Why water matters
An acid is a substance that ionises in water to produce hydrogen ions, H+(aq). An alkali is a soluble base that ionises in water to produce hydroxide ions, OH−(aq). The hydrogen ion does not float alone — it bonds to a water molecule to form the hydroxonium ion, H3O+. It is these free ions that give a solution its acidic or alkaline behaviour.
When hydrogen chloride is dissolved in water it ionises completely:
Key idea / 要点
HCl(aq) → H+(aq) + Cl−(aq)
Only when the H+ ions are free and mobile can the substance turn blue litmus red, react with metals, and conduct electricity.
Evidence from dry conditions
Dry hydrogen chloride gas, or HCl dissolved in a non-aqueous solvent such as methylbenzene, contains only covalent HCl molecules. There are no free H+ ions, so it does not change the colour of dry litmus paper and does not react with magnesium. The same molecule becomes strongly acidic the moment water is added.
| Substance | Effect on dry blue litmus |
|---|---|
| Dry HCl gas | No change |
| HCl in methylbenzene | No change |
| HCl in water | Turns red (acidic) |
Alkalis behave the same way. Solid sodium hydroxide only shows alkaline properties after it dissolves and ionises: NaOH(aq) → Na+(aq) + OH−(aq). Aqueous ammonia forms OH− by reacting with water: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq).
Worked example
Dry ethanoic acid crystals are added to dry blue litmus paper — no colour change is seen. When a few drops of water are added, the paper turns red. Explain.
Answer: Dry crystals have no free H+ ions. Water lets the acid ionise, releasing H+(aq), which is acidic and turns litmus red.
Remember
- Acidic property ⇒ free H+(aq); alkaline property ⇒ free OH−(aq).
- Water is the solvent that makes ionisation possible.
- No water ⇒ no free ions ⇒ no acidic/alkaline behaviour and no conduction.