The reaction may start very slowly, but at a critical moment it speeds up and is complete within about twenty seconds. What to Expect and Why Initially, O 2 and NO will meet in the tube connecting the two smaller flasks, due to gas diffusion, and wisps of brown NO 2 may become visible. Air in the connecting tube may slow this process. As NO and O 2 react, the volume starts to decrease, and brown NO 2 is seen to increase in amount. The decreasing volume causes a decrease in pressure, so water starts to flow towards the middle flask.
Oxygen flows towards the NO flask as the pressure falls and water enters the middle flask.
As bulk flow of oxygen enters the NO flask, the reaction becomes fast, and the gas in the end flask suddenly becomes quite brown. When the middle flask is full of water, the process would stop if only NO 2 were formed, since the total gas volume would have decreased to that of the end flask. However, some of the NO 2 dimerizes to N 2 O 4 ,causing a further decrease in volume and pressure, so water continues to flow towards the500 mL flask. If any gas is still present in the 500 mL flask when the water stops flowing, it may be either air trapped from the setting-up process, or excess O 2 or NO if the flask volumes were not exact. Slight pressurization of the flasks, as mentioned above, usually overcomes this problem.