Answer :
Final answer:
CO₂ and Dry Chemical extinguishers can be used on Class B and C fires but are not suitable for Class A, D, or K fires. Liquid CO₂ exists in extinguishers below its critical temperature of 31 °C but not above it.
Explanation:
CO₂ and Dry Chemical - Carbon Dioxide Extinguishers can be used on Class B and C fires. Class B fires involve flammable liquids such as gasoline, oil, and grease, whereas Class C fires involve electrical equipment. These extinguishers are not suitable for Class A fires, which involve ordinary combustibles like wood and paper, nor for Class D fires, which involve combustible metals, nor for Class K fires, which involve cooking oils or fats in a kitchen setting.
The critical temperature of carbon dioxide is 304 K or 31 °C. On a cool day, at 18 °C, the temperature inside the CO₂ fire extinguisher is below this critical temperature, allowing liquid CO₂ to exist. On a hot day, at 35 °C, the temperature exceeds the critical temperature, which means no amount of pressure can liquefy the CO₂, and thus no liquid is present in the extinguisher.
Chemicals such as carbon tetrachloride are no longer used in fire extinguishers due to the formation of toxic gases like phosgene when they are exposed to fire. This highlights the importance of chemical considerations when selecting firefighting agents and equipment.
