Liquid Cesium (melting point 28ºC) is cooling from an initial temperature of 35ºC when the readings on the thermometer stop going down in temperature. What is probably observed when this takes place?

a) The temperature will slow its descent at 29ºC as the initial flakes of solid form and continue to drop to room temperature as the cesium becomes solid.

b) The temperature will stop decreasing and stay at 28ºC until the mass of cesium becomes solid.

c) The temperature slows at 32.5º C as solid flakes begin to form, halfway between the initial temperature and the melting point.

d) The temperature stops briefly at 28ºC, and the cesium solidifies once the temperature begins dropping again.

Explanation

Heat can flow in or out of a system while that system is at constant temperature in two cases: during the phase change or if the system is doing work.

Here, heat is flowing out of the system as the Cs cools. If the temperature stops dropping, it is because the Cs is freezing. The temperature will remain constant during the entire phase change.

a) incorrect, temperature drop stops at the melting temperature where is stays until the phase change is complete, not above and it does not fall during solidification

b) correct

c) incorrect, the temperature drop stops at the melting temperature and solidification does not occur until the cesium reaches the melting temperature

d) incorrect, solidification occurs at the melting temperature, not below

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