Draw the structure of common form of ice.
Or
Discuss hydrogen bonding in water and ice. 
Or
Why does ice float on water?

In the liquid state, H₂O molecules are held together by intermolecular hydrogen bonds. Each oxygen atom can form two H-bonds using each of its two lone pairs of electrons. In other words, each water molecule is generally H-bonded to four other water molecules.

Thus, in the liquid state, H₂O exists as an associated liquid.
We know that in ice, each oxygen atom is surrounded by four hydrogen atoms in such a way that two hydrogen atoms are linked to an oxygen atom by covalent bonds whereas other two hydrogen atoms are linked by hydrogen bonds. In ice (solid state), a water molecule is associated with four other water molecules through hydrogen bonding in a tetrahedral manner. This gives rise to open cage-like structure which prevents the close packing of molecules (lower density). When ice absorbs heat and melts to form water, hydrogen bonds break and close packing of water molecules take place. Due to this close packing, the density of water is higher than that of ice and ice floats over water.