Question

At its melting point ice is lighter than water because

• A. ${H_2O}$ molecules are more closely packed in solid state
• B. Ice crystals have hollow hexagonal arrangement of ${H_2O}$ molecules.
• C. On melting of ice the ${H_2O}$ molecule shrinks in size
• D. Ice froms mostly heavy water on first melting.

Answer 1

Answer: (B)

Ice crystals have hollow hexagonal arrangement of ${H_2O}$ molecules.

Answer 2

In the structure of ice each molecule of $H_2O$ is surrounded by three $H_2O$ molecules in hexagonal honeycomb manner. On the other in water, each molecule is surrounded by four neighbouring molecules randomly which results an open cage like structure. As a result there are a number of 'hole' or open spaces. In such a structure lesser number of molecules are packed per ml. When ice melts a large no. of hydrogen bonds are broken. The molecules therefore move into the holes or open spaces and come closer to each other than they were in solid state. This result sharp increase in the density. Therefore ice has lower density than water.