Acids, Bases
& Salts

Indicators change colour to tell acids from bases. Litmus: acids turn blue litmus red; bases turn red litmus blue. Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base. Some substances (onion, vanilla) are olfactory indicators — their smell changes.

• Acid + metal → salt + hydrogen (the gas pops with a lit splinter).
• Acid + metal carbonate/bicarbonate → salt + water + CO₂ (turns lime water milky).
• Acid + base → salt + water — neutralisation.
• Base + metal → salt + hydrogen (only some metals, e.g. zinc with NaOH).

Acids produce H⁺ ions (as H₃O⁺) in water; bases produce OH⁻ ions. Acids only show acidic behaviour in water — the H⁺ ions need water to form. A strong acid/base ionises almost completely (HCl, NaOH); a weak one only partly (acetic acid).

The pH scale (0–14) measures how acidic or basic a solution is: below 7 acidic, 7 neutral, above 7 basic. Lower pH = more H⁺ ions. pH controls real life: soil for crops, acids in our stomach, tooth decay below pH 5.5, and treating stings.

A salt forms when an acid and base neutralise. Key ones to know:

• Common salt — NaCl; raw material for many chemicals.
• Baking soda — NaHCO₃; antacid and in baking (releases CO₂).
• Washing soda — Na₂CO₃·10H₂O; cleaning, removing hardness of water.
• Bleaching powder — CaOCl₂; bleaching and disinfecting water.
• Plaster of Paris — CaSO₄·½H₂O; sets hard with water — casts and moulds.