Final answer:
CO2 fire extinguishers are primarily used for Class B fires involving flammable liquids and can also be used for Class C electrical fires. They contain liquid CO2, which can change to gas depending on the temperature relative to the critical temperature. Modern firefighting foams that contain fluorinated solutions are now universally used for large fires involving organic liquids.
Step-by-step explanation:
CO2 portable fire extinguishers are primarily used for Class B fires, which involve flammable liquids. These extinguishers work by displacing oxygen and smothering the fire, which is extremely effective in the case of liquid fires. However, they can also be used for Class C fires, which are electrical fires, since CO2 is non-conductive. They are not the best choice for Class A fires (wood, paper) because they can fail to remove sufficient heat to prevent reignition, nor for Class D fires (metals), which require specialized extinguishing agents.
The use of carbon tetrachloride in fire extinguishers was discontinued because it forms phosgene, Cl₂CO, a toxic gas when exposed to heat. The Lewis structures for carbon tetrachloride and phosgene would show carbon at the center with four and two chlorine atoms bonded to it, respectively, along with a double bond to oxygen in the case of phosgene.
The critical temperature of carbon dioxide refers to the temperature above which CO2 cannot exist as a liquid, regardless of pressure. On a cool day, the internal temperature of a CO2 fire extinguisher cylinder is below this critical temperature, allowing liquid CO2 to exist. On a hot day, this temperature can exceed the critical temperature, causing all the CO2 to exist as a gas, which gives us the observation that there is no liquid sloshing around.
Modern firefighting foams contain fluorinated solutions which spread across the surface of burning liquids and extinguish fires effectively. These foams have become universally used for handling large-scale fires involving organic liquids due to their ability to create a barrier between the fuel and the oxygen.