Which white powders form acids when mixed with water? I have a mystery acid which isn't citric, boric, or oxalic acid. When mixed with water, it smells pungent, like hydrochloric acid--though I haven't compared them side by side.

Which other white powders form a strong acid upon being combined with water? Is there any powder that decomposes into HCl when mixed with water?

(Note: the mystery acid was sold as part of a set, along with sodium chlorite, for generating chlorine dioxide for use in aquariums.)
From the pH and solubility information alone, I would propose sodium bisulfate: cheap, readily available, safe in the powerfully oxidizing solution. (Possibly also potassium bisulfate.)

One way of manufacturing $\ce{HCl}$ produces sodium bisulfate as a byproduct:
$$\ce{NaCl + H2SO4 -&gt; HCl + NaHSO4}$$ The odor of $\ce{ HCl}$ could be a minor impurity in this mystery acid. The other impurities also suggest it is not a reagent grade product.