Iron sulfide or ferrous sulfide is a chemical compound with the formula FeS. Powdered iron sulfide is pyrophoric (ignites spontaneously in air)

                                           Iron sulphide is formed by chemical reactions that take place if an egg is heated for a long time. The protein in an egg yolk contains iron which is released as the protein uncoils on heating. On the other hand egg white protein contains the elements hydrogen and sulphur, which are released as the protein unfolds.       The hydrogen and sulphur combine to make the foul smelling substance hydrogen sulphide. And due to prolonged cooking of an egg in its shell (e.g. when immersed A boiling water for 15 minus or longer) egg white’s hydrogen sulphide and the egg yolk’s iron then react together to form green iron sulphide on the surface of the yolk. This doesn’t mean the egg is bad.  This It is also called "hard-boiled egg".

           To minimize this problem immersed egg in cold water immediately after cooking, the green colour is either not apparent at all, or is much lees marked than is the case when the egg is allowed to cool slowly.  Because then hydrogen sulphide tends to migrate to the colder shell and away from the yolk.

Experiments relating to the Membrane round the Yolk.

           To determine whether or not any constituent of the membrane round the yolk takes any part in the reaction, an experiment was made as follows.

 

1.      Take a fresh egg

2.      Break the egg and collect egg white and yolk separately

3.      Heat egg white in a test tube until coagulated

4.      After that pour egg yolk, free from membrane, into the test tube to form a layer above the white.

5.      On heating the test tube for about 15 minutes in boiling water, a green ring was obtained at the junction of the yolk and white.

6.      On examining a “hard-boiled" egg, the colorless yolk membrane can be seen.

 

           It shows that the green coloration was due to interaction between con­stituents of both the yolk and the white was obtained in the following way. Yolk of egg, separated from egg white, not produce green colour. Because iron content of egg white (0.0001 %) is only about one-eighty-fifth that of egg yolk, egg white is free from iron. The sulphur content of the yolk of an egg 0.187 % is not very much leas than that of the white 0.196 % this element is evidently in less stable combination in the later than in the former.

 Chemical reactions

            Iron sulfide reacts with hydrochloric acid, releasing the malodorous (rotten egg smell) and                       very toxic gas, hydrogen sulfide

FeS + 2 HCl → FeCl2 + H2S

FeS can be obtained by the heating of iron and sulfur:

Fe + S → FeS