Semiheavy water
Semiheavy water is a naturally occurring chemical variant of water. Other variants include heavy water, and HO. It shares most of its properties with common water, with its distinguishing feature being a single hydrogen atom per water molecule being of the heavier isotope deuterium, as opposed to the far more common protium. This difference of one more neutron in one of the hydrogen atoms does not notably change its chemical properties, as these are mostly dictated by the number of valence electrons an atom has. The only notable difference to common water is a larger mass, as a result of the extra neutron per atom.
In an amount of water, about 1 molecule in 3,200 is HDO. By comparison, heavy water occurs at a proportion of about 1 molecule in 41 million. This makes semiheavy water far more common than heavy water.
Semiheavy water cannot be isolated in its pure liquid form since, owing to hydrogen exchange in water, it is in equilibrium with HO and DO. If however it were possible to weigh the same volume of HDO and HO, the HDO would be heavier, lending it its name.