Question

Below are the heating-cooling curves for five different substance. Heat is added uniformly to the same mass of each substance at exactly the same rate.
Which substance has the greatest latent heat of fusion?

• A. 1
• B. 2
• C. 3
• D. 4
• E. 5

Answer 1

Answer: (B)

2

The latent heat of fusion of a substance is the amount of heat that must be added to or taken away from a substance to change a certain mass of a substance from solid to liquid or liquid to solid at constant temperature.

When looking at the heating-cooling curves, the slanted portions of the graphs represent the solid state (lowest), the liquid state (middle height), and the gas state (highest). The level parts represent changes of state. The lower part represents a change from solid to liquid as the graph goes from left to right. The higher part represents a change from liquid to gas as the graph goes from left to right. Longer level lines indicate higher latent heats (takes more heat to change state).

In this case, we are looking for the greatest latent heat of fusion, so we're looking for the substance whose graph has the longest low level line. The substance whose graph meets this condition requires the most heat to change the substance from solid to liquid, once the solid substance has been heated to the melting point.

The longest low level line is seen on the graph for substance 2,

so the correct choice is option "b"