We have a common concept that a matter in is solid state is heavier than that in liquid state. But there is an exception in case of water. When water freezes to form ice, then for the same volume, ice turns out to be lighter.

An object can float in liquid only when it is able to displace an amount of fluid whose weight is equal to its own weight.

In a water molecule, there are two Hydrogen (H) atoms and one Oxygen (O) atom. Oxygen is more electronegative then Hydrogen. The two H atoms are bonded to a single O atom by strong covalent bonds. Since, O atom is more electronegative then H atom, so it attracts the shared pair of electrons towards itself and gets a partial negative charge. As a result, the H atoms get partial positive charge. Due to this, an atom of a molecule attracts an atom with opposite charge in its neighboring molecules. When one of the involved atoms is Hydrogen, then the bond formed is known as Hydrogen Bond. Since the bond is formed between atoms of neighboring molecules, the bond is known as Intermolecular Hydrogen Bonding.

In liquid form, water makes and breaks these weaker H-bonds due to its mobility. But when it freezes, its mobility is negligible and the H-bonds once formed cannot be broken. As a result, the molecules on frozen form are at some distance apart equal to arm length of the H-bond. This leads to a cage like structure which is less dense than an equal volume of water and hence is lighter. Ice is 9% less dense than an equal volume of water or in other words takes 9% more space.

Since, ice is lighter than water so when placed in water it starts floating.