It is tasteless, colourless and odourless gas, which is exceedingly stable and inert.
The salts of the acid are colourless or faintly yellow.
The blood is colourless.
Examples of the first case are found among the colourless acridines and quinoxalines which give coloured salts; of the second case we may notice the colourless hydrochloride and sulphate of the deep yellow o-aminobenzophenone.
They are either colourless liquids, which boil without decomposition, or crystalline solids; and are both basic and acidic in character.