The left hand bottle contains more nitrogen dioxide than the Tetroxide, whereas the Second bottle's contents' state favors the formation of more Dinitrogen Tetroxide (colorless) than Nitrogen Dioxide (Red/Brown).
Nitrogen Dioxide is a reddish-brown gas with a distinct, pungent odor similar to that of Chlorine. It causes significant and damaging pulmonary effects when it is inhaled, due to the formation of both Nitric (HNO3) & Nitrous (HNO2) acids when it reacts with Water present in the lungs. When Nitrogen Dioxide is compressed or cooled to near 0° C, two molecules of the Dioxide bond, forming the colorless Dinitrogen Tetroxide. It is a strong oxidizing agent. Upon heating, Nitrogen Dioxide decomposes back into lower oxides of Nitrogen (NO) as well as Oxygen. It reacts with iodides to form nitrites and elemental iodine. The presence of NO2 in concentrated Nitric acid causes the acid to take on a yellow color, depending on the concentration of the dissolved gas.
