4
$\begingroup$

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?

New contributor
Maharsh Jani is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
$\endgroup$
6
  • 4
    $\begingroup$ 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. $\endgroup$ Nov 25 at 11:16
  • 1
    $\begingroup$ so that cough was due to tiny NaOH and Sodium Aluminate Vapor ? $\endgroup$ Nov 25 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
    Nov 25 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 ? $\endgroup$ Nov 25 at 12:13
  • 1
    $\begingroup$ 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
    Nov 25 at 12:37
7
$\begingroup$

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.

$\endgroup$
0
7
$\begingroup$

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.

$\endgroup$
3
  • 2
    $\begingroup$ 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
    2 days ago
  • 1
    $\begingroup$ @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! $\endgroup$
    – M. Farooq
    2 days ago
  • 1
    $\begingroup$ 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
    2 days ago

Your Answer

Maharsh Jani is a new contributor. Be nice, and check out our Code of Conduct.

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.