Final answer:
The presence or absence of liquid carbon dioxide in a fire extinguisher's cylinder is determined by the temperature. Below the critical temperature, liquid CO2 is present, while above the critical temperature, there is no liquid CO2.
Step-by-step explanation:
On a cool day, the temperature is below the critical temperature of carbon dioxide (CO2), which is 304 K or 31 °C. This means that liquid CO2 is present in the fire extinguisher's cylinder. However, on a hot day when the temperature is greater than the critical temperature, no amount of pressure can liquefy carbon dioxide, so there is no liquid CO2 in the extinguisher.