Acid–alkali indicators
4 marks · NormalComplete the table to show the colour each indicator would turn in a neutral solution (pH 7).
| Indicator | Colour in Neutral Solution |
|---|---|
| Litmus | |
| Methyl orange | |
| Phenolphthalein | |
| Universal indicator |
Suggest why universal indicator is more useful than phenolphthalein for measuring the pH of a weakly acidic solution.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Litmus: purple, Methyl orange: orange, Phenolphthalein: colourless, Universal indicator: green
(b) — · must show Universal indicator shows several colours for different pH values, Phenolphthalein is only colourless in acid so cannot distinguish strengths
Common mistakes: Stating 'colourless' for universal indicator
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Marking points
B1 (a) · 2 marks · All four correct colours (purple, orange, colourless, green). One mark for two correct, two for all.
B2 (b) · 2 marks · Universal indicator provides a range of colours for different pH values; phenolphthalein cannot distinguish between strongly and weakly acidic as both are colourless.
Worked solution
| Indicator | Colour in Neutral Solution |
|---|---|
| Litmus | purple |
| Methyl orange | orange |
| Phenolphthalein | colourless |
| Universal indicator | green |
Universal indicator shows shades from red to purple and green to blue for different pHs, making it possible to distinguish weakly acidic from strongly acidic. Phenolphthalein is just colourless in all acids, so cannot show the difference.