not calculations of pH from hydrogen-ion concentration.
Soil pH affects nutrient availability and plant growth. Calcium hydroxide can treat excessively acidic soil because it neutralises acid. Adding too much creates a different problem: overly alkaline soil. Determine the starting pH and add an appropriate amount rather than assuming more is better.