Chlorine

Situated on a high rock, Nottingham Castle commands spectacular views over the city and once rivalled the great castles of Windsor and the Tower of London. A historic site at the heart of the Robin Hood legend and the starting point for the English Civil War, its history is chequered with sieges, murders and intrigue.

Totally destroyed after the Civil War, the medieval castle was replaced by a magnificent ducal mansion in 1674. Then in 1875 it was converted into the first municipal museum and art gallery outside London. Award-winning cave tours tell this history and take you down into the passageways and tunnels beneath the building.

Chlorine has been in use for thousands of years in other forms, but it wasn't named until 1810 by Sir Humphry Davy.

Only fluorine is a lighter halogen than chlorine.

It is the second most common halogen on Earth.

It is a yellowy-green dense gas with a choking smell.

Chlorine kills bacteria – it is a disinfectant. It is used to treat drinking water and swimming pool water. Chlorine gas is very poisonous, and was used as a chemical weapon during the First World War.