Mysterious gas riddle

When demonstrating the acidity of nitrogen oxides (in connection with acid rain), I prepare NO2 by the reaction between concentrated nitric acid and a piece of copper:

Cu(s) +  4HNO3(aq) → Cu(NO3)2(aq)  +  2NO2(g)  +  2H2O(l)

Diagram outlining inversion of nitric acid into water with bromothymol blue.Apart from this gas, some students remember that both bromine and iodine monochloride gases are orange-brown. I tell them that as far as I know, there are no other orange-brown gases.

I then distribute to each group a stoppered test tube containing the gas; they are instructed to invert the test tube, remove the stopper and quickly immerse the tube into a slightly basic bromthymol blue solution.

They immediately see the change in colour of the solution, which rises into the test tube, while the colour of the gas becomes progressively fainter (1 above). If the test tube is carefully agitated, so that it is not removed from the water, the reaction is that much faster. Soon the colour of the gas completely disappears, all the solution turns yellow, but there remains a space above the liquid in the test tube (A in 2 above).

The question is: what gas is A?

Most students suggest that it is air, as they have seen that I attempt to sweep all the air out of the receptacle when I prepare other gases, such as ammonia or hydrogen chloride. They claim that I didn't sweep out all the air when preparing the nitrogen dioxide, and this is what remains in A.

Others suggest that it is hydrogen. It is true that copper does not release hydrogen with dilute (1 M) acids, but it does with hot concentrated HCl, so this suggestion is reasonable.

Now I tell the students to remove the test tube from the water and stopper it immediately, while holding it up against a sheet of white paper. Since the water runs out, air goes in, and to their amazement they see that a light orange-brown colour reappears in the test tube (3 above).

After a few seconds this colour again disappears, as the walls of the test tube are wet (4 above).

The riddle is to explain what caused the reappearance of the colour.

We will read all your responses and select some of your solutions for the December issue.

[If you decide to do this as a demo, remember that nitrogen dioxide may be the most hazardous substance that a high school student will encounter. It is corrosive to eyes, as well as nose, throat and lungs upon inhalation. The demo should be done in a fume hood and the production of nitrogen dioxide should be limited to a small amount. Use proper precautions and as always consult the MSDS information.]