Question

why only water is used for the comparison of relative density​

Answer 1

Answer:

We calculate relative density with respect to water. Why? Generally, the density of water at 4 °C (the temperature at which the density of water reaches its maximum value) is used as the reference to measure relative density.

Answer 2

Explanation:

Relative density, or specific gravity,[1][2] is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" is often preferred in scientific usage.