>>>
In my opinion, all the above answers have failed to notice
something crucial. It is not about the precipitate of AgCl coming
out all these stuffs.
Because: If you add silver nitrate after adding nitric acid,
THERE WILL BE NO NET REACTION between the two of them.
Let's see. Silver nitrate + nitric acid => ??? NET Change
There is no NET change at all.
This is why you MUST add silver NITRATE with nitric acid. Both
of similar anion base.
Whereas, if you added silver SULPHATE (which works, trust me)
with nitric acid, what happens is:
Ag2SO4 + 2HNO3 --> 2AgNO3 + H2SO4
A pretty screwed up anion test, huh?
When these two react, well maybe your
precipitate wouldn't be so visible. That's the way I'm taught. Btw
the maybe part is MY OPINION, don't blame my teacher.
I emphasised the "NET" because there is change. AgNO3 does
react with HNO3, but in the end, the overall change is nothing
because basically the products=reactants.
And you can use Ag2SO4, but you must use H2SO4 in place of
HNO3's function <<<
Good conceptual theorizing attempt, Garrick, and while you're
not "wrong", but the point is not so much having 'no net change' in
the sense of deliberately adding a 'nitrate' to another 'nitrate'
for the sake of keeping the anion as 'nitrate' only, but rather, it
is primarily the solubility that we're concerned about. So I'd say
you're right, but your phrasing needs elaboratio.
From the way the question is phrased, the best answer still has
to do with firstly the Ag+ (forming AgCl(s) with Cl-), and secondly
using 'nitrate' (not to prevent 'net change', but rather) to safely
avoid any false positive ppts, because nitrates are the safest to
use (since ALL nitrates are always soluble).
So again, what you mean to say is, we could use
alternative combinations of acid and silver salt, as long as they
do not react to cause false positives, by precipitating out the
Ag+(aq) ions before they have the chance to precipitate out with
the Cl-(aq) ions in the test solution.
>>> Whereas, if you added silver SULPHATE (which works,
trust me) with nitric acid, what happens is:
Ag2SO4 + 2HNO3 --> 2AgNO3 + H2SO4
A pretty screwed up anion test, huh? When these two react, well
maybe your precipitate wouldn't be so visible.
<<<
No, it's not 'screwed up' at all. First of all, your equation
isn't correct, for 2 reasons.
Silver sulfate is rather insoluble (ie. minimally soluble), so
you can't use it at all. (How can you test for ppt if what you're
adding is itself a ppt?)
And even if assuming it was soluble, you would not 'exchange
ionic partners' in a clean-cut fashion like your equation
implied, because both your RHS products are soluble. (ionic
partners only 'exchange' in an ionic precipitation reaction, where
one RHS product compound is insoluble).
Rather, it would be an aqueous mixture of all 4 possible
aqueous compounds, because all 4 compounds are soluble.
So again, good try, Garrick, but you will want to elaborate
further to say the reason why we use silver nitrate
acidified with nitric acid, is again :
1) Silver cations to precipitate out any Cl- ions present
2) Nitrate anions because all nitrates are SOLUBLE and
hence a) allows use to provide *aqueous* Ag+ ions, and b) won't
cause false positive ppts in the test solution (rather than
"no net change" as the reason.).
3) Adding protons (H+ ions) for this reason
http://www.sgforums.com/forums/2297/topics/320107 control-F
(ie. "find") "silver nitrate"
Garrick's essential point is point#2a, but he/you shouldn't
phrase it like "ensuring no net change" and "making the ppt
harder to see", but rather emphasize it's about *solubility*,
ie. ensuring that Ag+(aq) ions are still available (in
aqueous state) AND that no other ppt is formed before
adding the mixture to the test solion.
Also, you should note that silver chloride is a white ppt,
silver bromide is a cream ppt, and silver iodide is a yellow ppt.
So the colour must be stated precisely.
"After adding nitric acid and silver nitrate into the test
solution, if a white precipitate is observed, we conclude chloride
anions must have been present in the test solution, and that the
white ppt must be silver chloride."
Finally, again, I'd recommend everyone read this :
"Give two possible reasons why nitric acid must be
added, before adding AgNO3(aq)". For the answer, control-F (ie.
"find") "silver nitrate" here :
http://www.sgforums.com/forums/2297/topics/320107