O' Level TYS Chemistry question :)

iamapebble SG

Hello :) It's the first time I'm posting in SG forums though I've been reading for a long time already. Here's a chemistry TYS question which bugged me for very long and yet to have a answer, I hope you guys will help me out okay? Thank you :D

"When testing for a chloride ion with silver nitrate, why must we first acidify the compound with nitric acid?"

As we all know the test for chloride ion is first to acidify with nitric acid and then add silver nitrate, a white precipitate is then formed to indicate it is a chloride ion. But why must we add the nitric acid?

^tamago^ SG

Because silver hydroxide is also a white, insoluble substance, and could confuse your test for chloride ion.

iamapebble SG

Hey sorry but I'm quite slow. Means:

XCl + AgNO3 -> AgCl [white ppt] + XNO3

XOH + AgNO3 -> AgOH[white ppt] + XNO3

I get this part, but I still don't understand why we must acidify it.. >.<

secretliker SG

It's explained in the equations already. Because both AgCl and AgOH are white ppt, simply adding AgNO3 cannot be a complete test for presence of chloride ions.

It's because if there are XOH and XCl present in the solution, then the HNO3 added would undergo acid-base reaction with the HNO3 to form XNO3 and H2O. XCl would react with HNO3 to form XNO3 and HCl.

Now, after adding AgNO3, you can be sure that the white ppt is AgCl.

[I hope I'm right.]

iamapebble SG

OH, I think I get it, thanks so much to you both! (: