Question

density of water at 80 degree Celsius is less than its density at 30 degree Celsius,why?

Answer 1

when the temperature of a liquid is increased they tend to expand thus increasing there volume for the same amount of mass.
this decreases the density of liquid and it is the same case for water

Answer 2

Density is defined as mass per unit volume. When we increase the temperature, the volume increases. As we know, volume is inversely proportional to density. So at high temperature, the density is low.