In chemistry, we use the term ‘pure‘ to describe a substance that only contains one element or one compound.
Each symbol on the periodic table represents an element, such as ‘Mg’ for magnesium and ‘Cl’ for chlorine. If a substance is made up of identical atoms, it is a pure substance. In other words, all of the atoms in the substance have the same atomic number (number of protons). For example, a substance made up of just magnesium atoms is a pure substance.
A compound is made up of two or more separate elements that are chemically joined together. If a substance is made up of one type of compound, it is a pure substance.
A mixture contains two or more elements or compounds that are not chemically combined.
Most mixtures can be physically separated by various physical methods, such as:
We can determine whether a substance is pure or impure by examining the melting and boiling points of the substance. Pure substances have specific and fixed melting and boiling points. For example, gold has a melting point of 1063°C, while water has a boiling point of 100°C.
In contrast, mixtures have melting and boiling points that occur over a range of temperatures. This is due to the presence of different substances.
To identify the purity of a substance, we can use heating curves, such as the graph below for pure water.
As we heat pure water, the temperature gradually increases until it reaches the melting point of 0°C, where the temperature stops rising. Once the ice melts, the temperature increases again until it reaches the boiling point of 100°C, where the temperature stops rising again.
We can expect to see a similar graph with other pure substances. However, the melting and boiling points will be different to water.
In contrast, the heating curve for an impure substance, such as a mixture of water and another substance, will have a range of melting and boiling points. The graph below shows the heating curve for a mixture of water and an impurity.
As shown in the graph, the mixture melts and boils over a range of temperatures, indicating that the water is not pure. The melting and boiling points for impure substances will vary, but we can expect to see a similar heating curve pattern for other impure substances.