Question

Why is ice lighter than water?

Answer 1

Hint: Water in its solid state is called ice. Understand the way water molecules are arranged in ice. It is known that ice floats on water. Try to find if voids are present in the structure of ice.


Complete step by step answer:
When liquid water freezes, water molecules form a crystalline structure maintained by hydrogen bonding. Solid water, or ice, is less dense than water. The density of water is higher than the ice because the orientation of hydrogen bonds causes molecules to push farther apart, which lowers the density. Hence, water is heavier than ice.
As we all know, a water molecule is made of one oxygen atom combined with two hydrogen atoms as shown below. At normal temperature, the water molecules are held together in a liquid state because of the intermolecular attraction of the water molecules. In a liquid state, the water molecules are constantly whizzing around in the container and are rearranged constantly.
When the water freezing over into ice is held together not by the O-O attraction but by the O-H attraction. The lattice arrangement of ice prevents the movement of water molecules. Since the H-O is not quite as tight as the O-O bond, it experiences a little expansion once the O-H bond takes over. Hence Ice is lighter than water.



Note: It should be noted that ice has greater density at 4oC. At this point also, it starts freezing. When the temperature is further dropped to 0oC, the intermolecular hydrogen bonding leads to a three-dimensional cage structure leaving half of the space vacant. It doesn’t mean that the structure of water and ice is different. It is just that vacant space occupied by water in liquid state is more.