In simple terms
A friendly intro before the formal notes — no formulas yet.
The Proton Pass
Acids and bases are defined by their ability to 'pass' a proton (a hydrogen ion, H⁺) between them. The acid is the proton donor, and the base is the proton acceptor.
Imagine a game of catch where the ball is a proton (H⁺). The person throwing the ball is the acid (proton donor). The person catching the ball is the base (proton acceptor). After throwing, the first person is now ready to catch (becoming a conjugate base), and the catcher can now throw (becoming a conjugate acid).
- 1
Brønsted-Lowry: acids donate H⁺, bases accept H⁺. Identify the acid and base in the equilibrium and label the conjugate pairs.
- 2
Strong acids ionise fully; weak acids establish an equilibrium. Compare the pH of a strong versus a weak acid at the same concentration.
- 3
Ka measures weak acid strength; pKa = −log Ka. Observe how the equilibrium position relates to acid strength.
- 4
pH = −log[H⁺]. Learn how to calculate the pH of strong acids and make approximations for weak acids by linking pH meter readings to [H⁺].
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Brønsted-Lowry: acids donate H⁺, bases…
Brønsted-Lowry: acids donate H⁺, bases accept H⁺.
1 more simulation for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
1 simulation
- PhETCore9701 7.2
pH Scale
Test everyday liquids with a pH probe, dilute them with water and compare their H₃O⁺ and OH⁻ concentrations.
Try this
- On Macro test battery acid, then dilute: drain to 0.1 L and top up with water to 1.0 L.
- On Micro read [H₃O⁺] before and after a tenfold dilution.
- On My Solution drag the pH and watch both concentrations move on the logarithmic scale.
Look for pH = −log₁₀[H⁺]: each tenfold dilution of a strong acid raises the pH by one unit, and [H₃O⁺] × [OH⁻] stays constant, so as one concentration rises the other falls.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Brønsted-Lowry Theory: A Proton's Journey
The theory states that an acid-base reaction involves the transfer of a proton ( ion). A hydrogen atom contains one proton and one electron; a hydrogen ion, , is therefore just a proton.
- A Brønsted-Lowry acid is a proton donor.
- A Brønsted-Lowry base is a proton acceptor.
Consider hydrogen chloride gas dissolving in water. The molecule donates a proton to a water molecule. Thus, acts as the acid and acts as the base.
Acid + Base Conjugate Acid + Conjugate Base
Conjugate Acid-Base Pairs
When a Brønsted-Lowry acid donates a proton, it forms its conjugate base. When a Brønsted-Lowry base accepts a proton, it forms its conjugate acid. These pairs are always found together in an acid-base equilibrium. In the general reaction below, HA is the acid and A⁻ is its conjugate base. B is the base and HB⁺ is its conjugate acid.
A conjugate acid-base pair consists of two species that differ by exactly one proton ().
The acid has the proton; the conjugate base does not.
Every acid-base reaction involves two conjugate acid-base pairs.
Strength of Acids and Bases
The strength of an acid or base is determined by its extent of dissociation in water. It is not the same as concentration. A strong acid is an acid that dissociates (ionises) almost completely in aqueous solution. The reaction is considered to go to completion. In contrast, a weak acid only partially dissociates, establishing a dynamic equilibrium where the majority of molecules remain undissociated. The same principle applies to strong and weak bases.
Strong Acids: e.g., . Use a forward arrow () in equations.
Weak Acids: e.g., (ethanoic acid), (carbonic acid). Use an equilibrium arrow () in equations.
Strong Bases: e.g., Group 1 hydroxides like . Dissociate fully.
Weak Bases: e.g., (ammonia). Reacts partially with water.
Examiners are very strict on the use of reaction arrows. Use for strong acid/base dissociation to show the reaction goes to completion. Use for weak acid/base dissociation to show an equilibrium is established. Using the wrong arrow will lose you marks.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
In the equilibrium , identify the two conjugate acid-base pairs.
- 1
Pair 1: The ethanoic acid molecule, , donates a proton to become the ethanoate ion, . Therefore, is the acid and is its conjugate base.
Calculate the pH of a $0.0500,mol,dm⁻³$ solution of nitric acid (). Give your answer to two decimal places.
- 1
Identify the acid type: Nitric acid is a strong acid, so it dissociates completely in water.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- Brønsted-Lowry acid
A Brønsted-Lowry acid is a proton donor.
- Brønsted-Lowry base
A Brønsted-Lowry base is a proton acceptor.
- conjugate acid
When a Brønsted-Lowry base accepts a proton, it forms its conjugate acid. These pairs are always found together in an acid-base equilibrium.
- strong acid
A strong acid is an acid that dissociates (ionises) almost completely in aqueous solution.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
A conjugate acid-base pair consists of two species that differ by exactly one proton ().
The acid has the proton; the conjugate base does not.
Every acid-base reaction involves two conjugate acid-base pairs.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test your understanding of Brønsted-Lowry theory
Test your understanding of Brønsted-Lowry theory
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Test your understanding of Brønsted-Lowry theory on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
Discuss Brønsted-Lowry theory of acids and bases
Ask, share and discuss with other Chemistry students