Elements, Compounds & Mixtures
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
Elements, Compounds and Mixtures
All matter can be classified as either a pure substance (an element or a compound) or a mixture.
- Element: a substance made of only one type of atom (same atomic number throughout); cannot be broken down into simpler substances
- About 118 known elements, each with its own chemical symbol (e.g. H, O, Fe)
- Some elements exist as molecules (O₂, N₂, Cl₂), but they are still elements because only one type of atom is present
- Compound: two or more different elements chemically bonded together in a fixed ratio
- Can only be separated back into elements by a chemical reaction, not by physical methods
- Has completely different properties from the elements it contains - e.g. reactive sodium metal and poisonous chlorine gas combine to form harmless sodium chloride (table salt)
- Represented by a fixed chemical formula (H₂O, CO₂, NaCl)
- Mixture: two or more substances (elements and/or compounds) that are physically combined, not chemically bonded
- Components keep their own individual properties
- Composition can vary - there is no fixed ratio
- Can be separated by physical methods (filtration, evaporation, distillation, chromatography, using a magnet)
Whether particles are all identical, bonded in fixed pairs, or scattered independently distinguishes elements, compounds, and mixtures
Comparing Elements, Compounds and Mixtures
| Property | Element | Compound | Mixture |
|---|---|---|---|
| Composition | One type of atom only | Two+ elements chemically bonded | Two+ substances physically combined |
| Ratio | N/A | Fixed | Variable |
| Separation | Cannot be separated further | Only by a chemical reaction | By physical methods |
| Properties | Unique to that element | Different from its elements | Components keep their own properties |
| Examples | O₂, Fe, He | H₂O, CO₂, NaCl | Air, seawater |
A useful test for purity: pure substances (elements and compounds) melt and boil at one specific, fixed temperature. Mixtures melt and boil over a range of temperatures.
Worked Example: Iron and Sulfur
- Question: iron filings and sulfur powder are stirred together, then a separate sample is heated strongly until it glows red. Describe the difference between the two products, and how you could separate the components of the unheated sample. [3]
- Answer: the unheated (stirred-only) sample is a mixture - iron and sulfur are not chemically bonded, so a magnet can be used to remove the iron, leaving the sulfur behind [2]; the heated sample has reacted to form iron sulfide, a compound, which is not magnetic and has properties different from either iron or sulfur [1]
Common Mistakes
- ❌ Thinking air is a compound
- ✓ air is a mixture of gases (mainly N₂ and O₂) - they are not chemically bonded and can be separated by fractional distillation
- ❌ Thinking salt water is a compound
- ✓ salt water is a mixture - the salt (itself a compound, NaCl) is simply dissolved in water (itself a compound, H₂O); the two can be separated again by evaporation or distillation
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