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.

  • For example, water (H2O) is not viewed as three separate atoms (two hydrogen atoms and one oxygen atom) but rather as a single H2O molecule.

Molecules can also be made from two or more of the same element. For example, the oxygen we inhale is in the form of O2, where two oxygen atoms are chemically bonded, so it’s considered 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.

A round-bottomed conical flask containing illustrated H2O molecules. The depiction shows the chemical bond between the atoms.

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
A spoon pouring salt into a cup of water, illustrating a mixture. The salt and water molecules remain separate and aren't chemically bonded.

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

Red circles symbolise one element, and yellow circles represent another. An arrow indicates the mixing of the two coloured circles, illustrating a mixture. A subsequent arrow depicts the red and yellow circles transitioning to chemically bond. Another arrow shows the separation and subsequent bonding of the two different coloured circles within the same area.

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