Question

Why is a low-density liquid used as a manometric liquid in a physics laboratory?

Answer 1

We must be aware of the term manometer in order to understand this question. A look into term pressure and density and their relation with the manometer will be a better approach and a head start. The solution to this question resides within the formula of pressure in terms of density.

Formula used:
$P = \rho gh$
Where, $P$ = Pressure, $\rho$= density of manometric liquid, $g$ = acceleration due to gravity and $h$ = uprise in level of manometric liquid.

Complete step by step answer:
Manometer is an instrument used to quantify pressure. Pressure is calculated by manometer by use of manometric liquid filled in a U-shaped tube. Pressure applied in one column of U-shape tube will provide an uprise in another column of tube. Measurement of rise in level of liquid in another column and its application in formula of pressure and density will quantify the pressure.

Now according to the formula of pressure we know that pressure is directly proportional to density of manometric liquid and height of rise in level of manometric liquid. If we observe the density and height with a constant value of pressure, we can conclude that for a constant value of pressure, there can be two possibilities:
-A low-density liquid giving a visible large height gap.
-A high-density liquid giving a small rise in height.

In the laboratory we need to measure the pressure and are required to calculate the rise in height. If we use high density liquid, the rise in height will be basically so less that measurement will be either invisible or unmeasurable.

Hence, to get an evaluated rise in height a low-density liquid is used as a manometric liquid.

Note: Manometric liquid is the liquid that is filled in a manometer to measure the pressure of other liquid or gases. A manometric liquid, besides low density, should also have a defined meniscus, low surface tension and should be immiscible into fluid whose pressure is to be measured.