It is heavier than oxygen and covers the fire like a blanket.
Explanation
Combustion requires three essential components: fuel, heat, and oxygen (the fire triangle). To extinguish a fire, at least one of these components must be removed. Carbon Dioxide ($CO_2$) is widely used as a fire extinguisher for fires involving electrical equipment and inflammable materials (Class B and Class C fires) because it is non-conductive and does not damage electronic equipment.
Detailed Analysis:
- Density Comparison: Carbon Dioxide is significantly heavier than oxygen. The molecular weight of $CO_2$ is approximately 44 g/mol, whereas the molecular weight of Oxygen ($O_2$) is approximately 32 g/mol.
- The "Blanket" Effect: Due to its higher density, when $CO_2$ is sprayed onto a fire, it sinks and covers the burning material like a blanket.
- Mechanism of Action: By covering the fire, $CO_2$ cuts off the contact between the fuel and the atmospheric oxygen. Since oxygen is necessary for combustion, the fire is extinguished once the supply is interrupted.
- Secondary Effect: While the primary mechanism in this context is oxygen displacement, liquid $CO_2$ stored in cylinders also expands rapidly upon release, cooling down the fuel, though it does not lower the intrinsic ignition temperature of the material itself.
Key Takeaway:
Carbon Dioxide is an effective fire extinguisher because it is heavier than oxygen, allowing it to displace air and cut off the oxygen supply required for combustion.