Mustard gas (HD) is a chemical compound that was first used as a chemical weapon in World War I. In pure form, it is a colourless, odourless, viscous liquid at room temperature and causes blistering of the skin. The name comes from impure mustard gas, which is usually yellow-brown in colour and has an odour resembling mustard, garlic or horseradish. It is otherwise not related to mustard in any way.