Concept:A pure solid, when heated steadily, shows constant temperature during phase changes.
Explanation:The substance starts as a solid, so its temperature first rises with time.
At segment
P, the temperature becomes constant despite continuous heating.
This constant temperature indicates a change of state from solid to liquid, which is melting.
After melting is complete, the temperature of the liquid rises again.
At segment
Q, the temperature becomes constant once more.
This second plateau represents the change of state from liquid to gas, which is boiling.
Hence, the processes at
P and
Q are melting and boiling respectively.
Answer:C. Melting and boiling
