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MCQEasyJEE 2025Internal Energy & Enthalpy

JEE Chemistry 2025 Question with Solution

Given below are two statements : Statement I : When a system containing ice in equilibrium with water (liquid) is heated, heat is absorbed by the system and there is no change in the temperature of the system until whole ice gets melted. Statement II : At melting point of ice, there is absorption of heat in order to overcome intermolecular forces of attraction within the molecules of water in ice and kinetic energy of molecules is not increased at melting point. In the light of the above statements, choose the correct answer from the options given below:

  • A

    Statement I is true but Statement II is false

  • B

    Statement I is false but Statement II is true

  • C

    Both Statement I and Statement II are true

  • D

    Both Statement I and Statement II are false

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Two statements about melting of ice and heat absorption during phase transition.

Find: Which statement(s) are correct.

During melting, the heat supplied is absorbed as latent heat of fusion. At the melting point, temperature remains constant until the entire solid phase changes into liquid phase. Therefore, for Statement I, heat is absorbed but the temperature does not change until all the ice melts, so Statement I is true.

For Statement II, the absorbed heat is used to overcome intermolecular forces of attraction present in ice. Since the temperature remains constant at the melting point, the average kinetic energy of the molecules does not increase during this phase change. Hence, Statement II is also true.

Therefore, both Statement I and Statement II are true. The correct option is C.

Detailed Explanation

Given: A system contains ice in equilibrium with water, and the statements describe what happens on heating.

Find: Whether Statement I and Statement II are true or false.

  1. Statement I: When a system containing ice in equilibrium with water is heated, heat is absorbed by the system, and there is no change in the temperature of the system until the whole ice gets melted.

This is the standard behavior of a substance undergoing melting at constant pressure. The heat absorbed does not raise the temperature immediately.

  1. The supplied heat is used as latent heat of fusion, which converts ice into water.

  2. As long as both ice and water are present together in equilibrium, the temperature stays constant at the melting point.

So, Statement I is true.

  1. Statement II: At the melting point of ice, heat is absorbed to overcome intermolecular forces of attraction within the molecules of water in ice, and the kinetic energy of molecules does not increase.

In ice, water molecules are held in a solid lattice by intermolecular attractions, mainly hydrogen bonding. The absorbed heat weakens and overcomes these attractions so that the solid structure can change into liquid.

  1. Since temperature remains constant during the phase transition, the average kinetic energy of the molecules does not increase at that stage.

So, Statement II is also true.

Therefore, both Statement I and Statement II are true. The correct option is C.

Common mistakes

  • Assuming that whenever heat is supplied, temperature must increase. This is wrong because during a phase change, the absorbed heat can be used as latent heat instead of raising temperature. Check whether the substance is undergoing a change of state before linking heat input to temperature rise.

  • Confusing kinetic energy with potential energy during melting. This is wrong because at the melting point the absorbed heat mainly increases potential energy by overcoming intermolecular attractions. Use temperature constancy to infer that average kinetic energy does not increase.

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