Acids, Bases and Salts: Question 6
Syllabus 7.1
A pool-maintenance technician tests a sample of pool water using three different indicators. The results are shown in the table.
| Indicator | Colour observed |
|---|---|
| Litmus | blue |
| Methyl orange | yellow |
| Thymolphthalein | blue |
What can be concluded about the pool water sample?
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Worked solution
Step 1: Recall the colour behaviour of each named indicator
Each indicator has its own colour scheme, separate from the universal indicator’s full pH scale:
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| Litmus | red | blue |
| Methyl orange | red/pink | yellow |
| Thymolphthalein | colourless | blue |
Step 2: Apply this to the given results
- Litmus is blue → this indicates an alkaline solution.
- Methyl orange is yellow → this also indicates an alkaline solution.
- Thymolphthalein is blue → thymolphthalein is only blue in an alkali (it would be colourless in an acidic or neutral solution), so this again indicates an alkaline solution.
Step 3: Reach a conclusion
All three indicators independently point to the same conclusion, so the pool water sample must be alkaline.
Why the other options are wrong
- A (acidic): none of the three colours match the acidic response of any of the indicators (litmus would be red, methyl orange red/pink, thymolphthalein colourless).
- C (neutral): a neutral solution would leave thymolphthalein colourless, not blue, so this result rules out neutral.
- D (inconsistent): the three results are not contradictory. Each indicator, read using its own correct colour scheme, agrees that the solution is alkaline.
Final answers
- The pool water sample is , option B.