CO2 is a non-combustible gas and extinguishes fire in two ways:
(i) It is heavier than oxygen and covers the fire like a blanket. It cuts off the contact between oxygen and fuel by displacing oxygen.
(ii) In cylinders, CO2 is kept in the liquid form. As it is released from the cylinder CO2 expands enormously. This is an effective way to put out fires and also prevent fire from reoccurring because of the lack of oxygen and the ice cold temperature of the CO2 when released from the extinguisher.