Question

why we did not take oxygen and respire in water although it contains (H2O) both hydrogen and oxygen molecule ? give reason for answer

Answer 1

Humans can't breathe underwater. Chemically when hydrogen and oxygen gases combine we get liquid water. Oxygen is bound to two hydrogen atoms in water and so we cannot breathe .
then raises the doubt how do fishes breathe? Fishes have the capacity to remove the dissolved oxygen through their gills. But human lungs do not have this capacity.