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I mixed 2 spoons of baking soda with 1 spoon of salt and mixed them with water (but not mixing totally) and left them for several days. At first I saw some bubble but the water was clear, but then it got this layer of precipitation on top. I still use it to rinse my mouth, but then I notice it has this really bad odor that grows worse day by day. I threw it all away, but the mixture/precipitation stick to the side of the glass too and it's very hard to brush off.

What happened? Was there any toxicity? I googled everywhere, but they say there couldn't be reaction between $\ce{NaCl}$ and $\ce{NaHCO3}$. Could it be the air and or something in the water that added to it? Or the glass?

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    $\begingroup$ Fungus and mold growth. The layer of "precipitation" on the top, as you said, is fungal/bacterial growth. Common sense dictates that you should discard such solutions, let alone use them for rinsing your mouth. $\endgroup$
    – ACR
    Commented Aug 26, 2021 at 14:09
  • $\begingroup$ I have used that mouthwash for several years with no problem. I mix a cup of each dry powder together then use a small amount to make fresh water solution a few times per day. $\endgroup$ Commented Aug 26, 2021 at 15:08
  • $\begingroup$ It is not much different than seawater, and many things grow on that. $\endgroup$ Commented Aug 26, 2021 at 16:05
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    $\begingroup$ All aqueous solutions when maintained several days in the open air, get dust, batteries and pollen from the wind. This produces mold growth and bad oder. Nothing unusual. Discard these solutions ! $\endgroup$
    – Maurice
    Commented Aug 27, 2021 at 20:26

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First, per a reference on personal care products, to quote:

Many personal care and pharmacy products contain a mixture of citric acid and sodium bicarbonate.

So chemically, you may actually have a mix of citric acid and NaHCO3, which per an educational source can react as follows, to quote:

In the presence of water, citric acid and sodium bicarbonate (aka baking soda) react to form sodium citrate, water, and carbon dioxide. Students investigate this endothermic reaction...

So one possible explanation upon mixing NaCl, citric acid and NaHCO3 with a limited amount of water could form a citrate as discussed in this Wikipedia article on trisodium citrate.

Interestingly, its reported appearance is a white crystalline powder. Further, quoting the Wikipedia article:

Sodium citrate is employed as a flavoring agent in certain varieties of club soda. It is common as an ingredient in bratwurst, and is also used in commercial ready-to-drink beverages and drink mixes, contributing a tart flavor.

So your tasting experience may have help in the identification of this distinct unexpected salt.

I would also note as a possible explanation that adding CO2 and NaCl to tap water lowers dissolved oxygen content. This may foster, in time and exposure to dust, a more biological-based growth that would, however, not have a crystalline salt appearance.

I hope this helps.

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    $\begingroup$ How have you come to conclusion citric acid is involved in any way? Batches of baking soda or table salt do not contain citric acid. $\endgroup$
    – Poutnik
    Commented Aug 28, 2021 at 8:30
  • $\begingroup$ First, I cite the citric acid presence as a 'possibility'. I also vaguely recall reading the contents on bicarbonate mixtures in the grocery store. Per a source: "baking powder (a mixture of sodium bicarbonate and at least one other compound). Baking powder differs from baking soda in that it includes an acidic compound that reacts with sodium bicarbonate to produce carbon dioxide. One of the most common compounds mixed with sodium bicarbonate in baking powder is tartaric acid..." at encyclopedia.com/science-and-technology/chemistry/… $\endgroup$
    – AJKOER
    Commented Aug 28, 2021 at 14:56

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