In a sample solution, containing $\ce{Cl-}$, while conducting the test for detection of $\ce{Cl-}$ ion first it is acidified with conc. $\ce{HNO3}$ then cooled and then $\ce{AgNO3}$ solution is added. A curdy white precipitate appears which is soluble in $\ce{NH4OH}$ but reappears on adding $\ce{HNO3}$.

My question: is the $\ce{HNO3}$ added dilute or concentrated?

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    $\begingroup$ Could be either... In fact if the salt were say sodium chloride, then HNO3 (both dil and conc) would give HCl, which then reacts with silver nitrate to form silver chloride. $\endgroup$ – Varun Feb 17 '16 at 9:36
  • $\begingroup$ HCl + AgNO3 -> AgCl + HNO3? So the nitric acid is regenrated? $\endgroup$ – Tyto alba Feb 17 '16 at 9:59
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    $\begingroup$ The question clearly states the HNO3 is conc. The reason to add a few drops of conc. nitric acid is to remove any other anions contaminating the sample that might give a false positive to the silver nitrate test. $\endgroup$ – Spontification Feb 17 '16 at 14:47

To best understand, let us consider the reactions occurring.

First, we have $\ce{Cl-}$ ions in solution. Acidifying the solution serves to remove other anions that might give a false positive for the chloride test. Upon addition of $\ce{AgNO3}$, the following reaction occurs:

$$\ce{Ag+(aq) +Cl-(aq) -> AgCl (s)}$$

When enough ammonia is added to neutralize the $\ce{H3O+}$ in solution:

$$\ce{NH3(aq) +H3O+(aq) -> NH4+(aq) + H2O(l)}$$

The ammonia can then begin to form a metal complex with the $\ce{AgCl}$ that is soluble in water:

$$\ce{AgCl(s) +2NH3(aq)->Ag(NH3)2+(aq) +Cl-(aq)}$$

When $\ce{HNO3}$ is reintroduced, the ammonia ligand is protonated because it is more basic than water, and the metal complex is destroyed, reforming $\ce{AgCl}$: $$\ce{Ag(NH3)2+(aq) +2H3O+(aq) +Cl-(aq) ->AgCl(s) +2NH4+(aq) +2H2O(l)}$$

Since you will likely be working with relatively small amounts, it is likely that you will be using concentrated acid in stoichiometric amounts to produce these products.

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    $\begingroup$ I'd quibble with notion that the sentence "Acidifying the solution serves to remove other anions that might give a false positive for the chloride test." Since the acid is added to the same solution, no other anions are removed, with perhaps the exception of carbonate. The solution needs to be acidic so that silver hydroxide doesn't form when the silver nitrate is added. $\endgroup$ – MaxW Feb 17 '16 at 23:00
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    $\begingroup$ I can't find any indication that this reaction would occur. Silver hydroxide appears to be too unstable a compound to even form. $\endgroup$ – ringo Feb 17 '16 at 23:12
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    $\begingroup$ You're right. It isn't silver hydroxide, but rather silver oxide that forms in alkali solutions. en.wikipedia.org/wiki/Silver_oxide#Preparation $\endgroup$ – MaxW Feb 17 '16 at 23:15
  • $\begingroup$ @ringo What is the source of NH3 here? Is it NH4OH? But it gives NH4+. $\endgroup$ – Tyto alba Feb 18 '16 at 15:56
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    $\begingroup$ $\ce{NH4OH}$ is just another way of saying $\ce{NH3(aq)}$. $\endgroup$ – ringo Feb 18 '16 at 18:59

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