Acids, Bases and the pH Scale — O/L Science

Acids and bases are two fundamental classes of chemical compounds. An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water — examples include hydrochloric acid (HCl) and citric acid in lemons. A base is a substance that releases hydroxide ions (OH⁻) or accepts H⁺ ions — examples include sodium hydroxide (NaOH) and ammonia.

The pH scale (0–14) measures the concentration of H⁺ ions in a solution. pH 7 is neutral (pure water); below 7 is acidic; above 7 is basic (alkaline). Each unit on the scale represents a ten-fold difference in concentration. Indicators such as litmus, phenolphthalein, and universal indicator change colour to reveal pH.

When an acid and a base react, they neutralise each other, forming a salt and water. This reaction is called neutralisation: HCl + NaOH → NaCl + H₂O. Understanding acids and bases is essential for digestion (stomach acid pH ~2), soil management, water treatment, and medicine.