The “hypo” prefix is really just a way of telling you how many oxygen atoms are in sodium hypochlorite. It means only one oxygen atom. Thus, sodium hypochlorite is NaOCl. That’s the main ingredient in bleach. The “hypo” distinguishes this chemical from sodium chlorite, which is NaClO₂.

When you mix bleach with ammonia, the sodium hypochlorite reacts with the ammonia to produce a toxic gas, chloramine (NH2Cl). The full chemical reaction is:

NaOCl + NH₃ –> NH₂Cl + NaOH

The gas is toxic and should not be inhaled.