Elements
An element is a pure substance made of only one kind of atom. It cannot be broken down into simpler substances by chemical means. There are about 118 elements. Each has a symbol of one or two letters: the first letter is a capital and the second, if there is one, is small.
| Element | Symbol | Element | Symbol |
|---|---|---|---|
| Hydrogen | H | Sodium | Na (natrium) |
| Carbon | C | Potassium | K (kalium) |
| Nitrogen | N | Iron | Fe (ferrum) |
| Oxygen | O | Copper | Cu (cuprum) |
| Sulphur | S | Zinc | Zn |
| Chlorine | Cl | Lead | Pb (plumbum) |
| Calcium | Ca | Silver | Ag (argentum) |
| Magnesium | Mg | Gold | Au (aurum) |
Compounds
A compound is a pure substance made of two or more different elements chemically joined in a fixed ratio. A compound with only two elements is a binary compound. Its name ends in -ide, and the metal or first element is named first: sodium chloride (NaCl), magnesium oxide (MgO), iron sulphide (FeS), carbon dioxide (CO₂).
| Compound | Made from | How its properties differ from the elements |
|---|---|---|
| Water, H₂O | Hydrogen and oxygen | Hydrogen burns and oxygen helps burning, but water puts out fires |
| Sodium chloride, NaCl | Sodium and chlorine | Sodium is a soft reactive metal and chlorine is a poisonous gas; salt is a safe white crystal |
| Iron sulphide, FeS | Iron and sulphur | Iron is attracted by a magnet; the compound is not |
Mixtures
A mixture contains two or more substances that are only mixed together, not chemically joined. They keep their own properties and can be present in any proportion. Air, sea water, soil and soft drinks are mixtures.
| Compound | Mixture | |
|---|---|---|
| Joined chemically? | Yes | No |
| Proportions | Fixed | Any |
| Properties | Different from the elements | Same as the substances in it |
| Separation | Needs a chemical reaction | Physical methods |
| Example | Salt | Salt and sand |
Separation of mixtures
Because the substances in a mixture keep their own properties, we separate them by physical methods that use a difference in a property.
| Method | Property used | Example |
|---|---|---|
| Filtration | Solid does not dissolve | Sand from water |
| Evaporation | Solvent evaporates; solute does not | Salt from sea water |
| Distillation | Different boiling points | Pure water from salt water; ethanol from a fermented mixture |
| Chromatography | Different solubility and sticking | Dyes in ink |
| Magnetic separation | Magnetic and non-magnetic | Iron filings from sand |
| Decanting and separating funnel | Liquids that do not mix | Oil from water |
A student separates a 50 g mixture of sand and salt. Choose the mass of sand. The rest is salt.
Key terms
- Element
- A pure substance made of only one kind of atom.
- Compound
- A pure substance of two or more elements chemically joined in a fixed ratio.
- Mixture
- Two or more substances mixed but not chemically joined.
- Filtrate
- The liquid that passes through filter paper.
- Solute
- The substance that dissolves in a solvent.
Summary
- Elements have one kind of atom; each has a symbol.
- Compounds are chemically joined in a fixed ratio and their properties differ from the elements; binary compounds end in -ide.
- Mixtures are not joined; the parts keep their properties and can be separated by physical methods.
- Choose the separation method from the property that is different.