What does one pH unit mean?

A tenfold change in hydrogen ion concentration, and a smaller pH means a stronger acid.

How do strong and weak acids differ?

A strong acid ionises completely in water while a weak one only partly ionises, so the weak acid has the higher pH.

S4 Chemistry: Acids, Bases and Salts (Paper 1)

2026-11-01

This paper covers the definitions of acids and bases, indicators, pH and hydrogen ion concentration, and compares strong with weak acids.

Direct answer　Decide whether the acid or base is strong, then convert between pH and hydrogen ion concentration, working through pOH for alkalis.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry: Acids, Bases and Salts
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat is the pH of 0.010 M hydrochloric acid?
FoundationWhat is the pH of 0.0010 M sodium hydroxide solution?
FoundationWhich of these is an acid? A sodium chloride, B sulphuric acid, C sodium hydroxide, D ammonia
FoundationWhat colour does blue litmus paper turn in an acid?
IntermediateThe pH of 0.10 M acetic acid is about 2.9 while 0.10 M hydrochloric acid is 1.0. Why is the pH higher?
IntermediateA solution has a pH of 3. Find its hydrogen ion concentration.
IntermediateThe pH falls from 2 to 4. By what factor does the hydrogen ion concentration change?
ChallengeWhat volume of 0.10 M hydrochloric acid neutralises 25.0 cm3 of 0.10 M sodium hydroxide?
Challenge10 cm3 of a strong acid of pH 2 is diluted to 1 000 cm3. Find the new pH.
Error redoError repair: a student dilutes a pH 6 solution a hundred times and says the pH becomes 8 and the solution turns alkaline. Give the mistake and the correct pH.

03　Answers and one-line notes

#	Answer	One-line note
1	pH 2	A strong acid ionises completely, so the hydrogen ion concentration is 0.010 M and the pH is 2.
2	pH 11	The pOH is 3, so the pH is 14 minus 3, which is 11.
3	B	Sulphuric acid releases hydrogen ions in water, while sodium hydroxide and ammonia are bases and sodium chloride is a salt.
4	red	Litmus turns red in acid and blue in alkali.
5	acetic acid is a weak acid and only partly ionised	Acetic acid has a small Ka of 1.8 times ten to the minus five, so its hydrogen ion concentration is about 1.3 times ten to the minus three molar.
6	1 x 10-3 M	A pH of 3 means the hydrogen ion concentration is 10 to the minus three molar.
7	it becomes one hundredth	Each pH unit means a tenfold change, so two units means one hundredth.
8	25.0 cm3	The acid and alkali react one to one, so equal moles means equal volumes at the same concentration.
9	pH 4	There are 10 to the minus four moles in 10 cm3, and after dilution to one dm3 the concentration is 10 to the minus four molar, so the pH is 4.
10	dilution only brings the pH towards 7; the answer is about 6.98	After dilution the acid hydrogen ions are only 10 to the minus eight molar, so water contributes too and the total is about 1.05 times ten to the minus seven molar, giving pH 6.98.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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