Nature of Matter: Elements, Compounds, and Mixtures
Iron and sulfur: a mixture beside a compound
Compare a mixture and a compound made from the same two elements, iron and sulfur.
Activity 8.5 puts a mixture and a compound side by side, made from the same two elements. It's a teacher demonstration, done in a fume cupboard or a well-aired room: don't breathe in the gases.
Take 5.6 g of iron filings and 3.2 g of sulfur powder and mix them well: Sample A. Can you still see both? Yes: black bits and yellow bits.
Heat half of Sample A gently in a china dish, stirring all the time, until it forms a black mass. Let it cool and grind it: Sample B. Now compare the two.
| Test | Sample A (mixture) | Sample B (heated) |
|---|---|---|
| how it looks | black and yellow particles you can tell apart | the same colour and texture all through |
| a magnet moved over it | pulls the iron filings out | nothing is attracted |
| dilute hydrochloric acid added (waft the smell, never sniff directly) | the iron gives hydrogen: no colour, no smell, burns with a pop; yellow sulfur is left unchanged | gives hydrogen sulfide: no colour, but smells of rotten eggs |
Iron + dilute hydrochloric acid → Iron chloride + Hydrogen
Iron sulfide + dilute hydrochloric acid → Iron chloride + Hydrogen sulfide
So in Sample A the iron and sulfur are just mixed: each keeps its own properties, and a magnet separates them. Heating made them join chemically into a new substance, iron sulfide. It's a compound with completely new properties: there's no free iron left, so the magnet does nothing, and iron and sulfur can no longer be separated by physical means.
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