An element has one kind of atom. A compound has two or more kinds of atom chemically joined in a fixed ratio. A mixture has substances that are only physically combined.
This lesson belongs to SPM Science elements and materials and uses the atom-reading skill from reading atomic structure information.
How do you sort a substance in three questions?
Ask these in order and stop when you get a yes.
- Is it made of only one kind of atom? If yes, it is an element.
- Are two or more kinds of atom chemically joined in a fixed ratio? If yes, it is a compound.
- Are two or more substances just mixed, with no fixed ratio? If yes, it is a mixture.
Worked example: iron and sulfur
An invented class has 7 g of iron filings and 4 g of sulfur powder. First the two are stirred together. Then the same two are heated strongly until they react.
After stirring. A magnet pulls out the iron, and the sulfur stays behind. Each substance keeps its own properties, and the amounts can be any ratio. This is a mixture.
After heating. The product is grey-black iron(II) sulfide. A magnet no longer separates it, because the iron atoms are now chemically joined to sulfur atoms in a fixed ratio. This is a compound, with properties different from both starting substances.
The same atoms gave two different results, and the three categories explain why.
What separates a compound from a mixture?
| Feature | Compound | Mixture |
|---|---|---|
| Bonding | Atoms chemically joined | No chemical joining |
| Ratio | Fixed | Any ratio |
| Properties | Different from parts | Parts keep their own |
| Separation | Chemical reaction | Physical methods |
Use the table as a checklist. A reason that uses two rows scores better than one that only repeats the definition.
The mistake that costs marks
The slip is to call a substance a compound because it looks uniform. A student writes, “Salt water is a compound because it looks the same everywhere.”
| Claim | Right or wrong | Reason |
|---|---|---|
| Looks uniform so compound | Wrong | Appearance is not the test |
| Salt can be recovered by evaporation | Right | Physical separation shows mixture |
| Salt and water in any ratio | Right | No fixed ratio |
Salt water is a mixture. Boiling off the water leaves the salt unchanged.
Check yourself
Classify each as element, compound or mixture with a reason: (a) copper, (b) carbon dioxide, (c) sea water.
Answer
(a) Copper is an element: it contains only copper atoms.
(b) Carbon dioxide is a compound: carbon and oxygen atoms are chemically joined in a fixed ratio of 1 : 2.
(c) Sea water is a mixture: it contains water, salts and other substances in no fixed ratio, and the water can be recovered by distillation.
What to study next
Move on to explaining isotopes and their uses, then come back to classifying materials from properties to use the same reasoning on tables.
If you want a teacher to test your sorting with new examples, see online one-to-one Science tuition.