What are compounds?
A A substance formed by the chemical union of two or more elements. contains atoms of different A substance made of one type of atom only., chemically joined together. Compounds form in chemical reactions, and you need other chemical reactions to separate a compound into its elements. The diagrams show what happens when iron filings and sulfur powder react together in a chemical reaction, rather than just mix together.
The iron and sulfur atoms in iron sulfide are joined together, so they cannot easily be separated
Properties of mixtures and compounds
Mixtures have different properties from compounds. The table summarises these differences:
| Mixture | Compound | |
| Composition | Variable composition 鈥 you can vary the amount of each substance in a mixture | Definite composition 鈥 you cannot vary the amount of each element in a compound |
| Joined or not | The different substances are not chemically joined together | The different elements are chemically joined together |
| Properties | Each substance in the mixture keeps its own properties | The compound has properties different from the elements it contains |
| Separation | Each substance is easily separated from the mixture | It can only be separated into its elements using chemical reactions |
| Examples | Air, sea water, most rocks | Water, carbon dioxide, magnesium oxide, sodium chloride |
| Composition | |
|---|---|
| Mixture | Variable composition 鈥 you can vary the amount of each substance in a mixture |
| Compound | Definite composition 鈥 you cannot vary the amount of each element in a compound |
| Joined or not | |
|---|---|
| Mixture | The different substances are not chemically joined together |
| Compound | The different elements are chemically joined together |
| Properties | |
|---|---|
| Mixture | Each substance in the mixture keeps its own properties |
| Compound | The compound has properties different from the elements it contains |
| Separation | |
|---|---|
| Mixture | Each substance is easily separated from the mixture |
| Compound | It can only be separated into its elements using chemical reactions |
| Examples | |
|---|---|
| Mixture | Air, sea water, most rocks |
| Compound | Water, carbon dioxide, magnesium oxide, sodium chloride |