Carbon dioxide can be liquefied relatively easily, but not all gases are as liquefied as carbon dioxide. Each gas has its own critical temperature. Above this temperature, no matter how much pressure is applied, it is impossible to liquefy. The critical temperature of carbon dioxide is 31 degrees Celsius, which means that it can be liquefied under normal room temperature conditions. Sulfur dioxide and ammonia are also liquified easily. Oxygen, nitrogen, hydrogen and nitrogen are difficult to liquefy because their critical temperature is very low, ranging from -118 degrees Celsius to -268 degrees Celsius.