Acids, Bases and Salts: Question 1
Syllabus 7.1, 7.2, 7.3
A laboratory technician dips a strip of universal indicator paper into each of four unlabelled beakers of solution and records the colour produced.
| Beaker | Colour of universal indicator paper |
|---|---|
| P | red |
| Q | green |
| R | purple |
| S | orange |
Which beaker most likely contains a strong alkali?
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Worked solution
Step 1: Recall the universal indicator colour scale
Universal indicator gives a range of colours across the whole pH scale, not just a single acid/alkali colour change:
- Red to orange: strongly to weakly acidic (low pH, roughly 1–6)
- Green: neutral (pH 7)
- Blue to purple: weakly to strongly alkaline (high pH, roughly 8–14)
Step 2: Match each beaker’s colour to a region of the scale
| Beaker | Colour | Approximate pH region |
|---|---|---|
| P | red | very acidic |
| Q | green | neutral |
| R | purple | very alkaline |
| S | orange | weakly acidic |
Step 3: Identify the strong alkali
A strong alkali corresponds to the highest end of the pH scale, which universal indicator shows as purple.
Beaker R is the only beaker showing this colour, so beaker R contains the strong alkali.
Why the other options are wrong
- P (red): shows strong acidity, not alkalinity. It is at the opposite end of the scale.
- Q (green): shows a neutral solution (pH 7), neither acidic nor alkaline.
- S (orange): shows only weak acidity, not any kind of alkalinity.
Final answers
- The strong alkali is in , option C.