An understanding of the differences between elements, compounds and mixtures form the basis for understanding chemical reactions. However, for many students, identifying the difference between the particles in an element, compound or mixture proves tricky; especially when the images they see are often very similiar, as already discussed in the core idea. As teachers, our job is to try to remove stumbling blocks that may hinder a student's learning in this area. A good place to start is to think about how we present the ideas to students and the images we use.

INSERT [NOT_EU_02_ima] [images A and B from https://www.rsc.org/cpd/resource/RES00001530/classifying-materials/RES00001529#!cmpid=CMP00004903]

In each image, what does the single circle represent?

In Image A, the circle is representing a particle of water (H₂O – three atoms). In Image B, a circle is representing the particle of a helium atom. 

How useful is this type of representation?

The different colours suggest thatyou are looking at the particles of two different substances, so you can conclude that they will have different properties. They don't show if you are looking at an element or compound.

We could extend our ideas about the particle theory further by using different shapes to represent the particles of different substances. There are plenty of other shapes available to represent particles that mirror the shape of molecules. A water molecule could be a triangular particle. A methane molecule could be a square particle. 

INSERT [NOT_EU_02_ima]   [image of water, oxygen and methane molecules - 2nd image on page from https://www.rsc.org/cpd/resource/RES00001530/classifying-materials/RES00001529#!cmpid=CMP00004903]

Using this graphic representation, it is easier for students to distinguish between elements, compounds and mixtures and the following definitions:

1. The particles of an element contain only one type of atom.

INSERT  [NOT_EU_02_ima]  [image from point 3 an element https://www.rsc.org/cpd/resource/RES00001530/classifying-materials/RES00001529#!cmpid=CMP00004903]

2. The particles of a compound contain more than one type of atom. 

INSERT  [NOT_EU_02_ima] [image from point 4 a compound https://www.rsc.org/cpd/resource/RES00001530/classifying-materials/RES00001529#!cmpid=CMP00004903]

3. A mixture consists of two or more particles of different substances. 

INSERT [NOT_EU_02_ima]   [image from point 1 a mixture https://www.rsc.org/cpd/resource/RES00001530/classifying-materials/RES00001529#!cmpid=CMP00004903]

So what is the the difference between a pure and impure substance?

A pure substance has only one type of particles. It can be either an element or a compound.

An impure substance is a mixture of two or more different types of particles. It can be either a mixture of elements, a mixture of compounds or a mixture of elements and compounds.