Red phosphorus and chlorine. High-speed footage of red phosphorus burning in chlorine gas. Phosphorus is a reactive element that is only ever found in compounds in nature, but exists as several distinct allotropes as an element. Red phosphorus burns in chlorine gas to produce phosphorus trichloride (PCl3), a colourless fuming liquid. The equation for the reaction is P4 + 6 Cl2 ? 4 PCl3