Final answer:
A fire extinguisher receives a class C rating for electrical fires because the non-conductive CO₂ gas ensures safety. Liquid CO₂ is present in the extinguisher below its critical temperature of 31 °C, but above this temperature, CO₂ remains a gas irrespective of pressure.
Step-by-step explanation:
An extinguisher receives a class C rating because it is suitable for fighting electrical fires, where the extinguishing substance needs to be non-conductive. A carbon dioxide (CO₂) fire extinguisher can achieve this because CO₂ gas does not conduct electricity and can safely be used on electrical fires.
Regarding the behavior of CO₂ in a fire extinguisher, on a cool day (18 °C), you can hear liquid CO₂ sloshing around inside the cylinder because the temperature is below the critical temperature of CO₂, which is 304 K or 31 °C. In this state, CO₂ can exist as a liquid under pressure. On a hot day (35 °C), the extinguisher contains no liquid CO₂ because the temperature is above its critical temperature. Above this temperature, CO₂ cannot be liquefied regardless of the pressure applied.