Halogen physical properties and identification

Iodine’s solid crystals can look shiny. Its vapour is purple, which is different from the colour of the solid at room temperature.

These descriptions are for the elements themselves, not their solutions. For example, red-brown liquid bromine is not the same description as bromine dissolved in water.

Connecting the states to melting and boiling points

Both the melting points and boiling points increase down Group 7. To connect this trend to physical state, compare room temperature with each substance’s melting and boiling points:

• Below its melting point, a substance is solid.
• Between its melting and boiling points, it is liquid.
• Above its boiling point, it is gas.

Room temperature is above chlorine’s boiling point, so chlorine is a gas. It lies between bromine’s melting and boiling points, so bromine is a liquid. It is below iodine’s melting point, so iodine is a solid.

Halogens form molecules containing pairs of atoms, such as $\mathrm{Cl_2}$, $\mathrm{Br_2}$ and $\mathrm{I_2}$. Down the group, the molecules become larger and the forces between molecules become stronger. More energy is needed to overcome these forces, so melting and boiling occur at higher temperatures.

Using the pattern to make predictions

A prediction should connect an element’s position in the group to the known trend.

Fluorine is above chlorine, so it has lower melting and boiling points. Since chlorine is already a gas at room temperature, fluorine is also expected to be a gas.

Astatine is below iodine, so it is expected to have higher melting and boiling points. Since iodine is solid at room temperature, astatine is predicted to be solid too. The colour trend also suggests a darker appearance further down the group, although the trend alone cannot give an exact shade or numerical melting point.

Identifying chlorine chemically

Chlorine’s yellow-green colour is a physical property. A chemical test provides a more reliable way to identify it.

Expose damp blue litmus paper to the gas. If chlorine is present, the paper is bleached white. It may turn red briefly before becoming white, because chlorine produces acidic substances when it comes into contact with the water on the paper. The bleaching is the identifying result.

The paper must be damp so that chlorine can react with the water. Chlorine is toxic, so this test should be carried out under supervision in a fume cupboard; smelling the gas is not a safe identification method.

• Colours become darker.
• Melting and boiling points increase.
• The states of chlorine → bromine → iodine are gas → liquid → solid at room temperature.

Fluorine, above chlorine: lower melting and boiling points; gas at room temperature.

Astatine, below iodine: predicted higher melting and boiling points; solid at room temperature.

Chlorine test

Damp blue litmus paper is bleached white. It may turn red briefly first. Chlorine must be handled in a fume cupboard.