Phase changes occur at constant temperature during heating

Steps:

• Constant heat input raises temperature linearly in single phases (solid, liquid).
• Temperature plateaus when energy causes phase transition (melting, boiling) instead of temperature rise.
• Graph sections: W (solid heating), X (melting plateau), Y (liquid heating), Z (boiling plateau).
• Constant temperature sections are X and Z, indicating phase changes.

Why C is correct:

• Latent heat of fusion/vaporization absorbs energy without temperature change (first law of thermodynamics: Q = mL, no ΔT).

Why the others are wrong:

• A: W involves rising temperature in solid phase.
• B: W and Y involve rising temperatures in solid and liquid phases.
• D: Y involves rising temperature in liquid phase.

Final answer: C