A chemist slowly heats a 50g sample of an unknown pure crystalline solid at atmospheric pressure and records the temperature every 30s. The data would produce the following qualitative graph if plotted:
Graph description (verbal):
• The plot of temperature (y-axis) vs. time (x-axis) begins with a straight, positively sloped line from 15∘C to 38∘C.
• A horizontal plateau then extends for 4.0min at 38∘C.
• The curve resumes a positive slope, rising from 38∘C to 92∘C.
• A second horizontal segment lasts 6.5min at 92∘C.
• Finally, the temperature again increases linearly from 92∘C to 120∘C.
Assume no mass is lost and no chemical reaction occurs during the experiment.
(a) Identify each of the five distinct segments (two sloped regions before, between, and after the plateaus, plus the two plateaus) as either heating of a single phase or a phase change. For each segment, state the physical phase(s) present.
(b) For each segment, classify the process occurring in the system as endothermic or exothermic. Justify your choices using energy transfer principles.
(c) The student claims that the longer plateau indicates the substance has a larger molar enthalpy of phase change at 92∘C than at 38∘C. Evaluate this claim, citing specific evidence from the graph and thermodynamic reasoning.