Heating Curve Phases

Steps:

• Examine graph slope: rising line means temperature increases during heating of a phase.
• Identify flat sections: constant temperature indicates phase change (melting or boiling).
• Sequence from solid: first heating (solid), melting (flat), liquid heating (rising), boiling (flat), gas heating (rising).
• Match points: X in initial rise (solid heating), Y at second flat (boiling), Z in final rise (gas heating).

Why D is correct:

• Boiling occurs at constant temperature as heat supplies latent heat of vaporization, breaking liquid-gas bonds without temp change.

Why the others are wrong:

• A: Places boiling at Y too early, skipping melting and liquid heating.
• B: Starts at liquid (X), ignoring initial solid phase.
• C: Positions melting at X (flat, not rising) and misses solid heating before it.

Final answer: D