BPSC 70th CCE Prelims Q34: Dalton’s atomic theory successfully explained i. Law of

Q: What is the answer to BPSC 70th CCE Prelims Q34?

D. Correct — D, i, ii and iv. John Dalton set out his atomic theory in A New System of Chemical Philosophy (1808) as a short list of postulates: matter is made of indivisible atoms; all atoms of an element are identical in mass and properties; atoms of different elements differ; atoms combine in simple whole-number ratios; and atoms are neither created nor destroyed in a chemical change. Each of the three correct statements follows from those postulates. (i) The law of conservation of mass, stated by Lavoisier in 1789, follows directly from the last postulate — if a reaction only rearranges indestructible atoms, the total mass cannot change. (ii) The law of constant composition, Proust's law of 1799, follows from atoms of each element having a fixed mass and combining in a fixed ratio, so pure water is 1 : 8 hydrogen to oxygen by mass whatever its source. (iv) The law of multiple proportions is the strongest case of all, because Dalton did not merely explain it — he deduced it from his own theory in 1803 and then found it confirmed: for a fixed mass of nitrogen, the masses of oxygen in N2O, NO and NO2 stand in the simple ratio 1 : 2 : 4. Statement (iii) is the outsider. Radioactivity was discovered by Henri Becquerel in 1896 and named by Marie Curie, eighty-eight years after Dalton wrote, and it is not a law of chemical combination at all; worse, it demolishes Dalton's central postulate, because a nucleus that emits an alpha or beta particle and turns into a different element is neither indivisible nor unchangeable. Since statement (iii) is false and it appears in options (a), (b) and (c), a single judgement eliminates three options and leaves (d).

BPSC 70th CCE Prelims General Studies · Question 34 of 143

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Answers derived, not from the official key. BPSC published an answer key for BPSC 70th CCE at the time, but it is no longer available from the Commission or in any web archive. The answers here are our best-supported answers — solved from the question itself and cross-checked against primary sources — and each card says how certain it is. The question text is verbatim from the official booklet. These are not the Commission's answer key.

Question order, option order and answers follow the official BPSC Booklet Series E, paper code 09/C/GO/CC/PT-2024(Q).

Dalton’s atomic theory successfully explained i. Law of conservation of mass. ii. Law of constant composition. iii. Law of radioactivity. iv. Law of multiple proportion.

(a)i, ii and iii
(b)ii, iii and iv
(c)i, iii and iv
(d)i, ii and iv

Answer

Why

Correct — D, i, ii and iv. John Dalton set out his atomic theory in A New System of Chemical Philosophy (1808) as a short list of postulates: matter is made of indivisible atoms; all atoms of an element are identical in mass and properties; atoms of different elements differ; atoms combine in simple whole-number ratios; and atoms are neither created nor destroyed in a chemical change. Each of the three correct statements follows from those postulates. (i) The law of conservation of mass, stated by Lavoisier in 1789, follows directly from the last postulate — if a reaction only rearranges indestructible atoms, the total mass cannot change. (ii) The law of constant composition, Proust's law of 1799, follows from atoms of each element having a fixed mass and combining in a fixed ratio, so pure water is 1 : 8 hydrogen to oxygen by mass whatever its source. (iv) The law of multiple proportions is the strongest case of all, because Dalton did not merely explain it — he deduced it from his own theory in 1803 and then found it confirmed: for a fixed mass of nitrogen, the masses of oxygen in N2O, NO and NO2 stand in the simple ratio 1 : 2 : 4. Statement (iii) is the outsider. Radioactivity was discovered by Henri Becquerel in 1896 and named by Marie Curie, eighty-eight years after Dalton wrote, and it is not a law of chemical combination at all; worse, it demolishes Dalton's central postulate, because a nucleus that emits an alpha or beta particle and turns into a different element is neither indivisible nor unchangeable. Since statement (iii) is false and it appears in options (a), (b) and (c), a single judgement eliminates three options and leaves (d).

Why the others are wrong

(a)i, ii and iii — Gets conservation of mass and constant composition right but includes radioactivity — a nuclear phenomenon discovered in 1896 that lies outside chemical combination entirely and contradicts Dalton's indivisible atom. It also drops the law of multiple proportions, the one law Dalton actually predicted.
(b)ii, iii and iv — Includes radioactivity and, in exchange, omits the law of conservation of mass — the very law that Dalton's fifth postulate exists to account for. Any option that drops conservation of mass while retaining a nuclear phenomenon has the chronology exactly backwards.
(c)i, iii and iv — Again carries radioactivity, and this time discards the law of constant composition. Proust's law is one of the two laws Dalton set out to explain in the first place — his theory was built on the observed regularity that a compound's composition never varies.

