Mixtures, Elements and Compounds
Introduction
In the Junior School CBC (KICD), Integrated Science studies matter. Grade 8 learners classify substances and learn how to separate mixtures in daily life.
Elements
An element is a pure substance made of only one kind of atom. Examples: hydrogen (H), oxygen (O), carbon (C), iron (Fe), gold (Au). They cannot be broken down into simpler substances by chemical means.
Compounds
A compound is formed when two or more elements chemically combine in a fixed ratio.
- Water (H2O): hydrogen and oxygen.
- Carbon dioxide (CO2): carbon and oxygen.
- Common salt (NaCl): sodium and chlorine.
A compound has properties different from its elements.
Mixtures
A mixture contains two or more substances physically combined, not chemically joined, in any proportion. Examples: air, sea water, soil, sand and sugar.
- The parts keep their own properties.
- They can be separated by physical methods.
Separating mixtures
| Method | Separates | Example |
|---|---|---|
| Filtration | Insoluble solid from liquid | Sand from water |
| Evaporation | Dissolved solid from solution | Salt from salt water |
| Sieving | Different-sized solids | Stones from flour |
| Winnowing | Light from heavy particles | Husks from grain |
| Magnetic separation | Magnetic from non-magnetic | Iron filings from sand |
| Decanting | Liquid from settled solid | Water from mud |
| Distillation | Liquids with different boiling points | Pure water from salt water |
Worked example
A mixture contains sand, salt and iron filings. Step 1: use a magnet to remove iron. Step 2: add water to dissolve the salt. Step 3: filter to remove sand. Step 4: evaporate the filtrate to obtain salt.
Try it
- Classify as element, compound or mixture: oxygen, sugar, milk, copper.
- Describe how to get salt from sea water.
- Name two ways farmers use separation, for example winnowing and sieving.