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I was doing an experiment with $\ce{NaOH}$ dissolved in some $\ce{H2O}$ and aluminium. My textbook says that the following reaction takes place

$$\ce{2 NaOH + 2 H2O + 2 Al -> 2 NaAlO2 + 3 H2}$$

It also says that hydrogen gas ($\ce{H2}$) is colorless, odorless, tasteless and highly combustible. But when I did the experiment the gas was kind of having a strong smell (not of rotten eggs) but indeed it was a strong smell which got me coughing really hard.

When I was doing some research about this I found this video with the same results:- https://www.youtube.com/watch?v=nG0qUBxcvko&ab_channel=ChanKaiWee ( After the 30 second mark)

Can someone please explain why this happens?

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    $\begingroup$ Tiny droplets of the solution carried into the air along with the evolved gas. May want to to this reaction in a better-ventilated area or even under a laboratory hood. $\endgroup$
    – Oscar Lanzi
    Commented Nov 25, 2021 at 11:16
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    $\begingroup$ so that cough was due to tiny NaOH and Sodium Aluminate Vapor ? $\endgroup$
    – Maharsh Jani
    Commented Nov 25, 2021 at 11:41
  • $\begingroup$ Proper enumeration of chemical reactions requires just ability to count and compare small integer numbers, what is taught early in elementary school. 3=2 is not true just because it is in the reaction. // Traces of other elements may create volatile smelly hydrides, as the result of intensive reduction process. $\endgroup$
    – Poutnik
    Commented Nov 25, 2021 at 11:53
  • $\begingroup$ it might be because I used tap water which might have some impurities and trace substances but is it enough for that strong of a smell ? $\endgroup$
    – Maharsh Jani
    Commented Nov 25, 2021 at 12:13
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    $\begingroup$ If we can exclude effect of aerosol of hydroxide solution, human nose is quite sensitive for some volatile compounds of heavy elements and these traces can have origin in Al. ( See e.g. telluric breath, being perceived months after tellurium exposure - episodictable.com/tellurium ) $\endgroup$
    – Poutnik
    Commented Nov 25, 2021 at 12:37

3 Answers 3

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Oscar Lanzi's answer addressed the points correctly. This reaction of Al + NaOH pellets is used in drain openers for obvious reasons. The reaction is so exothermic that solution can start boiling. It generates a lot of heat, efferevesence and steam.

The key point is that only chemically pure hydrogen is odorless and colorless, otherwise it is not. The OP mainly smelled the caustic mist that is generated by vigorously evolving hydrogen bubbles which bursting over the solution surface. This reaction should be carried out in a fume hood.

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    $\begingroup$ I used to make hydrogen this way, when I was a kid and didn’t have manganese available. I always thought the stinky smell was due to impurities. In any event, it is easy to do an experiment: filter out all droplets with a drying tube, etc., and smell the dry gas. $\endgroup$
    – Ed V
    Commented Nov 25, 2021 at 20:53
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    $\begingroup$ @EdV, the garlic like stench of hydrogen is usually present when an acid is used with iron or zinc. With NaOH and Al, there is no stench, it is just the caustic spray from hydrogen bubbles that severely irritates the nose. You also reminded of childhood experiments! $\endgroup$
    – ACR
    Commented Nov 26, 2021 at 0:23
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    $\begingroup$ Interesting. I know from several experiences that powdered iron, reacting with either hydrochloric acid or sulfuric acid, produces bad smelling hydrogen. The last time I used the Al and NaOH method, as a high school kid, I underestimated the time required to flush out enough air in the generator, so lighting the hydrogen resulted in a loud bang and a piece of glass tubing embedded into a ceiling beam in my basement lab. I would not have thought glass could do that! $\endgroup$
    – Ed V
    Commented Nov 26, 2021 at 2:19
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The consensus is that the odor could come from multiple causes:

  • Droplets of solution: Evolved gas (or even gas being blown over the liquid, as with winds blowing over a lake or ocean) can carry tiny particles of the liquid, which in this case is a caustic solution. In general, any gas-evolving reaction likely involves a toxic or corrosive medium that reacts to produce the gas, so it is good practice to carry out chemical reactions that evolve gas in a laboratory hood or in a well-ventilated setting.

  • Impurities: Most likely, the aluminum was an alloy whose al-loying elements may also react with the sodium hydroxide or even with the highly active nascent hydrogen that forms transiently during the main reaction. Products of such side reactions may trigger odors even at low levels. These may also be toxic and the same safety precautions noted above apply.

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Actually, the reason for the smell is very important as a related reaction (Sodium hydroxide added to an Aluminum soda can with residual amount of a phosphate based drink) is reported in Bretherick's Handbook of Reactive Chemical Hazards, Volume 1, page 23-24, to quote:

"A student mixed aluminium foil and drain cleaner in a soft drink bottle, which started emitting gas. Another student carried the bottle outside and was claimed to have been overcome by the toxic fumes [1]. Most drain cleaners are alkalis, so that aluminium will dissolve to produce hydrogen. The bleach that it is is suggested may have been present will produce no toxic fumes in alkali, and one would be surprised to find arsenic or antimony compounds present. If the collapse was not purely hysterical, the remaining, though remote, possibility would be phosphine. The soft drink the bottle had contained was one of the many perhaps best described as impure dilute phosphoric acid [2]. "

Basically, nascent hydrogen or more precisely, the potentially effects of reducing hydrogen atom radical acting on impurities and anions present.

Associated metal impurities occurring in the case of this question, can also produce very problematic compounds, as further noted in Bretherick's handbook.

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