Pure Substances and Impurities

Definition of Pure Substances

A pure substance contains only one type of element or compound throughout. This means it is made up of identical particles, either atoms of a single element or molecules of a single compound.

Pure substances have fixed melting and boiling points. For example, pure water melts at exactly 06C and boils at exactly 1006C at standard atmospheric pressure. This is because all the particles have the same structure and bonding, so they change state at the same temperature.

There are no other substances mixed within a pure substance. If any other chemicals or impurities are present, the substance is no longer pure.

Impurities in Substances

An impurity is any substance that is mixed with the pure substance but is not chemically bonded to it. Even small amounts of impurities can affect the physical properties of a substance.

• Impurities cause the melting point to decrease (melting point depression). This means the substance melts at a lower temperature than the pure substance.
• Impurities cause the boiling point to increase (boiling point elevation). The substance boils at a higher temperature than the pure substance.
• Impurities can also affect other physical properties such as appearance, taste, and texture.

This happens because impurities disrupt the regular arrangement of particles, making it easier to break the solid structure (lower melting point) and harder to turn into gas (higher boiling point). Specifically, the impurities interfere with the lattice structure, requiring less energy to melt and more energy to boil.

Identifying Purity

One way to check if a substance is pure is by measuring its melting point:

• A sharp melting point (melting over a very narrow temperature range) indicates a pure substance.
• If the melting point is lower than expected and the melting occurs over a wider temperature range, the substance contains impurities.

Similarly, the boiling point of a pure substance is sharp and at a fixed temperature. If impurities are present, the boiling point will be higher and less precise.

For instance, pure benzoic acid melts sharply at 1226C. If a sample melts between 1186C and 1226C, it suggests impurities are present.

Effect of Impurities

Impurities affect the physical properties of substances in the following ways:

• Lower melting point than the pure substance.
• Higher boiling point than the pure substance.
• Changes in appearance, such as colour or clarity.
• Changes in taste or smell (important in food and pharmaceuticals).

These changes occur because impurities disrupt the regular structure and bonding in the pure substance.

For example, salt mixed with pure water lowers the waters melting point, which is why salt is used to melt ice on roads in winter.

A sharp melting point is a key sign of purity, so chemists use melting point determination to assess the quality of substances.

For example, if a sample of aspirin melts sharply at 1356C, it is pure. If it melts over a range from 1306C to 1356C, it contains impurities.

For instance, pure water melts at 06C, but if salt is added, the melting point decreases.