In my school chemistry book, the experiment about the reduction of metal oxides with ammonia has a diagram of the set up as follows:

enter image description here

$$\ce{NH₃ + CuO → 3Cu + 3H₂O + N₂}$$

The tube entering the U-tube from combustion tube A is not dipped into the condensed water.

So doesn't that mean that nitrogen (density: $\pu{1.165 kg/m^3}$), being lighter than water(density: $\pu{1000 kg/m³}$) will push the water up on reaching a sufficiently high pressure and the water will flow up through Tube B?

I believe nitrogen gas won't pass through as bubbles as the tube is not under the water level. Nitrogen is also almost totally insoluble in water, hence the nitrogen doesn't dissolve in the water either(which can also be seen by nitrogen gas obtained in the gas jar by downward displacement of water) at the right side of the setup.

So is there a flaw in the diagram? Or am I missing something?

  • 1
    $\begingroup$ It is rather a problem of mechanics and hydrostatics. With sufficient pressure, gas will push liquid down until it bubbles through. But I agree the experiment is not well designed, they should avoid water accumulation in the gas path. $\endgroup$
    – Poutnik
    Sep 19, 2021 at 8:58
  • $\begingroup$ I suspect the purpose of the U-tube is to trap water (the label says "condensed water" and the tube is surrounded by a coolant). If so, the picture is very badly drawn as the volume condensing would not block the tube and prevent gas flow. $\endgroup$
    – matt_black
    Sep 19, 2021 at 11:57
  • $\begingroup$ @matt_black Additionally, there is not addressed "sucking up" gas volume because of gaseous ammonia dissolution in water. The reportedly condensed water could as well be remnant of sucked up water from the end cylinder. :-) $\endgroup$
    – Poutnik
    Sep 19, 2021 at 12:09

1 Answer 1


No, the experiment as depicted is basically correct, see my comment below, however, on what is likely understated.

Remember ammonia is highly soluble in water, so without Tube 'B', the water in Tube 'C' could backflow.

In reality, the volume of Tube 'B' is much larger than depicted to address backflow issues. I would personally replace it with a large heavy jar that sits in the cold water basin (while a light vessel could problematically float, hence the original design of secured tubing).

Don't believe, see what happens upon removing the backflow trap (aka, Tube 'B') when all the CuO is consumed leaving just highly soluble ammonia gas in the system.

  • $\begingroup$ So is the diagram drawn considering the unreacted ammonia proceeding further into the system? Because the explanation provided in our book avoids any mention of unreacted ammonia and assumes that a perfect mass of ammonia is being passed into the setup, just enough to complete the reaction without any residual ammonia left. Also, can you just elaborate slightly more why will the backflow take place if tube B is removed? $\endgroup$ Sep 19, 2021 at 14:49
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    $\begingroup$ This would be a bad setup if the purpose if to avoid backflow. Moreover the U-tube is labelled "condensed water" and is surrounded by what seem to be coolant. This implies it is designed to condense out the surplus water from the reaction not to avoid backflow. But it incorrectly shows the tube to be overfull with the condensate which would be unlikely in this reaction. $\endgroup$
    – matt_black
    Sep 19, 2021 at 20:20

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