Final answer:
A compressed gas is classified as such if its critical temperature is higher than the ambient temperature, allowing it to be liquefied at room temperature under pressure. Ammonia, with a critical temperature above room temperature, can be a compressed gas, while oxygen cannot since its critical temperature is below room temperature. Therefore the correct answer is 2) less than 50 degrees C.
Step-by-step explanation:
To be classified as a compressed gas, a substance must have a critical temperature that is higher than the ambient temperature around it, which allows the substance to remain in the gas phase under pressure. The critical temperature is the maximum temperature at which a substance can exist as a liquid, regardless of the pressure applied.
Substances with a critical temperature above room temperature can be liquefied at room temperature by applying sufficient pressure. For example, ammonia has a critical temperature of 405.5 K, which is above room temperature, making it capable of being a compressed gas since it can be liquefied under pressure at room temperature.
On the other hand, oxygen has a critical temperature below room temperature, making it impossible to liquefy at room temperature even when compressed. Substances like carbon dioxide have a critical temperature of 304 K (31 °C), and thus they cannot exist as liquids above this temperature, regardless of the pressure.
This characteristic is critical for handling and storage, for example, CO₂ fire extinguishers are advised not to be stored above its critical temperature to prevent the transition to a supercritical state where no amount of pressure can return it to a liquid form.