Mercury is generally used in thermometers because it has
A. High fluidity
B. High density
C. High coefficient of expansion
D. High specific heat

Hint: The metal used in the thermometer should be a good conductor of heat as well as it should have a high coefficient of expansion. Only metal that is in the liquid state at room temperature is mercury. Recall the properties of mercury.

Complete step by step answer:
We know that solids expand when they are heated. The metal used in the thermometer should be a good conductor of heat as well as it should have a high coefficient of expansion. The high coefficient of expansion implies that the liquid expands at a greater rate with increase in temperature. Only metal that is in the liquid state at room temperature is mercury.

We know that mercury is in liquid state at room temperature and it has a high expansion coefficient. Therefore, a slight change in temperature will result in a noticeable rise in the height of mercury in the thermometer column. Also, the melting and boiling point mercury is very high. Therefore, the mercury remains in liquid state for a wide range of temperature.

The viscosity of mercury is greater than water at room temperature. This implies that the mercury does not have high fluidity. Therefore, the option (A) is incorrect.

High density materials expand at a relatively slower rate than the materials with lower density. Therefore, the high density of mercury is of no avail. Therefore, the option (B) is incorrect.

Mercury has relatively lowest specific heat. Its temperature does not change rapidly with heat. Therefore, the option (D) is incorrect.

So, the correct answer is “Option C”.

Note:
Note that mercury is a solid which does not expand after freezing. Therefore, mercury is the most promising metal which can be used in thermometers. The metal used in thermometers should not stick to the walls. Therefore, the cohesive forces should be stronger than the adhesive forces for that metal.