Question

At its melting point ice is lighter than water because

(a) H₂O molecules are more closely packed in solid state

(b) ice crystals have hollow hexagonal arrangement of H₂O molecules.

(c) on melting of ice the H₂O molecules shrinks in size

(d) ice forms mostly heavy water on first melting.

Answer 1

Option b is correct. 

In ice crystal water molecules are linked though H-bond in hollow hexagonal arrangement as shown above. In liquid state  this hollow arrangement breaks in to more closer arrangement of molecules. Consequently the density is increased in liquid state. So the ice becomes lighter than water due to its hollow structure.

Answer 2

The correct option is: (b) ice crystals have hollow hexagonal arrangement of H₂O molecules.

Explanation:

When ice melts, its molecules move into the holes or open spaces and comes closer to each other than they were in solid state. Thus, ice has lower density than water and there is contraction in volume.