UPPSC 2022 Prelims GS Paper-I · Question 113 of 150

Correct — A, (A) is true but (R) is false. Read this paper's code before anything else, because not one of its four letters carries the meaning candidates are used to. In UPPSC 2022 Paper-I, (a) means '(A) is true but (R) is false', (b) means 'Both (A) and (R) are true and (R) is the correct explanation of (A)', (c) means '(A) is false but (R) is true', and (d) means 'Both (A) and (R) are true but (R) is not the correct explanation of (A)'. The familiar 'both true and R explains A' sits at (b) here, not at (a). Now the substance. The Assertion is true: dissolve table salt in a glass of water and the sodium chloride separates into sodium and chloride ions that spread evenly among the water molecules, so every drop drawn from the glass has the same composition and the same taste, density and refractive index — that is precisely what 'homogeneous' means. The Reason is false, and it is false by definition rather than by degree: a mixture whose composition differs from one point to another is heterogeneous, not homogeneous. The Reason has the definition exactly back to front. With the Assertion standing and the Reason inverted, the answer is (a) on this paper's code.

• (b)Both (A) and (R) are true and (R) is the correct explanation of (A) — This is where 'both true and R explains A' lives in this paper, and it is the option most candidates would tick out of habit at (a). It fails on the Reason. A statement that a mixture of differing composition is homogeneous is not a true statement, so there is no question of it explaining anything. Note also that even a repaired Reason — 'a solution having the same composition throughout is homogeneous' — would merely restate the definition rather than explain why salt water satisfies it.
• (c)(A) is false but (R) is true — Both halves are wrong. The Assertion is true, since a salt solution is the standard textbook example of a homogeneous mixture, and the Reason is false, since it describes a heterogeneous mixture. This option inverts the correct verdict on both statements at once.
• (d)Both (A) and (R) are true but (R) is not the correct explanation of (A) — It rests on the Reason being true, which it is not. A candidate who half-notices that the Reason is odd, but does not stop to check the definition, will often settle here as a compromise. The discipline that avoids it is to grade the Reason on its own terms first, as a free-standing claim about chemistry, before ever asking whether it explains the Assertion.

Mixtures are classified by whether their composition is the same at every point. A homogeneous mixture has uniform composition and uniform properties throughout — a single visible phase, with the components no longer distinguishable. A heterogeneous mixture does not: its composition varies from place to place and the components remain identifiable, as in sand in water, oil and water, or a handful of mixed grain. A solution is simply a homogeneous mixture of a solute dissolved in a solvent. Sodium chloride in water qualifies because the ionic lattice breaks apart in the polar solvent and the resulting sodium and chloride ions are dispersed at the scale of individual particles. Size is the physical basis of the distinction: solution particles are below roughly one nanometre, so they neither scatter a beam of light nor settle out nor stay behind on filter paper; colloidal particles are roughly one to a thousand nanometres and scatter light, which is the Tyndall effect seen in milk or fog; suspension particles are larger still and settle under gravity.

An assertion-reason item is three separate judgements, and they must be made in order — is the Assertion true, is the Reason true, and only if both stand, does the Reason explain the Assertion. The commonest error is to read the two statements as a single argument and grade them together. Here the Reason is a bare definition that can be checked without looking at the Assertion at all, and it is stated backwards, so the question is decided at the second step. The second discipline, which matters more in this particular paper than the chemistry does, is to read the printed code every time. UPPSC 2022 shifted all four assertion-reason options: the meaning candidates hunt for at (a) is at (b), '(A) true, (R) false' is at (a), '(A) false, (R) true' is at (c), and 'both true but R does not explain A' is at (d). Seven questions in this paper use that code — Q33, Q38, Q96, Q100, Q113, Q122 and Q131 — and a candidate answering any of them from memory of the usual layout will mark the wrong letter with the right reasoning.

• A homogeneous mixture has the same composition and the same properties at every point; a mixture whose composition differs from point to point is heterogeneous. The Reason in this question states the second and labels it the first, which is why it is false.
• Table salt in water is a true solution — the sodium chloride lattice dissociates into sodium and chloride ions dispersed uniformly through the polar solvent — so the Assertion is true.
• Particle size separates the three cases: solutions below about 1 nanometre (no scattering, no settling, not filterable); colloids about 1-1000 nanometres (Tyndall effect); suspensions larger, and they settle out.
• In UPPSC 2022 Paper-I the assertion-reason code reads (a) '(A) true, (R) false'; (b) 'both true and (R) IS the correct explanation'; (c) '(A) false, (R) true'; (d) 'both true but (R) is NOT the correct explanation'.

• Answering from the usual assertion-reason layout: in this paper (a) is '(A) true, (R) false', (b) is 'both true and R explains A', (c) is '(A) false, (R) true', and (d) is 'both true but R does not explain A'
• Grading the Reason only in relation to the Assertion instead of testing it as a free-standing claim — here it is a definition, and it is stated backwards
• Assuming a false Reason drags the Assertion down with it; the two are judged independently, and here the Assertion stands on its own

UPPSC uses assertion-reason for elementary science far more than UPSC does, and its Reasons are often definitions that have been quietly inverted — the test is whether the candidate knows the definition, not whether the causal link holds. UPSC asks the same content as a statement pair or a single-answer question about why a property exists, such as why water dissolves more substances than any other liquid.

• practice — not a real PYQ

  Which one of the following is a heterogeneous mixture?

  • (a)Sugar dissolved in water
  • (b)Sand stirred into water
  • (c)Air free of dust and droplets
  • (d)Brass

  Answer(b) Sand stirred into water — the sand stays distinguishable, settles on standing and can be filtered off, so the composition differs from point to point. Sugar solution, clean air and brass are all homogeneous, brass being a solid solution of copper and zinc.

• practice — not a real PYQ

  A beam of light is passed through two transparent liquids; its path is clearly visible in the first and invisible in the second. It follows that

  • (a)the first is a true solution and the second a colloid
  • (b)the first is a colloid and the second a true solution
  • (c)both are true solutions
  • (d)both are suspensions

  Answer(b) the first is a colloid and the second a true solution — colloidal particles of roughly 1 to 1000 nanometres scatter light and make the beam visible (the Tyndall effect), while the far smaller particles of a true solution do not.