pH and indicators
4 marks · NormalThe table shows the effect of three indicators in different solutions.
| Solution | Litmus | Methyl orange | Phenolphthalein |
|---|---|---|---|
| Vinegar | red | red | colourless |
| Soapy water | blue | yellow | pink |
State which solution is acidic. Explain your answer using the indicator results.
State which solution is alkaline. Explain your answer using the indicator results.
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.
Show answer
(a) Vinegar; all indicators show acid (red litmus, red methyl orange, colourless phenolphthalein).
(b) Soapy water; all indicators show alkali (blue litmus, yellow methyl orange, pink phenolphthalein).
Common mistakes: Only mentioning one indicator; Not tying indicators to acid/alkali definitions
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies vinegar as acidic.
A1 (a) · 1 mark · Cites at least two indicators to justify choice (e.g., red litmus/red methyl orange).
M1 (b) · 1 mark · Identifies soapy water as alkaline.
A1 (b) · 1 mark · Cites at least two indicators to justify choice (e.g., blue litmus/pink phenolphthalein).
Worked solution
a) Vinegar is acidic. Litmus is red in acid, methyl orange is red in acid, and phenolphthalein is colourless in acid; all match vinegar.
b) Soapy water is alkaline. Litmus is blue in alkali, methyl orange is yellow in alkali, phenolphthalein is pink in alkali; all match soapy water.