Nitrogen dioxide  -  NO2

 

   

 

Nitrogen dioxide is a brown gas. This is a sample of nitrogen dioxide in a completely sealed glass apparatus at approximately normal pressure.

The intensity of the color of the gas depends on the temperature. The higher the temperature, the darker the gas is looking. This is due to the equilibrium, which shifts to the right at lower temperature.

   2NO2 N2O4

The compound N2O4 is a colorless gas. When this glass apparatus with NO2 is put in a fridge, then the gas in it becomes completely colorless. When it is taken out, then it soon darkens again. At the temperature in a common fridge (-18 ºC or so), virtually all NO2 is converted to N2O4.