Question

At its melting point ice is lighter than water because?

Answer 1

Water in solid state is known as ice. It is known that the density of ice is lesser than that of liquid water, because of this reason ice floats on water. Ice has a hexagonal framework made by the Hydrogen Bonds.

Complete answer:
The structure of ice is found to be very unique. When water freezes it forms a Lattice structure which doesn’t occur in any substances other than ice. Ice has a rigid lattice structure in a tetrahedral crystalline form, which comes together to form a giant molecule. Unlike in liquid water, hydrogen bonds play a very important role in the density of ice. The covalent hydrogen bonds in ice cause the water molecules to be packed together very closely and tightly. The repelling nature of like-like charges is required to overcome this strong attraction, because of which the ice becomes less dense as it freezes.
Hydrogen bonds are present in liquid water as well, but water being a fluid and due to it being in motion always, the Hydrogen bond tends to break and form continuously. As the temperature falls, this motion of molecules slows down and the molecules start coming closer to each other. Where basically, in such a case the density increases, in water as the temperature drops it starts to form a hexagonal network, with empty space in the middle of the hexagons. Hence, ice has a greater number of holes or empty spaces.
When ice melts, the hydrogen bond present between the molecules starts breaking and the molecules start moving into these empty spaces or holes. Because of this, the molecules come closer to each other, than they were in ice form (because there is no empty space left). Since the no. of molecules increases drastically per mL, the density also increases.
Therefore, because of the above-mentioned reasons, ice is lighter than water.

Note:
At ${4^ \circ }C$ the density of ice is still greater than water, as it approaches ${0^ \circ }C$ it starts freezing. The intermolecular Hydrogen bonds form a hexagonal pattern. It doesn’t mean that ice and water have different structures. The difference is just that the empty space left in between is greater in ice but is minimum in liquid water.