Mustard gas. Molecular molecule of mustard gas, which is not a gas in actuality but a viscous liquid. Mustard gas (C4.H8.Cl2.S) has the chemical name bis-(2-chloroethyl)-sulfide. It is a nerve agent and a strong vesicant (blister causing chemical) used in chemical warfare. Atoms are colour-coded: hydrogen (white), carbon (grey), sulphur (yellow) and chlorine (green).