What is meant by the self dissociation of water? Define the ionic product of water and give its value at 25 °C. How do [H⁺] and [OH⁻] compare in neutral, acidic and basic solutions?

1. 1 · Define. Water splits very slightly into ions by itself: H₂O ⇌ H⁺ + OH⁻. This is self dissociation.
2. 2 · Pure water. At 25 °C, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol L⁻¹.
3. 3 · Ionic product. Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. It stays constant at a given temperature.
4. 4 · Compare.
   • Neutral: [H⁺] = [OH⁻]
   • Acidic: [H⁺] > [OH⁻], so [H⁺] > 10⁻⁷
   • Basic: [H⁺] < [OH⁻], so [H⁺] < 10⁻⁷
5. 5 · Close in one line. So adding an acid raises [H⁺] and lowers [OH⁻], and Kw stays the same.

Scoring tip: Write the power of ten clearly: 1.0 × 10⁻¹⁴. A wrong sign in the power loses the mark.