During the melting of a slab of ice at 273 K at atmospheric pressure :

Solution

Physics sir · 4 steps

Ice shrinks when it melts, so the atmosphere pushes in and does work on the system.

1. Volume change. Ice is less dense than water: about 917 kg m⁻³ against 1000 kg m⁻³. So the same mass takes up less volume after melting, and Δ V < 0.
2. Sign of the work. At constant pressure the work done by the system is W = PΔ V, which is negative here. The atmosphere does positive work P|Δ V| on the system, about 9 J for every kg of ice.

   Δ U = Q - PΔ V = mL + P|Δ V| > 0

3. Internal energy. The system absorbs latent heat Q = mL, about 3.34 × 10⁵ J per kg, and also receives work. Both add energy, so U rises even though the temperature stays at 273 K. The extra energy goes into breaking the bonds of the ice lattice.
4. So. Positive work is done on the ice-water system by the atmosphere, which is option (c).

The idea to keep. In a phase change, check the density first: a shrinking system has work done on it. And constant temperature does not mean constant internal energy, since latent heat raises molecular potential energy.

Answer from the NTA final answer key. Solution by PrepWiser.