Telling elements, compounds and mixtures apart
An element is a pure substance made of only one kind of atom, such as oxygen, iron or gold. Each element has a symbol, often taken from its English or Latin name: H for hydrogen, Na from natrium for sodium, and Fe from ferrum for iron. A compound forms when two or more elements combine chemically in a fixed ratio. Water, H₂O, always has two hydrogen atoms for every oxygen atom, and its properties are completely different from those of hydrogen and oxygen gases.
A mixture contains two or more substances physically mixed in any ratio, and each keeps its own properties. Air, sea water and a salad are mixtures, and their parts can be separated by physical methods. Changes in matter are of two kinds. Physical changes, like melting ice or tearing paper, change only shape or state and are often reversible. Chemical changes, like rusting, burning or cooking an egg, form new substances and usually cannot be reversed easily.
Why it matters
Knowing whether something is an element, compound or mixture helps us understand how to purify drinking water, extract metals and make medicines. Recognising chemical changes explains why iron rusts and why food spoils. Chemical symbols and formulae form the language of chemistry used in every later class.
Key terms
- Element
- A pure substance made of only one kind of atom.
- Compound
- A substance formed when two or more elements combine chemically in a fixed ratio.
- Mixture
- Two or more substances mixed physically, each keeping its own properties.
- Chemical change
- A change in which one or more new substances are formed.