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In my highschool practical examination yesterday, I was given an inorganic salt to identify using chemical tests. The test I performed for the presence of chloride was to heat salt in a test tube with $\pu{2-3 mL}$ of mildly concentrated sulphuric acid. I observed white fumes of $\ce{HCl}$ (or so I thought), so the procedure was to keep a glass rod dipped in $\ce{NH4OH}$ over the mouth of the test tube. I observed (denser?) white fumes of what I thought was $\ce{NH4Cl}$, but apparently not as my salt was actually aluminium nitrate as I confirmed later (brown $\ce{NO2}$ is unmistakable, and aluminium confirmed via cobalt nitrate (blue) ash test).

  • What exactly did I observe and why?
  • Could I prepare $\ce{H2SO4}$ by heating $\ce{NaHSO4}$ and $\ce{HCl}$? If not, why only the other way?
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2 Answers 2

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I agree with Maurice's answer, here I add some details:

  1. Moisture in the air may stabilize nitric acid against decomposition, which would keep the fumes white or at least more white.

  2. Ammonia can react to produce ammonium nitrate, which would form more intense fumes than the acid alone. The ammonium nitrate fumes would be more stable against decomposition and woukd remain white.

  3. A drop of silver nitrate solution (or a rod that has been dipped in such a solution) held over the reaction mixture would form a precipitate with a hydrochloric (or hydrobromic or hydriodic) acid fumes but not with nitric acid.

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  • $\begingroup$ I live in one of the most humid places in the world, just a few kilometres from the ocean. This makes a lot of sense, thank you. $\endgroup$ Commented Jan 19 at 2:14
  • $\begingroup$ I swore I could smell ammonia, is this produced by decomposition of ammonium hydroxide? $\endgroup$ Commented Jan 19 at 2:18
  • $\begingroup$ 3. This is indeed the confirmatory test, i shouldve done it but my lab teacher just told me it's nitrate lol $\endgroup$ Commented Jan 19 at 2:19
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    $\begingroup$ "Ammonium hydroxide" is not really so. It's just ammonia solution that has a few hydroxide ions in it because ammonia is a weak base. So your ammonia smell is directly from the solute. $\endgroup$ Commented Jan 19 at 2:28
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If your powder, mixed with "mildly" concentrated sulfuric acid, produces white fumes that react with ammonia, the white fumes may be $\ce{HCl}$ , $\ce{HBr}$, or $\ce{HNO3}$. But if it is $\ce{HNO3}$, the fumes are usually not white, as nitric acid is often partly decomposed into brown $\ce{NO2}$. Have you ever seen any slightly brownish fumes ?

Second question. $\ce{H2SO4}$ cannot be obtained from $\ce{NaHSO4 + HCl}$, because the reaction proceeds in the reverse direction. Heating a mixture of solid $\ce{NaCl + H2SO4}$ produce gaseous $\ce{HCl}$ plus $\ce{NaHSO4}$. So the only thing that can be made by heating a mixture $\ce{NaHSO4 + HCl}$ are vapors of $\ce{HCl}$.

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  • $\begingroup$ Thank you for the answer. Actually, the manual recommends concentrated H2SO4 but i doubt the one provided is very concentrated hence the 'mildly'. $\endgroup$ Commented Jan 18 at 17:02
  • $\begingroup$ No, I don't remember seeing brownish fumes, it was mostly white/offwhite. Its probably just my inexperience and I should've done the test again, but unfortunately in the exam writing matters more than performing. $\endgroup$ Commented Jan 18 at 17:05
  • $\begingroup$ Why does the reaction proceed in the reverse direction? Does it have anything to do with electrode potentials? $\endgroup$ Commented Jan 18 at 17:08
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    $\begingroup$ The reaction proceeds according to H2SO4 + NaCl -> NaHSO4 + HCl and not in the reverse direction, because HCl is the most volatile substance, and gets first out of the mixture, especially when heating a bit. This behavior has nothing to do with potentials, as the oxidation numbers of the atoms do not change during the reaction. $\endgroup$
    – Maurice
    Commented Jan 19 at 9:32

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