CDS (II) 2021 General Knowledge · Question 7 of 120

Correct — D, 1, 2 and 3. Statement 1 and statement 2 are the textbook definition almost word for word: a universal indicator is a mixture of several indicators, and it shows different colours at different concentrations of hydrogen ions in a solution. Combining several dyes with different transition ranges is exactly what lets a single strip run through a continuous sequence of colours rather than making one on-off switch, and matching that colour against a printed chart gives the pH. Statement 3 follows from what pH is for. Because the colour tracks hydrogen-ion concentration across the whole 0-to-14 range, a universal indicator lets you judge how strong a given acid or base is instead of merely telling acid from base — dilute hydrochloric acid and dilute ethanoic acid of the same concentration land on different colours because the first ionises far more completely than the second. That comparison is a routine school laboratory exercise, and it is the sense in which the paper's third statement is being scored as true.

• (a)1 only — Drops statement 2, which is part of the same textbook sentence. Showing different colours at different hydrogen-ion concentrations is precisely what a universal indicator is for.
• (b)1 and 2 only — The near-miss option. It keeps the two definitional statements and discards the use, but judging how strong a given acid or base is happens to be the reason the mixture was assembled in the first place.
• (c)2 and 3 only — Throws out the one statement that is a straight definition. A universal indicator is a mixture of several indicators, not a single dye.

A single acid-base indicator changes colour over a narrow band of pH, so litmus, methyl orange or phenolphthalein can each answer only a yes-or-no question. Blending several such dyes, chosen so that their transition ranges overlap in sequence, produces a mixture whose colour changes continuously from about pH 0 to pH 14. The pH scale itself measures hydrogen-ion concentration — the higher the hydronium concentration, the lower the pH, with 7 as neutral, below 7 acidic and above 7 alkaline.

Statements 1 and 2 are lifted from the textbook and are not in dispute. Statement 3 is the one worth pausing on, and it is where a well-prepared candidate can talk themselves out of the right answer. The textbook justification for it is the sentence that introduces the universal indicator in the first place — the question of how strong a given acid or base is, answered by making use of a universal indicator. What the textbook never does is mention titration, and in a quantitative titration a universal indicator is a poor choice, because its gradual colour drift gives no sharp end point; a chemist would reach for phenolphthalein or methyl orange instead. The key marks all three statements correct, so the item should be read as testing the pH-comparison role rather than end-point detection, and this card follows the key. Reading the third statement as loose wording for the standard school exercise of ranking acid and base strengths is the interpretation that makes the paper consistent with itself.

• A universal indicator is a mixture of several indicators, and it shows different colours at different concentrations of hydrogen ions in a solution.
• The pH scale runs from 0, very acidic, to 14, very alkaline, with 7 as neutral; a higher hydronium-ion concentration means a lower pH.
• Paper impregnated with universal indicator is the ordinary school instrument for measuring pH.
• A strong acid ionises almost completely in water while a weak acid ionises only partly, so equal concentrations of the two give different pH readings and different indicator colours.
• For a sharp titration end point a single indicator such as phenolphthalein or methyl orange is used, not a universal indicator.

• Assuming a universal indicator is one dye rather than a blend.
• Expecting a universal indicator to give a sharp titration end point; it gives a gradual colour change instead.
• Reading a low pH as a low hydrogen-ion concentration — the relationship runs the other way.

As a statements item on what a universal indicator is and does, as a pH-value-to-solution match, or as a comparison of the pH of equal concentrations of a strong and a weak acid.

• practice — not a real PYQ

  Five solutions tested with universal indicator show pH values of 4, 1, 11, 7 and 9. Which of these is the most acidic?

  • (a)The one with pH 4
  • (b)The one with pH 1
  • (c)The one with pH 7
  • (d)The one with pH 11

  Answer(b) The one with pH 1 — the lower the pH, the higher the hydrogen-ion concentration and the more acidic the solution.

• practice — not a real PYQ

  Which one of the following would be preferred for detecting a sharp end point in an acid-base titration?

  • (a)Universal indicator
  • (b)Phenolphthalein
  • (c)Turmeric paste
  • (d)Red cabbage extract

  Answer(b) Phenolphthalein — a single indicator with a narrow transition range gives a sudden colour change, while a universal indicator changes colour gradually.