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Physics Question 33 – JEE-MAIN 2025

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

During the melting of ice, the temperature remains constant, but the substance absorbs latent heat.

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Ninja Strategy•Volume Change and Latent Heat

Recognize that ice melting to water decreases volume (implying positive work on the system) and requires latent heat absorption (implying increased internal energy).

Step 1: Analyze Volume Change during Melting✦ Active

When ice melts into water at 273 K (or 0∘C), the volume of the system decreases. This is a unique property of water, where the density of liquid water is higher than that of ice.

ΔV=Vwater−Vice<0
Step 2: Determine Work Done○ Expand

Work done by the system on the atmosphere at constant pressure is given by W=PΔV. Since ΔV<0, the work done by the system is negative (W<0). This implies that the atmosphere does positive work on the ice-water system.

Wsystem=PΔV<0 ⟹Won system by atmosphere=−Wsystem=−PΔV>0
💡 Teacher's Secret Hint

Remember the sign convention for work: positive work done BY the system means expansion, negative work BY the system means compression (volume decrease).

Step 3: Determine Change in Internal Energy○ Expand

Melting is an endothermic process, meaning the system absorbs heat (Q>0, latent heat of fusion). According to the First Law of Thermodynamics, ΔU=Q−W. Since Q>0 and W<0, we have ΔU=Q−(−|W|)=Q+|W|. Therefore, the internal energy of the ice-water system increases during melting.

💡 Teacher's Secret Hint

Even though temperature is constant during phase change, internal energy changes because the potential energy associated with molecular bonds and arrangement changes.

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