This is a question from a 7th grade chemistry class.

Why does adding salt ($\ce{NaCl}$) to a cucumber soup makes it taste more sour?

My best guess is that the salt affects the dissociation levels of acids in the cucumbers - adding $\ce{NaCl}$ shifts the balance and more $\ce{H+}$ is being released, but I don't think thats the answer they are looking for. What affect does $\ce{NaCl}$ have on the sourness of food?

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    $\begingroup$ Salting is for enhancing taste in general. Most likely related to ( perhaps your) taste sensing and biology than chemistry. $\endgroup$ – Alchimista Sep 23 '19 at 8:29
  • $\begingroup$ That could be true, of course :) the question is from a 7th grade chemistry class - the teacher asked the studnets that assigment. Im not a chemist, but I work in medicine and I have no clue to be honest. $\endgroup$ – Mat Sep 23 '19 at 12:05
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    $\begingroup$ Adding NaCl will not shift the proton equilibrium at all. But on the other hand, I don’t know whether the sour taste comes from excess protons. $\endgroup$ – Jan Sep 24 '19 at 2:16
  • $\begingroup$ Note that taste is a subjective physiologic phenomena, influenced, but not determined by substance properties. While the activity of hydrogen ions may be affected by the salt presence, the acidity reporting taste sensors are affected much more. $\endgroup$ – Poutnik Feb 24 '20 at 11:10

This is a really interesting question. Mechanistically, it might all come down to flavor science/biology instead of simple chemistry.

However, I did find a really cool old paper that described how adding salts such as NaCl to acidic HCl solutions can increase the acidity (further lower the pH) of the solution.

The mechanism they suggest is that because the Na+ and Cl- ions are hydrated when dissolved (i.e., they are surrounded by a "shell" of neutral water molecules), those ions effectively end up competing with H+ (or H3O+ or however you want to depict solvated protons, which is a complicated question in itself) and therefore the acidic protons are less "protected" by the surrounding "shell" of water molecules, which increases the acidity.

I would be surprised if this is the answer that the teacher was expecting the 7th graders to come up with, though.

  • $\begingroup$ That's interesing, even for general knowledge. In all my teaching classes nobody metioned that, as probably it is either not know, or the outcome is so marginal. After some thinking I concluded that the nawser to my question may be coneccted not such with acid reaction as more with osmosis. I remembered that when you make sour cabbage you salt it to get the water out of the cells. So maby more water equals more protons from the organic acid from the cucumbers. Its just a guess thew. $\endgroup$ – Mat Sep 24 '19 at 5:49
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    $\begingroup$ @Mat your last comment is probably relates to what was in the mind of the teacher. Say that perhaps a salad preparation could be more appropriate as example. Removal of water might enhance taste of food, at least if a relationship "more concentration of something → more taste" holds. Honestly as formulated I don't think it has an answer. Cucumber aren't really sour first of all, and taste isn't a straightforward linear chemical parameter. I think the teacher created an exercise out of his personal experience. Perhaps an oversalted soup made it :) $\endgroup$ – Alchimista Sep 24 '19 at 8:05
  • $\begingroup$ That is quite possible :) $\endgroup$ – Mat Sep 24 '19 at 9:54

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