How do impurities affect the properties of ionic crystals?

Impurities in ionic crystals can alter their physical properties, such as colour, hardness, and melting point.

Impurities in ionic crystals can have a significant impact on their properties. This is because ionic crystals are made up of positive and negative ions arranged in a regular, repeating pattern. When an impurity ion is introduced, it can disrupt this pattern, leading to changes in the crystal's properties.

One of the most noticeable effects of impurities is a change in colour. For example, when a small amount of chromium is added to aluminium oxide, the resulting crystal (known as ruby) is red. This is because the chromium ions absorb certain wavelengths of light, giving the crystal its distinctive colour.

Impurities can also affect the hardness of ionic crystals. This is because the impurity ions can disrupt the regular arrangement of ions in the crystal, making it more difficult for the layers of ions to slide over each other. As a result, the crystal becomes harder.

In addition, impurities can alter the melting point of ionic crystals. The melting point of a crystal is determined by the strength of the forces between its ions. When an impurity ion is introduced, it can weaken these forces, causing the crystal to melt at a lower temperature.

Finally, impurities can affect the electrical conductivity of ionic crystals. Pure ionic crystals do not conduct electricity because their ions are not free to move. However, when an impurity ion is introduced, it can create vacancies or 'holes' in the crystal structure, allowing the ions to move and the crystal to conduct electricity.

In conclusion, impurities can significantly alter the properties of ionic crystals, affecting their colour, hardness, melting point, and electrical conductivity.