Atoms and elements

Mixtures and pure substances

Air is a mixture of gases — mainly nitrogen (~78%), oxygen (~21%), argon, and carbon dioxide — not a compound.

Separation techniques

Paper chromatography separates dissolved substances (e.g. food colourings, inks) based on how far they travel up the paper with the solvent.

The periodic table

Group 0 (or Group 18) elements are called the noble gases (e.g. helium, neon, argon); they are very unreactive because their outer electron shell is full.

Mixtures and pure substances

A mixture contains two or more substances that are not chemically combined and can be separated by physical methods.

Separation techniques

In distillation, a condenser cools the vapour back into liquid; the liquid collected is called the distillate.

The periodic table

Group 1 elements are called the alkali metals (e.g. lithium, sodium, potassium); they react vigorously with water to produce hydrogen gas and an alkaline solution.

Mixtures and pure substances

Because a mixture contains different particles, it melts and boils over a range of temperatures rather than at a single sharp point.

Separation techniques

Distillation separates a soluble solid from a liquid, or two liquids with different boiling points, by boiling and condensing the vapour.

The periodic table

Group 7 elements are called the halogens (e.g. fluorine, chlorine, bromine, iodine); they are reactive non-metals that form salts when they react with metals.

Mixtures and pure substances

A pure substance boils at a single fixed temperature because all its particles are identical and need the same amount of energy to break apart.