# DBT JRF 2022 — Question 26

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Thermodynamics - Phase Transitions · Difficulty: Medium

## Question

When ice absorbs latent heat at a constant pressure to melt from solid to liquid state at constant temperature, which of the following changes are happening? A. Entropy of the system is increasing. B. Cp (heat capacity at constant pressure) of the system is infinite or undefined. C. Enthalpy of the system is increasing. D. Cp of the system is zero.                                                                                                                                                                                                                                                                                                                                                        Choose the correct answer :

## Options

- **A.** A, B, C
- **B.** A, C, D
- **C.** A, B
- **D.** C, D

## Answer

Option C

## Explanation

Ice melting: thermodynamic changes at constant temperature and pressure: • Statement A (Entropy increases): Ice (highly ordered crystal) → liquid water (disordered). $\Delta S_{\text{system}} > 0$ ✔ TRUE • Statement B (Cp is infinite/undefined at phase transition): Heat capacity $C_p = \frac{dH}{dT}$ . At a first-order phase transition, heat is absorbed (latent heat) with NO temperature change ( $dT = 0$ ). Therefore $C_p = \frac{\Delta H}{0} \rightarrow \infty$ (undefined/infinite). ✔ TRUE • Statement C (Enthalpy increases): Melting is an endothermic process (absorbs latent heat = 6.01 kJ/mol for ice). $\Delta H_{\text{system}} > 0$ ✔ TRUE • Statement D (Cp is zero): FALSE — Cp is infinite, not zero. Answer: A, B, C are correct (Option 3).

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