The fizz when adding salt to cola (or any other sparkling drink, like
seltzer or even champagne) is as well a chemical as a physical
reaction, or better: A chemical reaction based upon physical reasons.
Cola or any other sparkling drink, like seltzer or even champagne,
contains carbon dioxide. This substance reacts with water and forms
carbonic acid, what is actually causing the bubbles in the liquid. So
far, this is clearly a chemical reaction.
However, when carefully examining a glass filled with a sparkling
drink, you will notice that the rising bubbles form streams each
beginning at a certain starting point in the glass. This is normally a
small, even invisible scratch or notch on the inner surface of the
glass. The bubbles can form easily around a sharp edge, and such a
point where the reaction between water and carbonic dioxide with the
result of carbonic acid takes place is something physical.
Salt consists of small crystals, each having numerous edges. So, when
salt is added to a sparkling drink, the carbon dioxide suddenly has
much more points to react with water and form carbonic acid. By this,
much more bubbles develop and rise to the liquids surface, giving the
impression of a fizz. The amount of carbon dioxide in the liquid will
soon be exhausted, and the reaction will expire very fast. Also, the
salt will dissolve in the liquid and the many sharp edges of the salt
crystals will be gone then.
Now you see: The fizz is a fugacious mega-sparkling (a chemical
reaction) which is caused not by the chemical properties of the salt
added, but by its physical properties (the edges of the crystals).
This whole process is also explained very intelligibly here:
Search terms used:
salt soda reaction fizz carbonic acid:
Hope this was what you wanted to know!