Identifying phase changes on a state diagram

Steps:

• Examine point P: gas turning to liquid indicates condensing.
• Examine point Q: liquid turning to solid indicates freezing.
• Examine point S: solid turning to liquid indicates melting.
• Cross-check option B matches these definitions for P, Q, and S.

Why B is correct:

• Condensing (P) is gas to liquid, freezing (Q) is liquid to solid, and melting (S) is solid to liquid, per standard phase transition definitions.

Why the others are wrong:

• A misassigns boiling to P (gas to liquid is condensing, not boiling).
• C misassigns freezing to P (gas phase change is condensing, not freezing).
• D misassigns melting to P (initial gas phase requires condensing first).

Final answer: B