JEE MAIN - Chemistry (2025 - 3rd April Evening Shift - No. 5)
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
Explanation
The correct choice is Option C: Both Statement I and Statement II are true.
Explanation:
Statement I is true because
During the ice–water phase change at 0 °C, any heat added goes into the latent heat of fusion.
The temperature remains fixed at 0 °C until all the ice has melted.
Statement II is also true because
The heat absorbed at the melting point is used to overcome (break) the hydrogen‐bonding forces between water molecules in ice—in other words, it increases the system’s potential energy.
Since no heat goes into raising the kinetic energy of the molecules, the temperature (which measures average kinetic energy) does not increase during melting.
Mathematically, for a mass m of ice, the heat needed is
$$ Q = m\,L_f $$
where $L_f$ is the latent heat of fusion of ice. This heat does not change the temperature but is entirely consumed in changing the phase (i.e., increasing potential energy).
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