<prompt> Critical temperature for liquid nitrogen is ?36.5degC-20degC-147degC-242degC  <response>: Ans. is c i.e., -147deg C Critical temperature Tc of a substance is the temperature at and above which vapour of that substance can not be liquified, no matter how much pressure is applied Note : Below critical temprature a substance can exist as a liquid or gas depending on pressure. Critical temperature of N2 is -146.9deg C; that means N2 can be liquified below -146.9deg C -- So, liquid nitrogen must be stored below -146.9deg C. 