The key idea
Acids release H⁺ ions in solution; bases accept them. Neutralisation: acid + base → salt + water.The pH scale runs from 0 to 14; below 7 is acidic, 7 is neutral, above 7 is alkaline.
H⁺(aq) + OH⁻(aq) → H₂O(l)
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
The pH scale and ions
The pH scale measures the concentration of H⁺ ions in solution from 0 to 14.Acidic solutions have a pH below 7 and contain more H⁺ ions than OH⁻ ions; alkaline solutions have a pH above 7 and contain more OH⁻ ions; pure water is neutral at pH 7.The scale is logarithmic: a change of 1 pH unit represents a tenfold change in H⁺ concentration.
Strong and weak acids
g. hydrochloric, sulfuric, nitric) is completely dissociated in water, releasing all its H⁺ ions. g.ethanoic acid, citric acid) is only partially dissociated, so only a small fraction of the molecules release H⁺ ions.At the same concentration, a strong acid has a higher H⁺ concentration and a lower pH than a weak acid.
Neutralisation reactions
Neutralisation is the reaction of an acid with a base (or alkali) to form a salt and water.The ionic equation for all neutralisation reactions is H⁺(aq) + OH⁻(aq) → H₂O(l). g.sodium + chloride → sodium chloride).
Salts from different acids
Hydrochloric acid forms chloride salts; sulfuric acid forms sulfate salts; nitric acid forms nitrate salts.Bases used in neutralisation include metal oxides, metal hydroxides, metal carbonates and ammonia.A carbonate reacts with an acid to produce a salt, water and carbon dioxide.
Definitions to learn
Acid
A substance that releases H⁺ ions in aqueous solution and has a pH below 7.
Base
A substance that neutralises an acid; a soluble base is called an alkali.
Alkali
A base that dissolves in water to produce OH⁻ ions; has a pH above 7.
Neutralisation
The reaction of an acid and a base to produce a salt and water.
Strong acid
An acid that is completely ionised in aqueous solution.
Weak acid
An acid that is only partially ionised in aqueous solution.
Write the ionic equation for a neutralisation reaction.
Identify the ions produced: H⁺ from acid, OH⁻ from alkali.
These ions combine to form water.
Write: H⁺(aq) + OH⁻(aq) → H₂O(l)
H⁺(aq) + OH⁻(aq) → H₂O(l)
For salt naming: the first word comes from the base (metal name), the second from the acid (chloride from HCl, sulfate from H₂SO₄, nitrate from HNO₃).
Do not write 'acid + base → salt + water' as the only answer for 'write an ionic equation'.The ionic equation for neutralisation is always H⁺ + OH⁻ → H₂O.
How to score full marks
- 1The ionic equation for neutralisation is always H⁺ + OH⁻ → H₂O — learn this and use it every time.
- 2Distinguish strong/weak (degree of ionisation) from concentrated/dilute (amount of solute) — these are different concepts.
- 3Name salts systematically: metal name first (from the base), then acid's anion (chloride, sulfate, nitrate).
Try these yourself
Open each answer only after you have explained the full chemical process.
1State the pH range of an acid.[1 mark]
- 1.Recall the pH scale.
2Write the word equation for the reaction of hydrochloric acid with sodium hydroxide.[1 mark]
- 1.Identify acid + base products.
3Give the name of the salt formed when sulfuric acid reacts with calcium hydroxide.[1 mark]
- 1.Identify the acid's name and the metal in the base.
4Explain what is meant by a strong acid.[2 marks]
- 1.Link to degree of ionisation.
5Explain why the pH of a strong acid is lower than that of a weak acid of the same concentration.[2 marks]
- 1.Compare H⁺ ion concentrations.
6Describe a simple test to show a substance is an acid.[2 marks]
- 1.State what you would add and what you would see.
7Write a balanced symbol equation for the reaction of magnesium with hydrochloric acid.[2 marks]
- 1.Identify the products of metal + acid.
8Explain the difference between a strong acid and a concentrated acid.[4 marks]
- 1.Link strength to ionisation and concentration to amount of solute.