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In the preparation of $\ce{NH3}$ from $\ce{NH4Cl}$ and $\ce{Ba(OH)2}$, my teacher said that when heating the test tube, the test tube faces downwards as opposed to upwards. Why is this so? Wouldn't the solid just fall out?

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  • $\begingroup$ This is from the Royal Society of Chemistry in 2016: edu.rsc.org/resources/making-and-testing-ammonia/433.article . The figures show the reaction test tube as horizontal and a suggestion is to put some CaO in that test tube to trap (some of) the water produced. $\endgroup$
    – Ed V
    Mar 6 '20 at 22:55
  • $\begingroup$ "As adverbs the difference between downward and downwards is that downward is toward a lower level, whether in physical space, in a hierarchy, or in amount or value while downwards is towards a lower place; towards what is below." and "In British English, downward (with no final -s) is normally only used as an adjective." Not sure which one is appropriate here. $\endgroup$ Mar 7 '20 at 21:46
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If you mean the test tube containing the solids (reaction tube), then that tube faces downwards to promote the flow of gas into the receiving tube. The idea is to let the gas sink away from the reaction zone and towards the receiving tube as it cools and becomes more dense.

The receiving tube, of course, faces down so that the ammonia gas, lighter than air, can float to the top after it meets the air. So you have the ammonia flowing down with cooling through the reaction tube and then flowing up with buoyancy into the receiving tube.

Now for my bonus question: Had you been producing carbon dioxide by heating magnesium carbonate, would you change the reaction tube? How about the receiving tube?

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    $\begingroup$ Since CO2 is heavier than air, the receiving tube would be right-side up. Something like this? imgur.com/jJh9Ann $\endgroup$ Mar 7 '20 at 0:45
  • $\begingroup$ Yes, the CO2 would just keep sinking in both tubes. $\endgroup$ Mar 7 '20 at 0:48

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