During the transformation of matter from the solid to the liquid state, the heat supplied does not produce temperature increase because

Analysis of the Question

The question asks why the temperature of a substance remains constant during the phase change from a solid to a liquid, even though heat is being continuously supplied to it. This is a fundamental concept in thermodynamics known as Latent Heat.

Explanation of the Concept

In physics, when you supply heat to a substance, it usually results in an increase in the kinetic energy of the molecules, which we measure as a rise in temperature. However, during a phase change (like melting or boiling), the substance undergoes a transition where the arrangement of its molecules changes.

• Solid state: Molecules are held in a fixed, rigid structure by strong intermolecular forces (bonds).
• Melting process: To turn a solid into a liquid, the substance must overcome these strong intermolecular forces.
• Latent Heat of Fusion: The energy supplied during this period is called the latent heat of fusion. Instead of increasing the kinetic energy (temperature) of the molecules, this energy is entirely consumed to break the bonds holding the solid structure together, allowing the molecules to move more freely as a liquid.

Step-by-Step Solution

1. Identify the process: The substance is changing state from solid to liquid (melting).
2. Apply the principle of energy conservation: Heat supplied ($Q$) = Energy used to increase kinetic energy + Energy used to change state.
3. Analyze the temperature: Since the temperature does not change, the kinetic energy of the molecules remains constant.
4. Conclusion: Therefore, all the heat supplied must be used to perform the work of breaking the intermolecular bonds (potential energy change) to transition the substance into a liquid state.

Identifying the Answer

Comparing this to the given options:

• Option A correctly describes that the heat is used to break the bonds.
• Option B is incorrect because heat capacity is not the factor causing the temperature plateau.
• Option C is incorrect because the heat is being absorbed by the substance, not conducted away.
• Option D is incorrect because this describes thermal equilibrium, not a phase change.

Exam Tip

Whenever you see a question about "constant temperature" during a phase change (melting or boiling), remember the keyword "Latent". Latent means "hidden"—the heat is "hidden" because it is doing the work of changing the state rather than raising the temperature.