-60.28 °C (212.87 K)

Hydrogen sulfide (or hydrogen sulphide) is the chemical compound with the formula H2S. It is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers (anaerobic digestion). It also occurs in volcanic gases, natural gas, and some well waters.

While the human body produces small amounts of H2S and uses it as a signaling molecule, larger amounts are extremely poisonous. A person in an industrial site or other location where it is likely present, should leave the area immediately and move to higher ground upon smelling the rotten egg smell. Do not attempt to rescue a person who has succumbed to it without a respirator. Also, don't assume an area is safe if you no longer smell it, as Hydrogen sulfide deadens the sense of smell.

Hydrogen sulfide has numerous names, some of which are archaic (see table).

Hydrogen sulfide is slightly heavier than air; a mixture of H2S and air is explosive. Hydrogen sulfide and oxygen burn with blue flame to form sulfur dioxide (SO2) and water. In general, it acts as a reducing agent.

At high heat or in the presence of catalysts, sulfur dioxide can be made to react with hydrogen sulfide to form elemental sulfur and water. This is exploited in the Claus process, the main way to convert hydrogen sulfide into elemental sulfur.

Hydrogen sulfide is slightly soluble in water and acts as a weak acid, giving the hydrosulfide ion HS− (pKa=6.9 in 0.01-0.1 mol/litre solutions at 18°C) and the sulfide ion S2− (pKa=11.96). A solution of hydrogen sulfide in water is initially clear but over time turns cloudy. This is due to the slow reaction of hydrogen sulfide with the oxygen dissolved in water, yielding elemental sulfur, which precipitates out.

Hydrogen sulfide reacts with metal ions to form metal sulfides, which may be considered the salts of hydrogen sulfide. Some ores are sulfides. Metal sulfides often have a dark color. Lead(II) acetate paper is used to detect hydrogen sulfide because it turns grey in the presence of the gas as lead(II) sulfide is produced. Reacting metal sulfides with strong acid liberates hydrogen sulfide.

If gaseous hydrogen sulfide is put into contact with concentrated nitric acid, it explodes.

It reacts with alcohols to form thiols.