Acids, bases and salts are among the most important classes of compounds in Chemistry, and they appear in many WAEC questions.
Acids
An acid is a substance that produces hydrogen ions (H⁺) in solution. Properties of acids:
- Have a sour taste (e.g. lemon juice, vinegar).
- Turn blue litmus paper red.
- React with metals to produce hydrogen gas.
- React with bases to form salt and water (neutralisation).
Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃).
Bases and Alkalis
A base is a substance that neutralises an acid to form salt and water. A base that dissolves in water is called an alkali and produces hydroxide ions (OH⁻).
- Have a bitter taste and soapy feel.
- Turn red litmus paper blue.
Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂).
The pH Scale
- pH < 7 = acidic; pH = 7 = neutral; pH > 7 = alkaline.
- Indicators change colour with pH: litmus, methyl orange, phenolphthalein.
Salts
A salt is formed when the hydrogen of an acid is replaced by a metal (or ammonium). Neutralisation: Acid + Base → Salt + Water.
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Salts can be prepared by neutralisation, reaction of acid with a metal, or by precipitation (double decomposition).
Summary
Acids produce H⁺, bases produce OH⁻ (in solution), and their neutralisation produces a salt and water. The pH scale measures acidity/alkalinity from 0 to 14, and indicators reveal pH by colour change.
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