The earth's crust consists of many different substances; its composition is shown in the following diagram.
The three most abundant elements on the earth's crust are oxygen, silicon and aluminium. Most of the elements are combined with one another to form compounds, ie, the combined forms of elements. For example, silicon is the second most abundant element but it does not occur freely in nature. However, its compound, silicon (IV) oxide, is very abundant.
Oxygen is the most abundant element in nature and exists on its own in elemental form on earth. It does not combine with other substances. This is unique among all planets in the solar system. Aluminium is the third most abundant element and is also the most abundant metal. It does not occur freely in nature. However, its compounds are plentiful and widely distributed, eg, aluminium oxide.
Compounds A compound is a substance which consists two or more elements chemically combined together - we assume that it contains more than one type of particle, and that the particles are chemically joined together. It can be decomposed into simpler substances by chemical means.
The properties of compounds are usually very different from those of the constituent elements. For example, salt (sodium chloride) is a compound of sodium and chlorine, and it exists as a white solid with a familiar taste. Sodium, however, is an extremely reactive silver-coloured metal which reacts vigorously with water while chlorine is a yellowish green gas with a choking smell.
Mixtures A mixture is a material consisting of two or more kinds of matter, each retaining its own characteristic properties - we assume that it contains different types of particles which are physically mixed rather than chemically joined.
Mixtures can be homogeneous or heterogeneous. Homogeneous mixture are known as solutions. For example, when sugar is dissolved in water, it forms a solution in which the sugar and water are uniformly mixed. Air is another homogeneous mixture which is really a solution of gases. Some of its constituents are nitrogen, oxygen, carbon dioxide, argon and water vapour. Heterogeneous mixtures are those which are not uniform in appearance and properties throughout. For example, concrete, which consists of cement, sand and rock aggregates is a heterogeneous mixture.