Question

why does weight of water become less when it's state changes in ice from liquid

Answer 1

Since mass is constant, but volume increases, it means that the density of frozen water is lower is lower than liquid water. The practical upshot is that the less dense ice floats on the more dense water. Everything in the world is made up of tiny molecules, and each molecule weighs a certain amount.

Answer 2

Because water expands when it freezes.
Since the volume of water increases when it freezes, its density must decrease (density = mass /volume). This means the density of ice is lower than the density of water. Naturally then ice floats in water, as do icebergs in the sea.