Iron is a silver-grey solid. When we sprinkle iron filings onto water, they sink showing that iron is a dense material. Iron is a magnetic material iron filings are attracted to a magnet and will stick to it. Like all metals iron conducts electricity. Sulphur is a yellow powder. It is much less dense than iron and if sprinkled on water it will float.
If iron is mixed with sulphur, the resulting mixture has the properties of both iron and sulphur. The mixture is grey like iron but has yellow specks of sulphur. If the mixture is sprinkled into water, the iron filings will sink and the yellow sulphur will float.
If the mixture is heated, a glow is seen to spread through the mixture due to the heat generated by the chemical reaction which takes place. A reaction which produces like this is called an exothermic reaction.
During this reaction the iron and sulphur combine together to form a compound called iron sulphide. This compound is quite unlike the iron or the sulphur it is made from. For example, the iron sulphide is not affected by a magnet. If powdered iron sulphide is sprinkled into water and it all sinks. The powder is almost black, with no signs of yellow sulphur. It doesn’t conduct electricity. The iron sulphide like all compounds is a single pure substance.