What mistakes do students make with Deﬁnition and determination of pH (A-level only)?

Saying a solution at pH 1 is twice as acidic as one at pH 2 rather than ten times. Believing pH cannot fall below 0 or rise above 14. Comparing a concentration in mol dm⁻³ with a pH value without converting one to the other.

What are the exam tips for Deﬁnition and determination of pH (A-level only) in Chemistry AQA A-Level?

Whenever a question compares acidity, convert everything to [H⁺] or everything to pH before you compare. A difference of n pH units is a factor of 10ⁿ in hydrogen ion concentration.

AQA · A-Level · Chemistry

Course version: 7405 · Version from Sept 2016

The concentration of hydrogen ions in aqueous solution covers a very wide…

Full specification wording

Deﬁnition and determination of pH (A-level only): The concentration of hydrogen ions in aqueous solution covers a very wide range.

Hydrogen ion concentrations in aqueous solution span an enormous range.

Checked against the AQA 7405 specification (opens in a new tab) on 20 September 2026.

Point 1 of 3
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Stating that [H⁺] covers many powers of ten, roughly 10 to 10⁻¹⁵ mol dm⁻³.
Explaining that a linear scale cannot conveniently display such a range.
Relating a tenfold change in [H⁺] to a change of one pH unit.
Comparing two solutions using the ratio of their hydrogen ion concentrations.

Common mistakes and how to put them right

Saying a solution at pH 1 is twice as acidic as one at pH 2 rather than ten times.
Believing pH cannot fall below 0 or rise above 14.
Comparing a concentration in mol dm⁻³ with a pH value without converting one to the other.
Assuming a strong acid and a weak acid of equal concentration have the same [H⁺].

Exam tips

Whenever a question compares acidity, convert everything to [H⁺] or everything to pH before you compare.
A difference of n pH units is a factor of 10ⁿ in hydrogen ion concentration.

Try the lesson

The start of the Deﬁnition and determination of pH (A-level only): The concentration of hydrogen ions in aqueous solution covers a very wide range. lesson, made from checked teaching notes for this specification point.

Have a go first

Which expression defines pH?

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Listen

Hydrogen ion concentrations in aqueous solution span an enormous range. Concentrated hydrochloric acid can exceed 10 mol dm⁻³; pure water at 298 K is 1.00 × 10⁻⁷ mol dm⁻³; a concentrated alkali can be below 1 × 10⁻¹⁵ mol dm⁻³. That is sixteen powers of ten, and no linear axis can show it. On a scale where 1 mol dm⁻³ is drawn a metre long, pure water would be a ten-millionth of a metre. The differences matter even between similar solutions: dissolve SO₂ in water and you reach pH 2 to 3, dissolve SO₃ and you reach pH 0 to 1, a hundredfold difference in [H⁺] from two oxides of the same element. Comparing quantities that differ by powers of ten is exactly the job a logarithm does. A strong answer earns marks for these points. stating that [H⁺] covers many powers of ten, roughly 10 to 10⁻¹⁵ mol dm⁻³. explaining that a linear scale cannot conveniently display such a range. relating a tenfold change in [H⁺] to a change of one pH unit. comparing two solutions using the ratio of their hydrogen ion concentrations. Exam tip: Whenever a question compares acidity, convert everything to [H⁺] or everything to pH before you compare.

See it

A top-down flowchart. The top node reads '[H⁺] covers many powers of ten' and branches to three examples: concentrated hydrochloric acid above 10 mol dm⁻³, pure water at 298 K at 1.00 × 10⁻⁷ mol dm⁻³, and a concentrated alkali below 1 × 10⁻¹⁵ mol dm⁻³. A fourth branch leads to 'Linear scale cannot conveniently display such a range', which leads to 'Logarithm compares quantities differing by powers of ten', then to 'Tenfold change in [H⁺] = one pH unit', and finally to 'Compare solutions using ratio of [H⁺]'. — It shows the wide range of hydrogen ion concentrations in aqueous solution, why a linear scale cannot display it, and how a logarithm relates a tenfold change in [H⁺] to one pH unit.

Key words

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range of hydrogen ion concentration

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need for a log scale

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See every definition

range of hydrogen ion concentration: The concentration of hydrogen ions in aqueous solution covers a very wide range, roughly 10 to 10⁻¹⁵ mol dm⁻³.
need for a log scale: A linear scale cannot conveniently display such a range, so a logarithm is used.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node reads '[H⁺] covers many powers of ten' and branches to three examples: concentrated hydrochloric acid above 10 mol dm⁻³, pure water at 298 K at 1.00 × 10⁻⁷ mol dm⁻³, and a concentrated alkali below 1 × 10⁻¹⁵ mol dm⁻³. A fourth branch leads to 'Linear scale cannot conveniently display such a range', which leads to 'Logarithm compares quantities differing by powers of ten', then to 'Tenfold change in [H⁺] = one pH unit', and finally to 'Compare solutions using ratio of [H⁺]'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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