Why is mercury used as a barometer liquid? Give three reasons.

Hint: Barometer is an instrument which is used to measure pressure of a system. It consists of an inverted glass tube standing in a bath of mercury. When air pushes down the surface of the mercury, it will rise by some amount inside the tube. With greater pressure, the rise in mercury inside the tube will also be high.

Complete answer:
In the barometer mercury is used as the barometric liquid. Mercury has many advantages when used as a measuring liquid in a barometer tube.

Mercury has high density. Because of this property, when there is a change in pressure the change in height of the mercury in the barometer tube will be very reasonable. Because of this property of mercury, we can measure very high pressure with the help of a barometer while if we use other lower density material, the change in height with pressure will be very high and building such apparatus will be very difficult.

Again, mercury is shining metal which can be easily identified inside the barometer tube for taking measurements. Because of this shining property of mercury, it can highlight its readings clearly.
Also, comparatively mercury has a lower specific heat than other elements. Mercury is also a good conductor of heat. So, if the temperature of the system changes, mercury can come to that temperature more quickly.

Note:
Barometer is used to measure air pressure and the pressure tendency can forecast short term changes in the weather.
Again, mercury is also the only metal which is in liquid state at room temperature. It has a high coefficient of expansion and hence we can also use it as thermometric fluid.