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Cambridge International AS & A Level Chemistry – Physical Chemistry (A Level)

Topic 25
Equilibria

Topic 25 takes equilibrium beyond Kc and Kp. The first pages cover conjugate acids and bases, pH, Ka, pKa and Kw with calculations for strong acids, strong alkalis and weak acids, and buffer solutions: how they are made, how they control pH, their uses and the role of HCO₃⁻ in blood, and buffer pH calculations. The last two pages cover the solubility product, Ksp, with the common ion effect, and partition coefficients, Kpc.

Exam Paper
Paper 4
9701 A Level
Learning Outcomes
25.1.1 – 25.2.3
Acids, buffers, Ksp, partition
Sub-topics
8 lessons
Revision notes available
Exam Board
Cambridge
9701 (2025 onwards)

Revision Notes

8 revision notes, in the order the specification teaches them.

A proton passing from an acid to a water molecule
1

Brønsted–Lowry Acids, Bases and Conjugate Pairs

Brønsted–Lowry acids and bases: proton donors and acceptors, proton transfer, conjugate acid-base pairs and water as an acid and a base.

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A drop of indicator falling into a flask above a pH colour scale
2

pH and Strong Acids

PH and strong acids: pH = −log[H⁺], [H⁺] = 10⁻ᵖᴴ, strong and weak acids, and calculating the pH of strong acids.

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The hydronium and hydroxide ions formed from water
3

The Ionic Product of Water, Kw, and Strong Bases

The ionic product of water: Kw = [H⁺][OH⁻], pKw, the effect of temperature on Kw and neutral pH, and the pH of strong bases.

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A beaker of mostly whole acid molecules with a few split ones
4

Weak Acids: Ka and pKa

Weak acids: the Ka expression, pKa, calculating the pH of a weak acid, finding Ka from pH, and the limits of the approximations.

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A drop falling into a solution that keeps a steady level
5

Buffer Solutions: How They Work and Why They Matter

Buffer solutions: making acidic and basic buffers, how a buffer resists pH change, and the blood buffer system.

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An acid and its salt mixed in a beaker with a pH probe
6

Calculating the pH of a Buffer Solution

Calculating buffer pH: [H⁺] = Ka × [HA]/[A⁻], buffers by partial neutralisation, making a buffer of a given pH, and pH after adding acid or alkali.

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A saturated solution with solid and dissolved ions
7

Solubility Product, Ksp, and the Common Ion Effect

The solubility product: Ksp expressions and units, calculating Ksp and solubility, and the common ion effect with calculations.

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A separating funnel with two layers
8

Partition Coefficients, Kpc

Partition coefficients: the meaning of Kpc, calculations for a solute between two immiscible solvents, and the effect of polarity.

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Summary Map

Download, print and revise the key facts of this topic at a glance.

Summary maps

Download, print and revise the key facts of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Acids, Bases and pH Map

Cambridge International AS & A Level Chemistry · Topic 25 Equilibria · 25.1 Acids, bases and pH

The whole section on one A4 page: Brønsted–Lowry acids and bases, pH and strong acids, Kw and strong bases, weak acids and their pH.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Partition Coefficients Map

Cambridge International AS & A Level Chemistry · Topic 25 Equilibria · 25.2 Partition coefficients

The whole section on one A4 page: what Kpc means, writing Kpc, Kpc from data, solvent extraction, several portions, and polarity.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Buffers Map

Cambridge International AS & A Level Chemistry · Topic 25 Equilibria · 25.1 Buffer solutions

The whole section on one A4 page: what a buffer is, how it works, calculating buffer pH, and buffers in blood, cells and food.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Solubility Product Map

Cambridge International AS & A Level Chemistry · Topic 25 Equilibria · 25.1 Solubility product

The whole section on one A4 page: solubility equilibria, writing Kₛₚ, Kₛₚ from solubility, solubility from Kₛₚ, precipitation and the common ion effect.

Download the map6 key ideas + a data table · A4 portrait · print ready

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Specification

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Official Cambridge documentsSyllabus (opens the Cambridge website in a new tab)

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25.1.1

  1. a

    understand and use the term conjugate acid

    Revise
  2. b

    understand and use the term conjugate base

    Revise

25.1.2

  1. 25.1.2

    define conjugate acid–base pairs, identifying such pairs in reactions

    Revise

25.1.3

  1. a

    define mathematically the term pH and use it in calculations

    Revise
  2. b

    define mathematically the term Ka and use it in calculations

    Revise
  3. c

    define mathematically the term pKa and use it in calculations

    Revise
  4. d

    define mathematically the term Kw and use it in calculations (Kb and the equation Kw = Ka × Kb will not be tested)

    Revise

25.1.4

  1. a

    calculate [H+(aq)] and pH values for strong acids

    Revise
  2. b

    calculate [H+(aq)] and pH values for strong alkalis

    Revise
  3. c

    calculate [H+(aq)] and pH values for weak acids

    Revise

25.1.5

  1. a

    define a buffer solution

    Revise
  2. b

    explain how a buffer solution can be made

    Revise
  3. c

    explain how buffer solutions control pH; use chemical equations in these explanations

    Revise
  4. d i

    describe and explain the uses of buffer solutions

    Revise
  5. d ii

    describe and explain the role of HCO3– in controlling pH in blood

    Revise

25.1.6

  1. 25.1.6

    calculate the pH of buffer solutions, given appropriate data

    Revise

25.1.7

  1. 25.1.7

    understand and use the term solubility product, Ksp

    Revise

25.1.8

  1. 25.1.8

    write an expression for Ksp

    Revise

25.1.9

  1. a

    calculate Ksp from concentrations

    Revise
  2. b

    calculate concentrations from Ksp

    Revise

25.1.10

  1. a

    understand and use the common ion effect to explain the different solubility of a compound in a solution containing a common ion

    Revise
  2. b

    perform calculations using Ksp values and concentration of a common ion

    Revise

25.2.1

  1. 25.2.1

    state what is meant by the term partition coefficient, Kpc

    Revise

25.2.2

  1. 25.2.2

    calculate and use a partition coefficient for a system in which the solute is in the same physical state in the two solvents

    Revise

25.2.3

  1. a

    understand the factors affecting the numerical value of a partition coefficient in terms of the polarity of the solute

    Revise
  2. b

    understand the factors affecting the numerical value of a partition coefficient in terms of the polarities of the solvents used

    Revise