Question

Why does no2 gas changes into solid n2o4 why cooled?

Answer 1

Heat shifts the equilibrium in favor of NO2 and the tube becomes darker. Since the formation of N2O4 is an exothermic reaction, lowering the temperature shifts the equilibrium in favor of colorless N2O4. In a dry ice - acetone bath, the N2O4 will crystallizes as a white solid.