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Name: Scott
Status: educator
Grade: 9-12
Location: CA
Country: N/A
Date: August 2006

Question:
When salt dissolves, the sodium is separated from the chlorine by the polar water molecules. Why does it still taste "salty". The salt has not undergone a phase change if it was not liquefied. I do not understand.



Replies:
Scott,

I am going to take a stab at this, although I am not an expert in flavors.

This is kind of a tricky question because it is difficult to pinpoint what exactly in a particular molecule makes it taste the way it does. Is it the combination of NaCl that makes for the salty taste, or is it the individual ions of Na(+) and Cl(-) that makes the salty taste?

We have some clues. We know for example that when MgCl2 is found in rock salt that the presence of MgCl2 makes the rocks taste bitter. We also know that baking soda (NaHCO3) does not taste as salty as NaCl. If we tabulate a list of Na(+) and Cl(-) containing compounds, we do find that the particular saltiness of NaCl seems to come from that particular combination of Na and Cl. This would seem to suggest that it is NaCl that is salty and not Na(+) or Cl(-).

However, we also know that NaCl immediately ionizes in the presence of water, and that saliva is mostly water and is particularly good at carrying ions to our taste buds. So, even if we were to put rock salt on our tongues, it is still the saliva (and the water in it) that carries the flavors to our senses. Thus, one would have to say that it is the ions that we are tasting, but it is the particular combination of Na(+) and Cl(-) ions that give us the "salty" taste.

Greg (Roberto Gregorius)


Scott,

The dissolved salt is in fact what you are tasting in the first place. As soon as the solid salt touches your tongue, your saliva dissolves the salt to form sodium and chloride ions. Our taste perception of salty depends solely on the fact that sodium and chloride ions are present. There are other types of salts, (potassium chloride, magnesium chloride etc) that will also taste salty, but it is primarily the chloride that gives off the salty and bitter taste sensations. Do not confuse chloride with chlorine, which of course, tastes different. Your tongue can tell the difference between a chloride ion and a chlorine molecule. But per your question, your tongue detects chloride for salty and if the chloride is tied up as a solid it can not taste it. Sodium chloride is hygroscopic, meaning that it will pick up water from the air and will start to dissolve itself naturally. The bonds between the sodium and chloride ions are weak and break easily.

Matt Voss


The question you pose is difficult and not well understood. "Taste" is a physiological response of the mouth, tongue, and throat. "Taste" also involves odor as well but that is even more complicated and not well understood. The mouth, tongue, throat produces a physiological response by "receptors" located in various parts. So NaCl, NaBr, NaI have a different "taste" because the physiological response depends upon the presence of both ions. So, LiCl, NaCl, KCl, RbCl, CsCl also have a different "taste" even though the anion is the same. So "salty" is not so much related to a phase change but rather to a complicated signal sent from the mouth, tongue, throat to the brain that depends upon both the cation and anion. Of course, this interaction also occurs with other cations such as Mg, Ca and anions such as (SO4), (PO4) etc.

I don't think the precise mechanism is understood -- but do not experiment with a "taste test" because some of these combinations may be toxic as well.

Vince Calder



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