The characteristic properties of acids and bases
Reactions of acids, indicators and the pH scale, and the H⁺/OH⁻ definition.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 7.1.1Describe the characteristic reactions of acids with metals, bases and carbonates.
- 7.1.2Describe the use of indicators and the pH scale.
- 7.1.3Define acids as proton donors and bases as proton acceptors, and explain strong and weak acids.Supplement
7 minute read
Acids, bases and pH
The three reactions of acids
Learn these as word equations first — they generate most of the salt questions in the paper.
- acid + metal → salt + hydrogen
- acid + base (metal oxide or hydroxide) → salt + water
- acid + carbonate → salt + water + carbon dioxide
The salt formed is named from the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.
The pH scale
pH runs from 0 to 14.
- pH < 7 is acidic — the lower the number, the more acidic.
- pH = 7 is neutral.
- pH > 7 is alkaline.
Universal indicator gives a colour range; litmus, methyl orange and thymolphthalein each give a single sharp change and are used in titrations.
What actually makes something acidic
An acid in solution produces H⁺ ions. An alkali produces OH⁻ ions. Neutralisation is the reaction between them:
H⁺(aq) + OH⁻(aq) → H₂O(l)
That ionic equation is worth memorising — it is the same for every acid–alkali neutralisation.
Strong and weak
This is about degree of dissociation, not concentration:
- A strong acid is fully dissociated into ions in solution (hydrochloric, sulfuric, nitric).
- A weak acid is only partially dissociated (ethanoic, carbonic).
A concentrated weak acid and a dilute strong acid are entirely different things. Strong/weak describes how completely the acid splits into ions; concentrated/dilute describes how much acid there is per volume.
Think of it like this
Strong vs weak is about how many of the acid molecules "let go" of their hydrogen ions. Concentrated vs dilute is about how many molecules are in the glass in the first place.
Worked examples
Method, step by step
Write the word and symbol equations for the reaction between hydrochloric acid and calcium carbonate.
- 1Pattern: acid + carbonate → salt + water + carbon dioxide
- 2Hydrochloric acid gives a chloride salt: calcium chloride
- 3Balance the symbol equation.
hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide; 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
Common misconceptions
- Using "strong" and "concentrated" interchangeably. They describe different things.
- Thinking all acids react with all metals. Metals below hydrogen in the reactivity series, like copper, do not react with dilute acids.
- Forgetting that a carbonate produces carbon dioxide as well as salt and water.
In the exam
- Learn the three acid reactions as word equations — they let you predict products without memorising individual cases.
- For "explain the difference between a strong and a weak acid", the answer must contain the word "dissociated" (or "ionised").
- The test for carbon dioxide is limewater turning milky/cloudy. Say both words.