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Some substances are pure. There are 92 naturally occurring elements that can exist in a pure form such as gold, copper, carbon or oxygen. These can be found in the Periodic Table illustrated below. You will learn more about this in Year 8.
These elements can join together chemically in thousands of combinations to create compounds. Water for example is a compound (H2O) and so is table salt (Sodium Chloride or NaCl). Compounds are also pure substances. Even though they are a combination of elements, they are chemically bound to form molecules that are all the same. Hence rainwater with no impurities is pure water.
Pure does not mean natural. Saltwater is natural, but it is not pure. It is a mixture of salt and water. Any substance that has another substance mixed in it is impure. This includes a birthday cake, a concrete block or a glass of cordial.
Air that we breath is a mixture of gases. Some of these gases, such as oxygen and nitrogen, are elements, while others are compounds such as carbon dioxide (CO2) and methane (CH4). Air also has small amounts of particles from smoke and car exhausts.
Many more impure substances than pure substances exist.
Figure 1: Types of substances.
Figure 2: Percentage of gases in air.
Figure 3: The Periodic Table of Elements
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