Hydrogen sulphide
(sulphuretted hydrogen, H₂S). A colourless very poisonous gas having an adour of bad eggs. Hydrogen sulphide could be obtained by reacting hydrochloric acid with iron (II) sulphide. It is tested for by mixing with lead nitrate, with which it gives a black precipitate. Its aqueous solution is weakly acidic.
Hydrogen sulphide reduces iron (III) chloride to iron (II) chloride, forming hydrochloric acid and a yellow precipitate of sulphur. Hydrogen sulphide precipitates insoluble sulphides, and is used in qualitative analysis. It burns with a blue flame in oxygen to form sulphur (IV) oxide and water. North sea gas is having some hydrogen sulphide, which is removed before supply to the consumer.

Hyperons
Elementary particles of mass 2185 times that of the electrons; intermediate between a proton and a deuteron. The best established hyperson is the lambda hyperon of zero charge but it is very likely that charged hyperons exist. Hypersons are found in cosmic radiation.

Hypochlorite
CIO₃, a negative ion derived from hypochlorous acid. HCIO; the ion is an oxidizing agent and a constituent of bleaching agents.

Hyposulphurous acid
Dithionous acid H₂S₂O₄. It is prepared by adding zinc dust to saturated sodium hydrogen sulphite.