Final answer:
The critical temperature of carbon dioxide determines whether liquid CO₂ is present in a fire extinguisher based on the surrounding temperature.
Step-by-step explanation:
The critical temperature of carbon dioxide (CO₂) is 304 K or 31 °C. On a cool day with a temperature below the critical temperature, liquid CO₂ is present in a fire extinguisher.
However, on a hot day with a temperature greater than the critical temperature, no amount of pressure can liquefy CO₂, so no liquid CO₂ exists in the fire extinguisher.