Molecules, Compounds and Mixtures


Molecules are formed when atoms chemically bond together. This forms a new substance with properties that are different from the individual atoms. The atoms in a molecule cannot be separated by physical means, such as filtration.

  • An example of a molecule is water (H2O), which is not considered three separate atoms (two hydrogen atoms and one oxygen atom). Instead, it is looked at as one H2O molecule.

Molecules can also be made from two or more of the same element. For example, the oxygen we inhale is chemically bonded in pairs (O2), so it is classed as a molecule.


Compounds are pure substances that contain two or more elements. All compounds can be classified as molecules, but not all molecules are compounds. For example, oxygen (O2), is not a compound because it only contains one type of element. Carbon dioxide (CO2) is a compound because it contains the elements carbon and oxygen.

Some other examples of compounds are:

  • Salt or sodium chloride (NaCl, an ionic compound)
  • Nitrogen gas (N2, a covalent molecule)
  • Water (H2O, a covalent molecule)

All the elements in a compound are always in fixed proportions. For example, the composition of water will always have two hydrogen atoms and one oxygen atom. So, we can write formulas using the chemical symbol of each element and their number of atoms.

For example, there are two oxygen atoms in carbon dioxide (CO2), so we write the two in subscript (O2). There is only one carbon atom, so we do not need to put a number next to the C.


A mixture is made up of two or more substances that are not chemically combined together. Although the substances mix, they do not react with each other. So, we can easily separate the substances in the mixtures using physical methods, such as:

  • Filtration
  • Crystallisation
  • Simple distillation

An example of a mixture is salt water, which is made by mixing salt and water molecules. After mixing, they do not form new molecules.

Forming Mixtures and Compounds