Acids, Alkalis, Ions and pH

Acids and alkalis are common in daily life. Lemon juice is acidic; soapy water is alkaline. Their behaviour in water is linked to tiny charged particles called ions, and we compare their “strength” using the pH scale.

Key ideas

• Acid: a substance that produces hydrogen ions, $\mathrm{H^+}$, in water; a proton donor.
• Base: a metal oxide or hydroxide; a proton acceptor.
• Alkali: a soluble base that produces hydroxide ions, $\mathrm{OH^-}$, in water (e.g. sodium hydroxide, aqueous ammonia).

Ions in aqueous solutions

Ions are atoms or groups of atoms with a charge. In water, acids release $\mathrm{H^+}$ ions and alkalis release $\mathrm{OH^-}$ ions.

• Strong acid (complete dissociation): $$\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$$
• Weak acid (partial dissociation): $$\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}$$

The pH scale (0–14)

• $\mathrm{pH = 7}$: neutral (pure water).
• $\mathrm{pH < 7}$: acidic; lower pH means higher $\mathrm{H^+}$ concentration.
• $\mathrm{pH > 7}$: alkaline; higher pH means higher $\mathrm{OH^-}$ concentration.
• Universal indicator: red/orange (stronger acid), green (neutral), blue/purple (stronger alkali).

Common indicators and colours

• Litmus: red in acid, blue in alkali.
• Methyl orange: red in acid, yellow in alkali, orange near neutral.
• Thymolphthalein: colourless in acid/neutral, blue in alkali.

Neutralisation

When an acid and an alkali react, $\mathrm{H^+}$ and $\mathrm{OH^-}$ combine to make water; a salt is also formed.

$$\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$$

Example: $\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}$.