Clean water is a basic need for everyone. But this drinking water lake, consists of dirt particles too. In terms of chemistry, water = H2O molecule.
They both have same molecules of water but some dirt particles are mixed in one of them which is unfit to drink.
Regardless of clean or dirt, the chemical formula of water remains the same. This idea is called "Law of constant proportion" i.e. hydrogen & oxygen combine in a fixed 2:1 proportion forming water molecule. If the ratio changes, we get a different molecule i.e. hydrogen peroxide with ratio 2:2 by combining 2 hydrogen & 2 oxygen atoms, which isn't water!