The chemical composition of the substance remains unchanged.
Explanation
Changes in matter are classified as physical or chemical based on whether the fundamental identity of the substance is altered. A physical change affects the form or state of a substance without changing its chemical composition or molecular structure.
Detailed Analysis:
- The change is reversible and temporary. is incorrect as the primary reason: While many physical changes are reversible, reversibility is not the exclusive or defining criterion. Certain chemical reactions are reversible (e.g., equilibrium reactions), and some physical changes are not easily reversible (e.g., breaking a crystal).
- There is a change in the state of matter only. is incorrect as the primary reason: A change in state is a specific example or manifestation of a physical change. However, the underlying reason it is classified as "physical" is that the substance itself has not changed identity.
- The chemical composition of the substance remains unchanged. is correct: In the transition from liquid water to water vapour, the molecules remain \(H_2O\). The intermolecular distances change, but the intramolecular bonds (chemical bonds) remain intact. Since no new substance is formed and the chemical identity remains constant, this is the primary reason it is a physical change.
- Heat is absorbed during the process. is incorrect: The absorption or release of heat (energy transfer) occurs in both physical changes (latent heat) and chemical changes (enthalpy of reaction). Thus, energy exchange does not distinguish between the two types of changes.
Key Takeaway:
The definitive characteristic of a physical change is that the chemical composition of the substance remains unchanged throughout the process, regardless of changes in state, shape, or size.