Mysterious gas riddle — a discussion

In the September issue, Yehoshua Sivan from Safed, Israel had a mysterious gas riddle for Chem 13 News readers. Below is a quick recap of the riddle.

Air is swept out of the test tube and NO2 is prepared 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 for NO2 inverted into water with bromothymol blue.

Image 1:  The test tube containing NO2 (brown) is inverted into a beaker of water with bromothymol blue.

Image 2:  The volume of gas decreases and the water turns yellow. Gas A is colourless.

Image 3: Test tube is removed from the beaker and the test tube with the gas is stoppered immediately. A light colour of brown appears.

Image 4: The brown color of the gas eventually fades and water droplets collect on the sides of the test tube

The riddle is to explain what caused the reappearance of the colour and to identify gas A.

[Note: a more detailed version that includes safety considerations — such as collecting the NO2 in a fume hood — is given in the September issue].

Yehoshua Sivan`s solution

The only orange-brown gas that is likely to reappear is NO2, since the halogens were not involved in the reaction in any way.
If so, then some colourless precursor of NO2 must have been present. Students may need to have access to a source, such as on wikipedia, where they would find that colourless nitrogen monoxide, NO, not only has a fairly low solubility in water, but also reacts with oxygen to form NO2:

2NO(g)  +  O2(g)  → 2NO2(g)

Now the question is: what is the source of the NO? Copper can also react with dilute nitric acid to form NO:

3Cu(s)  +  8HNO3(aq)  → 3Cu(NO3)2(aq)  +  2NO(g)  +  4H2O(l)

Presumably both reactions take place simultaneously, or maybe as the concentrated acid reacts, it becomes dilute.

So A certainly contained NO, which reacted with the air that entered the tube as the water was poured out. This formed the orange-brown NO2. Most certainly, it didn't contain air, which would have reacted with the NO before more air was admitted. Some students had suggested the gas was hydrogen because of the concentrated acid, but this experiment neither proves nor disproves its presence.

I strongly recommend that teachers do this experiment with their students and not merely relate the phenomenon!

Edwin Chong, Ancaster ON (retired)

Here is my take on the gas riddle. In preparing the nitrogen dioxide a small amount of nitric oxide was also formed so when water was allowed to enter the test tube, nitrogen dioxide dissolved (forming nitrous and nitric acids) and the solution turned yellow/gold but the nitric oxide remained due to its low solubility. Gas A is nitric oxide. In test tube 3, the nitric oxide reacted with oxygen in the air forming nitrogen dioxide — hence the brown colour — and finally in test tube 4 the nitrogen dioxide dissolved in water giving a colourless solution.

I do have a question. You seem to emphasize using basic bromothymol blue (bb); why not just neutral bb which would have a green colour?

Nice demo, definitely in a fume hood for the initial production then the rest could be in a well-ventilated room.

David Cash, Mohawk College, Hamilton ON (retired)

The mysterious gas labeled A at position 2 in the graphic is nitric oxide (NO), perhaps mixed with residual atmospheric nitrogen gas (N2).

First, the required data: web searches or perusal of textbooks of descriptive inorganic chemistry, the CRC handbook and MSDS information will inform the reader of the following:

(1)  Copper metal reacts with nitric acid in at least two ways; to form nitrogen dioxide, brown (NO2) (conc. nitric acid) and nitric oxide, colourless (NO) (dil. nitric acid):

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

3Cu(s)  +  8HNO3(aq) → 3Cu(NO3)2(aq) +  2NO(g)  +  4H2O(l)  (dilute)

(2)  Any nitric oxide formed reacts readily with oxygen (O2) in the air to form nitrogen dioxide:

2NO(g)  +  O2(g)  → 2NO2(g)

(3)  Nitrogen dioxide reacts readily with water to form both nitrous (HNO2) and nitric acids:

2NO2(g)  +  H2O(l)  → HNO2(aq)  +  HNO3(aq)

This reaction makes nitrogen dioxide highly corrosive to human tissue. The STEL (short term exposure limit, 15 minutes) is 5 ppm.

(4)  Nitric oxide is inert to and relatively insoluble in water, especially cold water; the solubility is about 8 mL of the gas in 100 mL of water at 0°C.

Next, the story (refer to the original graphic, previous page).

Left image: The initial reaction produces a mixture of nitric oxide and brown nitrogen dioxide; any atmospheric oxygen reacts with nitric oxide, producing more nitrogen dioxide.

Second to left image: The students open the tube, a bit more air enters, but the oxygen reacts with nitric oxide producing more nitrogen dioxide.

Image 1: The nitrogen dioxide readily reacts with the water, forming acids, and the colour of the indicator changes from the basic blue to the acidic yellow of bromothymol blue indicator.

Image 2: The water rises in the tube, since the nitrogen dioxide has all reacted into the water. The residual gas is a mixture of nitric oxide and nitrogen gases.

Image 3: The students remove the tube from the water. Air enters the tube, and the oxygen in the air reacts with the nitric oxide, producing more of the brown nitrogen dioxide.

Image 4: As suggested by the author, the water adhering to the inside walls of the test tube dissolves the newly produced nitrogen dioxide, and the gas in the tube becomes colourless.

[Although the answers in all three responses are basically the same, we decided to print them all because each discussion takes a slightly different approach. This is a good illustration of how different teachers approach the same topic.]