Reaction with metals
4 marks · NormalA student adds blue litmus paper to a solution of butanoic acid then tests its pH before and after adding sodium carbonate. Answer the following questions.
State what is observed when the blue litmus paper is placed in butanoic acid.
Give a likely pH value for butanoic acid.
State what you would observe after adding sodium carbonate to the butanoic acid.
Explain why the pH of the solution increases after adding sodium carbonate.
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(a) Blue litmus paper turns red.
(b) pH between 3–5
(c) Effervescence/fizzing (bubbles of gas) observed.
(d) Sodium carbonate reacts with butanoic acid, neutralising some of the acid and forming salt, water, and carbon dioxide, so the solution becomes less acidic and pH increases.
Common mistakes: Incorrect litmus colour; Giving an unrealistic pH value for a weak acid
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Marking points
B1 (a) · 1 mark · Blue litmus turns red in butanoic acid.
A1 (b) · 1 mark · States a correct weak acid pH: 3–5.
B2 (c) · 1 mark · Effervescence/fizzing (gas observed).
M1 (d) · 1 mark · Explains pH increases because carbonate neutralises the acid, reducing H+ concentration.
Worked solution
a) Blue litmus paper turns red in butanoic acid.
b) Likely pH value: between 3–5.
c) Adding sodium carbonate causes fizzing/bubbles (CO₂ gas is released).
d) The sodium carbonate reacts with the acid, removing hydrogen ions from solution (neutralisation), which makes the solution less acidic and so pH increases.