If same mass of liquid water and a piece of ice is taken, then why is the density of ice less than that of liquefied water ?

Explanation

In ice, molecular of `H_(2)O` are not packed so closely as in liquid water. There exists vacant space in the crystal lattice. This results in large volume and lower desnity (density = mass/volume)
In other words, density of ice is lower than liquied water and hence ice floats on water.