I have some sodium carbonate. But it's over 20 years old and was not kept in tight bottle. I wonder if this compound absorbs $\ce{H2O}$ and $\ce{CO2}$ from air.
I guess I'd need to burn it a little to turn it back in case the answer was yes.
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$\begingroup$ The hydroxide tag would have been much too localised (therefore pretty soon be deleted again) and I do not see any fit for this question anyways. $\endgroup$– Martin - マーチン ♦Commented Jun 25, 2014 at 2:01
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$\begingroup$ Hmm...The "burn it a little" link covers conversion of sodium bicarbonate to sodium carbonate. The current question, as I read it, is starting with the carbonate. Over time at ambient, the hygroscopic sodium carbonate just absorbs water, no carbon dioxide, and forms different hydrates. So it can be "burned back" (dry it) to the anhydrous carbonate. $\endgroup$– ronCommented Jun 25, 2014 at 2:15
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$\begingroup$ I thought that absorbing water turns it into the bicarbonate, rather than just wet powder. $\endgroup$– Tomáš ZatoCommented Jun 25, 2014 at 5:51
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Industrially, sodium bicarbonate is produced by reacting sodium carbonate with water and carbon dioxide. However, under normal household conditions sodium carbonate simply reacts with water or moisture in the air (it's hygroscopic) to form a hydrate - it doesn not form sodium bicarbonate. The water of hydration can be removed by mild heating (~100 C). At high temperatures (>850 C) sodium carbonate can decompose according to the following equation.$$\ce{Na2CO3(s) -> CO2(g) + Na2O(s)}$$
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$\begingroup$ I fear you've understood my question inversely. I asked if $\ce{Na2CO3 + CO2 + H2O -> NaHCO3}$ over time. Your answer only tells me that sodium bicarbone releases it's water. I thought however, that this only happens in heat over 150°C. $\endgroup$ Commented Jun 24, 2014 at 21:16
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$\begingroup$ Oh, I see that I've stated wrongly what's in the bottle. $\endgroup$ Commented Jun 24, 2014 at 21:17
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$\begingroup$ See my sentence beginning with "The sodium carbonate", it says that the sodium carbonate can NOT be converted back to sodium bicarbonate. The bicarbonate can release its water and carbon dioxide (slowly) at 25 C, especially at high humidity. $\endgroup$– ronCommented Jun 24, 2014 at 21:20
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$\begingroup$ Yes, I've read that and I've understood that. I just wanted the question and answer to make sense according to the title. Otherwise I thank you for the answer. $\endgroup$ Commented Jun 24, 2014 at 21:34
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$\begingroup$ I'm aware of the SE system. I was hoping that you'll edit the answer so that it's not confusing. Though it's my fault that I had a mistake in the text of the question, the title is explanatory. The edit would be too radical for me to do it myself. But to give you a guide, After 20 years I'm sure some of it has degraded does not match question does sodium carbonate degrade? unless it indeed degrades to $\ce{Na2O}$ at room temperature. $\endgroup$ Commented Jun 24, 2014 at 21:43