Elements, Compounds & Mixtures

Explained

Elements, compounds and mixtures

Everything is built from atoms. What separates these three categories is not what the atoms are, but how they are put together: one type of atom, several types chemically joined, or several substances simply sitting together.

The three categories

An element contains only one type of atom. Copper is an element. So is oxygen, even though its atoms go about in pairs, because both atoms are the same.

A compound contains two or more elements chemically combined, in fixed proportions. Water is always two hydrogen atoms to one oxygen atom. You cannot have slightly more hydrogen in some water than in other water, and that fixed ratio is what makes it a compound rather than a mixture.

A mixture contains two or more substances that are not chemically bonded. Because nothing is joined, the parts keep their own properties and can be separated by physical means such as filtering, distilling or using a magnet.

The test that separates compound from mixture

Ask whether the proportions are fixed and whether a chemical reaction would be needed to separate the parts. Air is a mixture: the proportion of oxygen varies, and you can separate it by fractional distillation without any chemistry. Water is a compound: the ratio never changes, and splitting it needs electrolysis.

Sea water is a mixture of water and dissolved salts. Note that water itself is still a compound inside that mixture. Something can be a compound and part of a mixture at the same time, which is a distinction questions like to probe.

Purity, and why melting point is the test

In chemistry, pure has a narrower meaning than in everyday speech. A pure substance is a single element or a single compound with nothing else in it. Bottled water described as pure on the label is, chemically, a mixture.

A pure substance melts at one sharp, fixed temperature. Pure ice melts at exactly zero degrees Celsius. A mixture melts over a range of temperatures, and usually at a lower temperature than the pure substance would. That is why melting point is used as a test for purity: a sharp melting point suggests purity, and a broad range suggests something else is present.

Separating the parts of a mixture

Because a mixture involves no chemical bonding, every separation method exploits a physical difference. Filtration uses particle size. Evaporation and crystallisation use volatility. Distillation uses boiling point. Chromatography uses how strongly substances are attracted to the paper compared with the solvent.

If a question asks you to choose a method, decide first which physical property differs between the parts. That property points straight at the technique.