The change of state from solid to liquid occurs

Analyze the question: It asks about the change of state from solid to liquid, which is melting (fusion). We need the statement that best describes it.

Concept: During melting, a pure solid absorbs heat energy called latent heat of fusion. This energy goes into breaking the bonds holding the particles in fixed positions. It does not increase the kinetic energy of the particles. So the temperature stays constant at the melting point until all the solid has melted.

Checking each option:

• A. at constant temperature: Correct. Melting happens at a fixed temperature, the melting point (for example, ice at 0 °C), and the temperature does not rise until melting is complete.
• B. when temperature is lower than the boiling point: This is true in a loose sense, since the melting point is below the boiling point. But it does not describe the process itself, so it is not the best answer.
• C. when temperature is just near freezing point: "Near" is vague. Melting occurs exactly at the melting point, which is the same as the freezing point for a pure substance. It is not just "near" it.
• D. when temperature is equal to the boiling point: Wrong. Equality with the boiling point applies to the liquid-to-gas change, not melting.

Answer: The key feature of melting is that it occurs at constant temperature while latent heat is absorbed, so the answer is A.

Tip: During any change of state (melting, boiling, freezing, condensation), the temperature stays constant while latent heat is being absorbed or released. On a heating curve, these changes show up as flat horizontal lines. If an option mentions "constant temperature" with a change of state, it is very likely correct.