Assertion (A) : Carbon dioxide is a compound. Reason (R) : Carbon and oxygen combine together in the ratio 3:8 by mass.

Assertion (A) is true because carbon dioxide (CO2) is a chemical compound formed by the combination of carbon and oxygen atoms [c1][c2]. Reason (R) is also true and provides the correct explanation for why CO2 is classified as a compound under the Law of Definite Proportions. According to this law, elements in a compound are always present in a fixed ratio by mass [t4][t10]. For carbon dioxide, the molecular formula is CO2, consisting of one carbon atom (atomic mass ~12) and two oxygen atoms (atomic mass ~16 each, totaling 32) [t7][t8]. The mass ratio of carbon to oxygen is therefore 12:32, which simplifies to 3:8 [t8][t9]. This fixed chemical composition by mass is a defining characteristic of a compound, confirming that R explains A.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > QUESTIONS > p. 61
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.3.1 Combustion > p. 69