Assertion (A) : A pure piece of diamond, on burning stronly, changes into CO2. Reason (R) : Diamond is made up of only carbon atoms.

Assertion (A) is true because diamond, an allotrope of carbon, undergoes combustion when heated strongly in the presence of oxygen to produce carbon dioxide gas [2][t3]. This chemical reaction is represented as C + O2 → CO2 + heat and light [2]. Reason (R) is also true as diamond is composed entirely of carbon atoms arranged in a rigid three-dimensional covalent network [1][c3]. Because a pure diamond consists solely of carbon atoms, its complete oxidation leaves no residue or ash, resulting only in the formation of carbon dioxide [t2][t3]. Therefore, the fact that diamond is made only of carbon atoms (R) directly explains why its combustion product is exclusively CO2 (A). Historically, this was the method used by Antoine Lavoisier to prove diamond's elemental composition [t2].

Sources

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