Why is Ice less dense than water?
To answer this question we need to think about this in two parts. In the first part, we need to understand how anything can float in water. The second part is to understand why ice floats in water without sinking. Any object floats if it has fewer mass per unit volume, or if it is less dense. When you put some handful of stones into a bucket of water, all the stones will sink. The stones are denser than the water and hence can displace the water easily. If an object has to float, it has to displace the amount of water same as that of its weight. Only in that situation the object can float, otherwise it sinks.
Coming to the reason why ice floats in water is due to the anomalous expansion of water. Water has maximum density at the temperature of four degrees centigrade. If the temperature becomes down and reaches zero degrees centigrade then it becomes less dense. Many objects or substances will have more density in their solid state than in liquid state. But, in the case of water the condition is different. Water is denser at 4 degrees centigrade and less dense at zero degrees Celsius. This is due to the peculiar hydrogen bonding that exists between the positively charged hydrogen and negatively charged oxygen atoms of two different water molecules.
The hydrogen and oxygen atoms in water are attracted to form a covalent chemical bonding in between them. If the water cools below 4 degree C, the hydrogen bonds are adjusted to bind with the oxygen atoms. The regular arrangement of oxygen atoms will give a lattice structure to the water known as ice’.
As ice is less dense than water, it floats. It is found that ice occupies 9 percent more volume than water and hence it weighs lesser than water. As ice displaces the heavy weight water, it floats on the top of water. This is the reason why at the places like rivers and lakes several fresh water animals survive below the ice inside waters, as ice on the top floats. It prevents the water underneath not to freeze and create an environment for the life, to survive. Otherwise, at cold temperatures rivers and lakes would have been filled with ice killing the creatures inside.