Question

why mercury is bad conductor of heat

Answer 1

Elemental mercury is a liquid metal (one of the very few liquid metals). Like all metals, mercury is an excellent conductor of heat. This is one of the reasons why mercury is used in a traditional mercury thermometer. The thermometer bulb is brought in close contact in the test object (e.g. under the arm pit of a person), and mercury expands upon heating, and pushes the level up in the capillary tube of the thermometer. Due to the goose-neck construction of the capillary, the liquid does not automatically pour back down into the bulb upon cooling. This is how you are able to “read” the temperature of an object using a mercury thermometer.

In short, mercury is an excellent conductor of heat.