Purity of Substances

In chemistry, we use the term ‘pure‘ to describe a substance that only contains one element or 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 entirely of magnesium atoms is a pure substance.

A compound consists of two or more different elements that are chemically combined. If a substance contains only one type of compound, it is considered a pure substance.

  • For example, a substance containing only water molecules (H2O) is a pure compound. However, natural water often contains dissolved substances. Therefore, truly pure water, often referred to as distilled water, is usually artificially produced.

A mixture contains two or more elements or compounds that are not chemically combined.

Most mixtures can be separated by various physical methods, such as:

  • Filtration
  • Crystallisation
  • Distillation
  • Fractional Distillation
  • Chromatography

Determining the Purity of a Substance

We can determine the purity of a substance by examining its melting and boiling points. Pure substances have specific, 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 display melting and boiling points over a range of temperatures due to the presence of various substances.

Heating curves

Pure 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 similar graphs for other pure substances. However, their melting and boiling points will differ from that of water.

Impure substances

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.

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