Liquid oxygen
Care should be taken when using a cold trap not to condense liquid oxygen (a light blue liquid) into the cold trap. Liquid oxygen is potentially explosive, and this is especially true if the trap has been used to trap solvent. Liquid oxygen can be condensed into a cold trap if a pump has sucked air through the trap when the trap is very cold, e.g. when cooled with liquid nitrogen. Besides oxygen, many hazardous gases emitted in reactions, e.g. sulfur dioxide, chloromethane, condense into cold traps.