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I heard a claim in the context of soapmaking that a stray grain of sodium hydroxide will decay into soda ash, i.e. sodium carbonate, on contact with air. This reaction happens in an aqueous solution, but I'm wondering how fast it would happen with only the air moisture absorbed by the hydroxide grain.

More to the point, some of the sodium hydroxide grains I have for soapmaking seems to have stuck together into a solid lump by absorbing air moisture inside the closed container. Should I assume a nontrivial fraction of that has turned into sodium carbonate by now?

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    $\begingroup$ Rather reacts with aerial CO2, as compounds not containing carbon cannot decompose to compounds containing it. $\endgroup$
    – Poutnik
    Jul 12, 2022 at 20:18
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    $\begingroup$ a) nobody can guess how tight your closed containers are. b) what does carbonate do when you add a few drops of vinegar? $\endgroup$
    – Karl
    Jul 12, 2022 at 21:25
  • $\begingroup$ Related: chemistry.stackexchange.com/questions/27221/… $\endgroup$
    – Nilay Ghosh
    Jul 13, 2022 at 3:55
  • $\begingroup$ Consider that, even with vastly increased CO2 in the atmosphere since the Industrial Revolution, it's << one part in a thousand. Depending on the rate of infiltration into the container, the biggest contaminant is just water, fusing the pellets together -- NaOH is very hygroscopic. $\endgroup$
    – DrMoishe Pippik
    Jul 13, 2022 at 18:31

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