Acid

Definition

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water, producing a solution with a pH below 7. Acids have a sour taste, can corrode metals, and react with bases to form salts and water in a process called neutralisation.

Key Characteristics

• Acids have a pH less than 7; the lower the pH, the stronger the acid.
• Acids turn blue litmus paper red and can cause universal indicator to turn shades of red or orange.
• Acids react with metals to produce a salt and hydrogen gas (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂).
• Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas.
• Acids react with bases/alkalis in neutralisation reactions to form a salt and water.
• Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).
• Weak acids (e.g., citric acid, ethanoic acid) only partially dissociate in water; strong acids (e.g., HCl, H₂SO₄) dissociate fully.

Real-World Example

Vinegar contains ethanoic acid (acetic acid, CH₃COOH) at about 4–8% concentration. When you add vinegar to baking soda (sodium bicarbonate), a neutralisation reaction occurs that produces carbon dioxide bubbles — this is used in baking to make cakes rise. The same acid is responsible for the sour taste of vinegar and pickled foods, and it is also used as a natural household cleaner because it can dissolve mineral deposits.

The pH Scale

The pH scale runs from 0 to 14:

• 0–6: Acidic (0 = most acidic, e.g., battery acid; 6 = weakly acidic, e.g., milk)
• 7: Neutral (pure water)
• 8–14: Alkaline/Basic (14 = most alkaline, e.g., drain cleaner)

Each step on the pH scale represents a tenfold change in hydrogen ion concentration.