PHYSICS · Multi-part · CBSE · Physics · Thermal Properties of Matter · Thermal Properties of Matter
Problem
A liquid is cooled from 30∘C to −15∘C at constant pressure. The temperature-time record yields the following qualitative cooling curve:
Graph description (verbal):
• The temperature decreases linearly from 30∘C to −4∘C in 6.0min.
• A brief dip occurs: the temperature overshoots to −8∘C, then rapidly rises to stabilize at −4∘C. This transient lasts 1.0min.
• A horizontal plateau follows at −4∘C for 7.5min.
• After the plateau, the temperature again decreases linearly from −4∘C to −15∘C over 5.0min.
Assume the sample mass is 60.0g and the rate of heat removal by the cooling apparatus is 85J s−1 throughout the experiment.
(a) Sketch a clearly labeled diagram of the described cooling curve. Indicate the segment corresponding to supercooling.
(b) Calculate the total quantity of heat removed during the 7.5min plateau. Explain what physical transformation is occurring in the sample during this interval.
(c) Provide a molecular-level explanation for the transient temperature dip (supercooling) observed just before the plateau, and discuss why the temperature subsequently rises without additional removal of heat.
(d) Considering the entire experiment, classify each segment of the curve (including the supercooling dip) as endothermic or exothermic from the system’s perspective, and justify each classification in terms of energy flow.