H 2 S gas and Iodine solution

Hydrogen sulfide, H 2 S(g), is made by reacting a metal sulfide with acid. The gas is poisonous and has an unpleasant smell (of rotten eggs), so this reaction is done in a fume cupboard.

White lead acetate paper can be used to detect the presence of H 2 S gas.

The lead acetate paper turns black, confirming the presence of H 2 S gas. Be grateful you can’t smell the gas too!

The gas is bubbled through iodine solution. The orange colour slowly fades. The final solution is cloudy.

Against a dark background it is obvious that the liquid contains a suspended white solid. There is a distinct odour of elemental sulfur. This substance is called colloidal sulfur. The particles are so fine they do not settle out and will pass through filter paper.

H 2 S contains sulfur in the -2 oxidation state. Sulfur cannot be reduced any further. The H 2 S is oxidised to solid sulfur: H 2 S(g) → S(s) + 2H + (aq) + 2e - The I 2 has been reduced to I - : I 2 (aq) + 2e - → 2I - (aq) H 2 S(g) + I 2 (aq) → S(s) + 2H + (aq) + 2I - (aq)