Final answer:
The pressure gauge on a fire extinguisher indicates if the pressure might be low, which can prevent proper discharge. CO2 behaves differently based on temperature, remaining as liquid below 31 °C and existing only in gaseous form above this critical temperature.
Step-by-step explanation:
The part of a fire extinguisher that indicates whether the pressure might be low is typically a pressure gauge. This gauge informs you of the internal pressure, and if the reading is below the recommended level, it suggests that the fire extinguisher may not discharge properly when needed. The behavior of CO2, the fire extinguisher's content, can differ depending on the temperature. On a cool day, the temperature of CO2 can be below its critical temperature of 304 K (31 °C), which allows for liquid CO2 to be present inside the cylinder. However, on a hot day where the temperature exceeds the critical temperature of CO2, no amount of pressure can liquefy the gas, indicating that no liquid CO2 exists in the fire extinguisher.