Concept

The laws of chemical combination were empirical generalisations discovered before anyone knew what an atom was: conservation of mass (Lavoisier, 1789), definite or constant proportions (Proust, 1799), multiple proportions (Dalton, 1803), reciprocal proportions (Richter), and Gay-Lussac's law of combining gaseous volumes (1808). Dalton's achievement in 1808 was to supply a single physical picture — small, hard, indestructible atoms with characteristic masses combining in whole-number ratios — from which the first four fell out as consequences. That is what 'successfully explained' means in this stem: not that Dalton discovered the law, but that his model accounts for it. The theory's limits are just as examinable. It cannot explain Gay-Lussac's law of gaseous volumes, which needed Avogadro's hypothesis of 1811; it has no room for isotopes, since it insists all atoms of an element share one mass; and the discovery of the electron in 1897 and of radioactivity in 1896 ended the indivisible atom for good.

Every option contains exactly three of the four items, so the question is a single-elimination puzzle rather than a memory test of the whole list. Find the one statement that cannot belong and the option set collapses. Two independent tests flag statement (iii). The first is chronological: Dalton published in 1808, Becquerel found radioactivity in 1896, and no theory explains a phenomenon discovered nearly a century after it. The second is categorical: the other three are laws of chemical combination, arrived at by weighing reactants and products, while radioactivity is a nuclear change in which one element becomes another — the opposite of everything Dalton assumed. Because statement (iii) sits in three of the four options, that single judgement is worth the whole mark, and option (d) is the only survivor. If a candidate instead tries to verify each of i, ii and iv from memory, the question takes four times as long for the same result.

Key facts

Dalton published his atomic theory in A New System of Chemical Philosophy, 1808; its key postulates are indivisible atoms, identical mass for atoms of one element, and combination in simple whole-number ratios
Law of conservation of mass — Antoine Lavoisier, 1789; law of definite/constant proportions — Joseph Proust, 1799; law of multiple proportions — Dalton himself, 1803
Multiple proportions illustrated: for a fixed mass of nitrogen, the oxygen masses in N2O, NO and NO2 are in the ratio 1 : 2 : 4
Radioactivity was discovered by Henri Becquerel in 1896 and investigated by Marie and Pierre Curie, who isolated polonium and radium in 1898 — long after Dalton, and incompatible with an indivisible atom
Dalton's theory fails on Gay-Lussac's law of gaseous volumes (1808), which required Avogadro's hypothesis (1811), and cannot accommodate isotopes, which are atoms of one element with different masses

Dalton's atomic theory (1808) — what it accounts for, and what it cannot

i. Law of conservation of mass (Lavoisier, 1789)→Explained — atoms are neither created nor destroyed in a chemical reaction, only rearranged

ii. Law of constant composition (Proust, 1799)→Explained — a compound always contains the same elements in the same fixed mass ratio

iii. 'Law of radioactivity'→Not explained — radioactivity was found by Becquerel in 1896 and requires a divisible atom that can change into another element

iv. Law of multiple proportions (1803)→Explained — Dalton deduced this law from the theory itself; N2O : NO : NO2 gives oxygen ratios 1 : 2 : 4

[Gay-Lussac's law of combining gaseous volumes, 1808 — not in the stem]→Not explained — needed Avogadro's hypothesis of 1811, which distinguished the atom from the molecule

The three highlighted rows are statements i, ii and iv, which is option (d). Statement iii is the only false item, and because it appears in options (a), (b) and (c), rejecting it alone answers the question.

Study next

Laws of chemical combination — conservation of mass, definite proportions, multiple proportions, reciprocal proportions, and Gay-Lussac's law of gaseous volumesAvogadro's hypothesis (1811), the atom-molecule distinction, and the mole conceptDiscovery of subatomic particles — Thomson's electron (1897), Rutherford's nucleus (1911), Chadwick's neutron (1932) — and how each broke a Dalton postulateIsotopes, isobars and isotones, and why isotopes make Dalton's 'identical mass' postulate untenableRadioactivity — alpha, beta and gamma emissions, half-life, and Becquerel and the Curies

Common traps

Treating 'law of radioactivity' as a real law of chemical combination because it sounds like one — it is a nuclear phenomenon and there is no such law in Dalton's scheme
Assuming Dalton only explained pre-existing laws; the law of multiple proportions was his own prediction from the theory
Forgetting the theory's limits — it does not explain Gay-Lussac's law of gaseous volumes, allotropy, or isotopes

BPSC builds these as four-statement lists in which every option offers exactly three items, so the marks turn on spotting the single false statement rather than on recalling the full set — a design that rewards speed. UPSC frames the same chemistry as two- or three-statement 'which is/are correct' sets and is likelier to test the limitations of a model, or its history, than the postulates themselves.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
Which one of the following laws was proposed by John Dalton himself and then explained by his atomic theory ?
- (a)Law of conservation of mass
- (b)Law of definite proportions
- (c)Law of multiple proportions
- (d)Law of combining gaseous volumes
Answer(c) Law of multiple proportions — Dalton put it forward in 1803; conservation of mass is Lavoisier's, definite proportions Proust's, and gaseous volumes Gay-Lussac's.
practice — not a real PYQ
Which of the following is a limitation of Dalton's atomic theory ?
- (a)It cannot explain the law of conservation of mass
- (b)It cannot explain the existence of isotopes
- (c)It cannot explain the law of constant composition
- (d)It cannot explain why compounds have fixed formulae
Answer(b) It cannot explain the existence of isotopes — the theory insists that all atoms of an element have identical mass, which isotopes contradict.