What is relative density?

We compare the density of different substances with the density of water at 4°C.

Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance".

4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only.

There is no unit of relative density as it is  the ratio of two similar quantities.

The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7.
It means that the substance is 7.7 times heavier than water.

LAW OF FLOATATION

"For a floating body, the upthrust experienced by it is equal to its weight."
Net force  on the floating  body is zero

What is Regelation?

The melting point of ice is 0°C at normal pressure but when you increase the pressure on ice it melts at the temperature below 0°C( i.e. melting point of ice decreases with pressure).
When the pressure applied is released then water formed from ice starts freezing. This phenomenon is called REGELATION.