Pure Substances

Before we look at pure substances, we have to remind ourselves of the definitions of elements, compounds and mixtures....

Element: A substance made up of only one type of atom.

e.g. sodium, chlorine, neon, copper, zinc.....

Elements can be given a chemical formula!

Compound: A substance made up of two or more types of different atoms that are chemically bonded together. Compounds cannot be separated back into their original elements easily.

e.g. pure water, carbon dioxide, magnesium oxide, potassium chloride.

Compounds can be given a chemical formula!

Mixture: Made of two of more substances (elements, compounds or both) which are NOT chemically bonded together and can easily be separated.

e.g. salt water, sand, chocolate, air

Mixtures CANNOT be given a chemical formula.

A pure substance is one which has definite chemical and physical properties.

e.g. a definite physical property means a defined melting or boiling point

e.g. a definite chemical property could be the way it reacts with water.

For a substance to be pure, ALL its particles must be identical. Pure substances can either be elements or compounds, but NEVER mixtures. Mixtures are NOT PURE!

Elements (made up of only one type of atom)

e.g. magnesium. In a sample of magnesium, all the particles are magnesium atoms. They are all the same. This means magnesium is a pure substance. It is an element AND it is pure.

e.g. 2. Chlorine. Chlorine particles exist as diatomic molecules. It goes around as Cl₂ molecules, however all the molecules are the same! They all have the formula Cl₂,so this means it is an element AND it is pure.

Compounds (two or more different types of atom that are chemically bonded)

e.g. water. Water particles exist as H₂O molecules. Every single particle in water has the formula H₂O, so water is a compound AND it is pure.

e.g. 2. carbon dioxide. Carbon dioxide particles exist as CO₂ molecules. Every single particle in carbon dioxide has the formula CO₂, so carbon dioxide is a compound AND it is pure.