# 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.)

• Does it dissolve in water completely? – Ivan Neretin Apr 6 '17 at 7:26
• @IvanNeretin It dissolves completely except for some chalky looking impurities. (The other powder, the sodium chlorite, also leaves impurities visible in the water when dissolved.) I'll measure the solubility when I get home. – piojo Apr 6 '17 at 9:38
• Is it sodium acetate? Because it is very soluble in water to form acetic acid. It has pungent smell and has ph of 2.4. – Nilay Ghosh Apr 6 '17 at 11:36
• @NilayGhosh Don't confuse people. Sodium acetate solution does not have pungent smell; more importantly, it has alkaline pH rather than acidic. True, it does contain some minuscule amounts of acetic acid resulting from hydrolysis, but that doesn't make it an acid. – Ivan Neretin Apr 7 '17 at 6:21
• Did I say "salts"? I meant "chlorides" (including covalent chlorides). $\ce{PCl5}$ would behave like that, but it is most likely too aggressive to be actually used. – Ivan Neretin Apr 7 '17 at 8:49

One way of manufacturing $\ce{HCl}$ produces sodium bisulfate as a byproduct: $$\ce{NaCl + H2SO4 -> HCl + NaHSO4}$$ 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.