The role of water: dissociation, strong and weak acids, and Kw
Acids and bases show their nature only in water. Water itself splits slightly into H⁺ and OH⁻; at 25 °C their product, Kw, is 1.0 × 10⁻¹⁴.
Why dry HCl gas does not change dry litmus
- 1Bring a dry strip of blue litmus near the mouth of a test tube of dry HCl gas.No colour change: dry HCl gas has no H⁺ ions
- 2★Wet the strip with a drop of water and bring it near again.It turns red: HCl dissolves in the water and gives H⁺(aq) ions
- 3Put a dry pellet of NaOH on dry red litmus in dry air.No change at first: the OH⁻ ions in the solid are not free
- 4Wait for some time.NaOH takes up moisture from the air (it is hygroscopic), and the litmus turns blue around the pellet
- 5★ConclusionAcidic and basic nature shows only when a substance is dissolved in water
★ key step
How water helps acids and bases split into ions
- Dissociation means splitting into ions. Acids and bases dissociate only in water.
- Dissolving sulphuric acid or sodium hydroxide in water gives out heat (it is exothermic). Part of this heat breaks the bond that holds the H atom or the OH group.
- Many bases, such as NaOH, are already made of ions. Water weakens the strong attraction between Na⁺ and OH⁻, so they move apart.
- H⁺ does not stay alone. It joins a water molecule to form the hydronium ion, H₃O⁺.
- Safety: to dilute concentrated sulphuric acid, add the acid slowly to water while stirring. Never add water to the acid: so much heat is released that the acid can splash and burn you.
Strong and weak acids and bases
| Kind | What happens in water | Examples |
|---|---|---|
| Strong acid | Splits completely: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq) | The seven in your NIOS lesson: HCl, HBr, HI, HClO₄, HClO₃, H₂SO₄, HNO₃ |
| Weak acid | Splits only partly; whole molecules remain: HF(aq) ⇌ H⁺(aq) + F⁻(aq) | Acetic acid CH₃COOH, HF, HCN, benzoic acid; all organic acids |
| Strong base | Splits completely: KOH(aq) → K⁺(aq) + OH⁻(aq) | LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂ |
| Weak base | Reacts with water only partly, giving few OH⁻ ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ | NH₄OH, Cu(OH)₂, Zn(OH)₂, Al(OH)₃ |
Self dissociation of water
Water splits into ions by itself, very slightly: H₂O(l) ⇌ H⁺(aq) + OH⁻(aq). This is self dissociation. The double half arrows (⇌) mean the change goes both ways and stops part way. At 25 °C, only about 2 in every billion molecules split. Each split gives one H⁺ and one OH⁻, so in pure water [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol L⁻¹. Square brackets mean concentration in moles per litre. Because Kw stays fixed, if [H⁺] goes up, [OH⁻] must go down.
Ionic product of water, Kw
Find Kw for pure water at 25 °C (298 K), where [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol L⁻¹.
- Formula: Kw = [H⁺] × [OH⁻]
- Substitute: Kw = (1.0 × 10⁻⁷ mol L⁻¹) × (1.0 × 10⁻⁷ mol L⁻¹)
- Multiply the numbers: 1.0 × 1.0 = 1.0. Add the powers: (−7) + (−7) = −14
Kw = 1.0 × 10⁻¹⁴ mol² L⁻² at 25 °C. It stays the same in every water solution at this temperature.
Neutral, acidic and basic solutions at 25 °C
| Solution | Compare the ions | [H⁺] in mol L⁻¹ | [OH⁻] in mol L⁻¹ |
|---|---|---|---|
| Neutral | [H⁺] = [OH⁻] | = 1.0 × 10⁻⁷ | = 1.0 × 10⁻⁷ |
| Acidic | [H⁺] > [OH⁻] | > 1.0 × 10⁻⁷ | < 1.0 × 10⁻⁷ |
| Basic | [H⁺] < [OH⁻] | < 1.0 × 10⁻⁷ | > 1.0 × 10⁻⁷ |
Car and inverter batteries hold dilute sulphuric acid, which conducts because the acid splits into ions in water. Battery shops dilute the acid by adding it slowly to water, never the other way round.
2 marks: “Why must an acid be dissolved in water before it can show acidic properties? Use dry and moist litmus with HCl gas as your example.” Acids give H⁺ ions only in water. Dry HCl gas has no H⁺ ions, so dry blue litmus stays blue. On moist litmus, HCl dissolves and gives H⁺(aq), which turns the litmus red.
2 marks: “Define the ionic product of water. Give its value at 25 °C, and say whether it changes when an acid is added.” Kw = [H⁺][OH⁻], the product of the concentrations of the two ions. At 25 °C it is 1.0 × 10⁻¹⁴. It does not change: if [H⁺] rises, [OH⁻] falls.
1 marks: “Is acetic acid a strong acid or a weak acid?” A weak acid: it splits only partly into ions in water.
No water, no ions, no acid. In pure water: 10⁻⁷ × 10⁻⁷ = 10⁻¹⁴.