What are Pure and Impure Substances?

A pure substance is a substance which is made up of only one type of element or compound. The chemical and physical properties of a pure substance do not change.

There are two parts of this definition to highlight:

1. One type of element or compound

2. Chemical and physical properties that do not change

We will cover the first part of the definition:

The Two Types of Pure Substance

The two types of pure substance are chemicals and compounds.

Elements

Elements are a type of substances that are made up of just one type of atom and all elements are pure substances.

Below you can see an old copper tea pot. This is an example of a pure substance, made purely out of the element copper (Cu).

Zooming in on a pure substance, such as this copper pot, you would see something similar to the diagram below.

The substance is made up of the same type of particle, in this case the same type of atom.

  • Another example of a pure substance made up of one type of atom is the lead of a pencil. When zooming in on the lead of a pencil, you would see that it is made up of carbon atoms.

Compounds

The other type of pure substance are the compounds, which are made up of only one type of molecule.

If you were to look at a pure substance made up of only one type of molecule, you would see something similar to the diagram below:

An example of a pure substance made up of only one type of molecule is water.

IF we were to zoom in on this glass of pure water, we would see water (H2O) molecules that make up this compound:

This is where the chemical formula (H2O) for water comes from. There is an oxygen atom in the middle and two hydrogen atoms attached to it.

  • The oxygen atom is in white and the hydrogen atom is in white

When you look at the particles that make up the compound water (pure water), that is what you’ll see, a bunch of water molecules floating around.

However, it is difficult to get substances which are completely pure. It will almost always include other substances.

Impure Chemical Substance

As you can see below, if you drop salt in to a beaker of pure water, this is what you’ll see.

When you zoom in to the sodium chloride salt (NaCl) at the bottom of the beaker, you can see it has not dissolved yet. The salt that hasn’t dissolved is a pure substance, it consists of just sodium chloride.

However, the water in the beaker is not pure because it has sodium and chlorine ions (from the salt) that have dissolved in the water. When you mix the contents of the beaker, there will be no pure substances and the solution will become saltwater.