A hot liquid X was allowed to cooled in air. The temp. was measured every 5 sec. Y axis shows temp. in deg. C of 10 deg. intervals & X axis shows time in sec. of 5 sec. intervals. Starting from 70 deg. it falls to 50 deg. in 5 sec., stayed same up to 15 sec., than starts falling again to 33 deg. in 25 sec., stayed same onwards. What is the melting point of X, room temp. & in which part of the graph is energy being given out?

Expert Answers

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What is happening here is that you have a liquid which has been heated above its melting point. As you allow the liquid to cool the temperature drops until it reaches its melting point. The entire time the temperature is falling, the liquid is radiating heat energy to the surrounding air.  At the melting point the temperature remains constant as the liquid starts solidifying and changing to a solid.  During this phase change from liquid to solid the temperature remains constant but heat energy is still continually being released to the surrounding air. Once the liquid has changed to a solid, you now have a solid at a temperature of 50 C.  Because this temperature is higher than the surrounding temperature the solid substance will continue to cool by again radiating heat energy to the surrounding air until it reaches room temperature at which point equilibrium conditions have been reached.

Thus, the melting point is 50 C and the room temperature is 33 C and energy is being released by the substance throughout the cooling process.

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