pH and indicators
2 marks · HardExplain how adding an acid to an alkaline solution changes its pH. State what happens to the indicator colour if universal indicator is used throughout.
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The pH of the solution decreases as acid is added. The universal indicator colour changes from blue or purple (alkali) through green (neutral) to yellow-orange-red (acid).
Also accept: pH goes down with acid added; universal indicator shows colour changes from blue/purple to green and then to yellow-orange-red
Common mistakes: Saying pH increases when acid is added; Only stating that it changes colour without direction
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Marking points
B1 · 1 mark · States that the pH decreases (goes down) as acid is added to alkali.
B2 · 1 mark · Describes universal indicator colour changing from blue or purple to green and then to yellow/orange/red as pH decreases.
Worked solution
As acid is added to an alkaline solution, the pH decreases. The universal indicator changes colour from blue or purple (alkaline) towards green (neutral) and finally to yellow, orange, or red (acidic) as more acid is added.