Chemistry
Acids, bases & pH — quick study summary
Acids donate H⁺ ions in water; bases accept them (Brønsted-Lowry definition). Strong acids/bases fully dissociate; weak ones reach equilibrium. pH measures H⁺ concentration on a log scale: pH = −log[H⁺]. Pure water at 25 °C has pH 7. Acids < 7, bases > 7, and a drop of one pH unit means 10× more H⁺. Neutralisation: acid + base → salt + water. Buffers resist pH change by combining a weak acid with its conjugate base.
Key points
- Brønsted-Lowry: acid donates H⁺, base accepts H⁺
- pH = −log₁₀[H⁺]; pure water at 25 °C has pH 7
- Strong acids (HCl, HNO₃, H₂SO₄) fully dissociate; weak acids (CH₃COOH) partially
- Neutralisation: acid + base → salt + water
- Buffer = weak acid + conjugate base; resists pH change
Practice quiz
Click each question to reveal the answer.
1. What is the pH of a solution with [H⁺] = 1 × 10⁻³ M?
- 3
- −3
- 10
- 13
Answer: 3
pH = −log(10⁻³) = 3. Acids have pH < 7.
2. How much more acidic is a solution at pH 4 than one at pH 6?
Answer: 100 times
Each pH unit is a factor of 10. From pH 6 to pH 4 is 2 units = 100×.
3. Which of these is a weak acid?
- HCl
- HNO₃
- H₂SO₄
- Acetic acid (CH₃COOH)
Answer: Acetic acid (CH₃COOH)
Acetic acid only partially dissociates in water — about 1% of molecules ionise at typical concentrations.
Last reviewed: May 2026