Final answer:
The maximum capacity of a portable CO2 extinguisher is 20 pounds (Option D). The presence or absence of liquid CO2 inside a fire extinguisher can be explained by its critical temperature, which allows it to exist as a liquid below 31 °C and only as a gas above this temperature.
Step-by-step explanation:
The maximum capacity, by weight, of a portable CO2 extinguisher is commonly found in different sizes. But as per the options provided, the answer is D. 20 pounds, which is a standard size for larger portable CO2 extinguishers often used in commercial environments for electronic and laboratory fires. However, smaller sizes like 5, 10, and 15 pounds are also available and are more often used in home or light commercial applications.Regarding the critical temperature of CO2, the phenomenon can be explained by the substance's change of state at different temperatures. On a cooler day at 18 °C, the temperature is below the critical temperature of CO2, which is 304 K or 31 °C. This allows CO2 to exist as a liquid under pressure inside the cylinder, which is why we can hear it sloshing around. However, on a hot summer day when the temperature exceeds 31 °C, CO2 cannot be liquefied regardless of the pressure. Thus, all the CO2 in the extinguisher is in the gaseous state, and no sloshing can be heard.