pH and indicators
3 marks · NormalExplain why a solution with pH 6 is described as weakly acidic and how you could confirm this using an indicator.
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Also accept: Explanation includes pH scale, weak acid range, and indicator choice
Common mistakes: Calling pH 6 strongly acidic; Not mentioning indicator colour changes
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Marking points
M1 · 1 mark · States that pH 6 is in the weakly acidic range (pH 4–6).
M1 · 1 mark · Correctly relates pH 6 to not being neutral or strongly acidic.
A1 · 1 mark · Describes how an indicator (such as universal indicator or litmus) would show an acid, e.g., orange/yellow for universal indicator or red for litmus.
Worked solution
A solution with pH 6 has a value between 4 and 6, which is classified as weakly acidic on the pH scale.
It is not neutral (pH 7) or strongly acidic (pH 0–3).
To confirm this, add universal indicator: it should show an orange or yellow colour, indicating a weak acid. Litmus would turn red, showing acid, but not its strength.