Question

What are two reasons why mercury rather than water is used in barometers?

Short answer

Mercury is denser than water and retains its liquid state over a wide range of temperature. These leads to preference of mercury over water in barometer.

Step-by-step solution

The conceptual Introduction

Fluid statics, often known as hydrostatics, is a branch of fluid mechanics that investigates the state of balance of floating and submerged body, as well as the pressure in a fluid, or imposed by a fluid, on an immersed body.

The first reason

The pressure is the product of height, density, and acceleration due to gravity.

This can be mathematically expressed as;

$\mathit{P}\mathbf{=}\mathit{h}\mathit{\rho }\mathit{g}$

Where, h is the height of the air column, ρ is the density of air and g is the acceleration due to gravity.

Mercury has a density that is around times that of water. Because density is inversely proportional to height, a barometer filled with water must be times higher than a barometer filled with mercury to measure the same pressure.

The second point

Water becomes vapour at a high temperature and turns to ice at a low temperature. So, it can’t be used in all climatic conditions. Mercury, on the other hand, remains liquid throughout a vast temperature range when compared to water.

So, mercury is preferred over water to fill in barometer due to its high density and ability to remain in liquid over a long range of temperature.