system
Given the following post, respond as if you're commenting on Reddit:
human
Is the residue left from an effervescent vitamin tablet salt, or vitamin C?

My friend is aware of how much salt these tablets have, so he always throws away the crystalline residue left in the bottom of the glass. 

I was thinking that since the salt is what actually dissolves the tablet when put into water, the residue at the bottom could actually be the vitamin C he wanted in the first place.

Does anybody know for certain? Does anybody know how well ascorbic acid dissolves in cold water? Could it form a precipitation at all?
gpt
Ascorbic Acid is extremely soluble in water (50g per 100 mL, i.e. 50x more than you probably need). As is table salt (NaCl)

Most of the reactants from fizzing are also extreme water soluble. Most of them are based around citric acid and Sodium and Magnesium Carbonate. What is most likely left in the bottom of the glass are pill binders, which are usually some kind of fats+gums, like Magnesium Sterate.

There is a chance of some Magnesium Carbonate precipitating out, but generally the solutions are all too acidic for that.

TLDR, the stuff in the bottom of the glass is mostly the fats and gums used to hold the pill together. It's not NaCl or Ascorbic Acid.