One method used to measure water pollution by organic waste is to calculate the biological oxygen demand, BOD. This determines the rate at which oxygen is used by micro-organisms in the water.

To determine the BOD of water, a sample of water is taken from the sea or river and the concentration of oxygen is measured.

The sample is then placed in a tube with an airtight seal and kept at 20*C for five days. The concentration of oxygen is then measured again.

The fall in oxygen concentration is the BOD.

Suggest why the airtight seals are placed on the tubes.