What is the ionic product of water? Derive the relation pH + pOH = 14 and explain how pH tells whether a solution is acidic, basic or neutral.

1. 1 · Ionisation of water. Water ionises slightly: H₂O ⇌ H⁺ + OH⁻.
2. 2 · Define Kw. The ionic product of water is Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ mol² L⁻² at 25 °C. In pure water [H⁺] = [OH⁻] = 10⁻⁷ mol L⁻¹.
3. 3 · Derive the relation.
   • [H⁺][OH⁻] = 10⁻¹⁴
   • −log[H⁺] − log[OH⁻] = −log(10⁻¹⁴) = 14
   • pH + pOH = 14
4. 4 · Draw the pH scale. [A line from 0 to 14: acidic from 0 to 7, neutral at 7, basic from 7 to 14; mark lemon juice, pure water and soap solution]
5. 5 · Real-life link. The stomach makes acid (low pH) to digest food, and antacids are bases that raise the pH.
6. 6 · Close in one line. Thus pH below 7 is acidic, 7 is neutral and above 7 is basic at 25 °C.

Scoring tip: Write the value of Kw with 'at 25 °C'. Show the log step clearly before writing pH + pOH = 14.