We first measure the mass and temperature of some water.  The water will be the source of the energy needed to evaporate liquid nitrogen.  By measuring the temperature of the water before and after the liquid nitrogen is evaporated, we will be able to determine how much energy was used.



We also need to measure the mass of liquid nitrogen.  We don't need to worry about the temperature, it's -320 F.  The liquid nitrogen can't get any warmer than that and still remain a liquid.


We pour the liquid nitrogen into the cup of water and wait.  Energy will flow from the warmer water into the very much colder liquid nitrogen.  We perform the experiment in a styrofoam cup and assume that no energy is flowing into the air in the room (because the water started out at room temperature there won't be much energy flowing from the water into the room anyway).



Now we remeasure the temperature of the water.  The water should be colder because some of its energy was used to evaporate the nitrogen.  We assume that the mass of the water has stayed the same.