Question

What do hydrometers test?

Answer 1

According to Archimedes principle, as liquid displaces its own weight, when it floats. This makes them dense or constitutes them with specific gravity. Specific gravity is the ratio of densities of a liquid with a reference, while density of any substance is the mass by volume. Hydrometer is used to measure specific gravity of any liquid that works on Archimedes principle.

Complete answer:
As we know, Archimedes' principle states that a solid body is able to displace its own weight. This displaced weight is due to the upthrust or upward pressure that any substance exerts in a liquid called buoyancy. The pressure differences at the bottom and the top layer of the medium creates this buoyancy. Hydrometer is a device that works on this same principle.
A hydrometer is used to detect the specific gravity or the density of any liquid. When any substance is added into the solution, there is change in the density relative to that substance and the solution, this change is the relative density or the specific gravity.
A hydrometer consists of a hollow glass tube with graduations with a wide round bottom that has lead or mercury for providing stability. The liquid to be tested is poured in a container and the hydrometer is immersed in the liquid that it is able to float. A point comes when the surface of the liquid is in contact with the stem of the hydrometer, at this point the relative density is measured. Hydrometer sinks deeper in liquids with low density, and sinks less deep in liquids with high density.
Hence, a hydrometer is used to test the specific gravity or relative density of any liquid.

Note:
A hydrometer has different applications, like it can be used as a lactometer for measuring the milk density, also used as a saccharometer to measure the density of sugars in any solution. Also, used as an alcohol meter to measure the density of alcohols in spirits. Its stem gives the reading on proper calibration.