Hydrofluoric acid
An aqueous solution of hydrogen fluoride, HF; colourless, fuming, poisonous liquid; extremely corrosive, it is a weak acid as compared to hydrochloric acid, but will attack glass and other silica materials; used to polish, frost, and etch glass, to pickle copper, brass, and alloy steels, to clean stone and brick, to acidize oil wells, and to dissolve ores.

Hydrogen
A colourless gaseous element. Hydrogen has some similarities to both the alkali metals (group 1) and the halogens (group 17) but is not normally classified in any particular group of the periodic table. It is the most abundant element in the Universe and the ninth most abundant element in the Earth’s crust and atmosphere (by mass). It occurs principally in the form of water and petroleum products; traces of molecular hydrogen are found in some natural gases and in the upper atmosphere.

Hydrogen chloride
A colourless fuming gas, HCI; m.p. – 1140C; b.p. – 850C. It can be prepared in the laboratory by heating sodium chloride with concentrated sulphuric acid (hence the former name spirit of salt). Industrially, it is made directly from the elements at high temperature and used in the manufacture of PVC and other chloro compounds. It is a strong acid and dissociates fully in solution (hydrochloric acid).

Hydrogen cyanide
A colourless liquid or gas, HCN, with a characteristic odour of almonds; r.d. 0.699 (liquid at 220C); m.p. – 140C; b.p. 260C. It is an extremely poisonous substance. With organic carbonyl compounds, it yields cyanohydrins.