Mustard gas is a chemical warfare agent that was first used during World War I. It is a yellowish-brown liquid at room temperature and can form a vapor that is harmful when inhaled or comes into contact with skin. The gas causes severe blisters and damage to the respiratory system, making it a dangerous weapon in warfare. The chemical name for mustard gas is sulfur mustard, and it belongs to a class of compounds known as blister agents. Its use is banned under the Chemical Weapons Convention, which prohibits the development, production, and stockpiling of chemical weapons.