Question

which of the following is a good conductor of heat?
(A) Plastic
(B) Mercury
(C) Water
(D) Alcohol

Answer 1

Hint : Thermally conductive materials tend to distribute temperature quickly about its entire volume even without the bulk movements of its molecules. Relative to non-metals, metals conduct heat better even in their liquid state. Higher thermal conductivities mean higher conductive capability.

Complete step by step answer
In general, whether a material is a good conductor or not is dependent on the value of its thermal conductivity. A low value of thermal conductivity denotes a non-conductive material called insulators or more precisely thermal insulators, and a high thermal conductivity value denotes very conductive material called thermal conductors.
Plastic is a class of materials generally considered as thermal insulators, due to their low thermal conductivity. On average the thermal conductivity of plastic is given as about $ 0.2W/m \cdot K $ . One reason plastic easily melts is because it doesn’t quickly distribute the heat absorbed to other parts of itself, hence making the part exposed to heat to reach melting point very quickly.
Mercury is a metal which happens to be its liquid state at room temperature. The thermal conductivity is about $ 8.30W/m \cdot K $ .
Water – this is also a very poor conductor of heat. It “conducts” its heat mostly through the macroscopic movement of its molecules, a process called convection. Fresh water has a thermal conductivity of $ 0.6W/m \cdot K $ .
Alcohol – this is another class of liquids also thermally non-conductive. They have a conductivity averagely around $ 0.2W/m \cdot K $
Hence, from these, we see that mercury has the highest thermal conductivity
Thus, option B is correct.

Note
Although, of all the options mercury is the best conductor but. In actuality, mercury is considered a poor conductor of heat relative to other metals. For example, copper has a thermal conductivity of about $ 385W/m \cdot K $ and aluminium of about $ 205W/m \cdot K $ .