At first, the temperature of liquid nitrogen ensures that the air in the balloon doesn't take much volume. The temperature being optimal for liquid nitrogen makes sure that the nitrogen stays liquid, however, as the temperature increase and the balloon begins to heat up the nitrogen in the balloon begins to boil due to its boiling point being very low and the volume beings to increase. This causes the balloon to grow in size. Using Charle's law it can be determined that the volume of a gas in any space is proportional to the absolute temperature of such gas.
[tex] \frac{ V_{1} }{T_1}= \frac{V_2}{T_2} [/tex]
