Which one of the following statements is NOT correct ? (a) The interconversion of states of matter is a physical change (b) During burning of a candle, both chemical and physical changes take place (c) During a physical change, the chemical composition of molecules changes (d) Rusting of iron is a chemical change

Statement (c) is incorrect because during a physical change, the chemical composition of molecules remains unchanged. A physical change involves alterations only in physical properties such as shape, size, or state, without the formation of new substances [3]. For instance, the interconversion of states of matter (melting, freezing, evaporation) is a physical change because the molecular identity stays the same [1]. Conversely, the burning of a candle involves both physical changes (melting and evaporation of wax) and chemical changes (combustion of wax vapor) [1]. Rusting of iron is a chemical change because it results in the formation of a new substance, iron oxide, through a chemical reaction [3]. Therefore, the assertion that chemical composition changes during a physical change contradicts the fundamental definition of such processes.

Sources

1. [3] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
2. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.4 Can Physical and Chemical Changes Occur in the Same Process? > p. 65
3. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.3.1 Rusting > p. 62