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.

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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 other options are wrong — (a) 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) 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) 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.

State PCS 2024 Q34 — Science & Technology PYQ with Answer

State PCS 2024 Q34 — Science & Technology PYQ with Answer