Q5(b). Phenolphthalein colour and fading
The phenolphthalein indicator gives pink colour in presence of dilute alkali, while in presence of excess of concentrated alcoholic alkali, the pink colour disappears. Explain by drawing the structures.
Model answer
The free solution for 2022 answers questions 1, 2, 4, 7, 8 and 11, the ones picked in its time plan. Question 5 is answered in full in the Geo-Scientist Chemistry Paper I, 2021 to 2025 book, with every part of every paper.
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UPSC Combined Geo-Scientist (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eleven questions of each paper, with the question printed above its answer
- Titration, gravimetric and nuclear numericals worked with units; structures drawn
- Topic-wise weightage and a topic-wise index
Coversphenolphthaleinacid-base indicatorquinonoid structurecolour fading excess alkaliindicator structure
All parts of Question 5
- 5(a) Strong acid-strong base titration10 marks
- 5(b) Phenolphthalein colour and fading10 marks
- 5(c) Weak base-strong acid titration10 marks