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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: alicia_tan2113 on December 30, 2010, 07:50:29 AM

Title: Identification of ions and gases
Post by: alicia_tan2113 on December 30, 2010, 07:50:29 AM
Describe a simple test, other than displacement, that can be used to show that sea water contains iodide ions.
Test:
Result:______________________________________________________________________________________________________________________________________________

I got stuck at this last question in my exam paper so I just need someone to go through this with me:)
Title: Re: Identification of ions and gases
Post by: Evaldas on December 30, 2010, 08:06:33 AM
Starch? Cos for example to prove that a potato contains starch you take tincture of iodine and put a few drops and the starch particles change color into a dark blue.
Title: Re: Identification of ions and gases
Post by: Fluorine on December 30, 2010, 08:14:50 AM
Silver nitrate test (http://www.chemguide.co.uk/inorganic/group7/testing.html) will give a precipitate...though if you have other halides it may cause a bit confusion. I did a lab on this though I recall very little of the procedure.
Title: Re: Identification of ions and gases
Post by: Evaldas on December 30, 2010, 08:20:28 AM
Silver nitrate test (http://www.chemguide.co.uk/inorganic/group7/testing.html) will give a precipitate...though if you have other halides it may cause a bit confusion. I did a lab on this though I recall very little of the procedure.
But "other than displacement".
Title: Re: Identification of ions and gases
Post by: alicia_tan2113 on December 30, 2010, 08:27:49 AM
Thats why i couldn't think of anything xD
Title: Re: Identification of ions and gases
Post by: Fluorine on December 30, 2010, 05:17:25 PM
I read over that part somehow - oops.
Title: Re: Identification of ions and gases
Post by: alicia_tan2113 on December 30, 2010, 08:32:35 PM
I learnt chromatography,decanting, filtration, evaporating to dryness, crystallization, sublimation... I wonder if that helps.
Title: Re: Identification of ions and gases
Post by: Twigg on December 31, 2010, 11:04:53 PM
Ooooh I used to know this. There's a soluble compound that changes color in the presence of different halogen ions but I can't remember it! I *think* it's AgNO3 or some other silver-based salt.

@Evaldas, that's what I was thinking. I don't think seawater would behave the same way as the tincture though. If it does... I am throwing out my Lugol's solution  ;D
Title: Re: Identification of ions and gases
Post by: Evaldas on January 01, 2011, 06:20:55 AM
Ooooh I used to know this. There's a soluble compound that changes color in the presence of different halogen ions but I can't remember it! I *think* it's AgNO3 or some other silver-based salt.

@Evaldas, that's what I was thinking. I don't think seawater would behave the same way as the tincture though. If it does... I am throwing out my Lugol's solution  ;D

But we said that it's not AgNO3, because "other than displacement".
Title: Re: Identification of ions and gases
Post by: vmelkon on January 01, 2011, 04:26:06 PM
Sea water doesn't contain much iodide ions. How could a visual chemical test show it? Most of it is chloride and to some minor extent, sulfate, some bromide, some nitrate, some borate.
It probably requires mass spectrometry which is much more sensitive and precise.
Title: Re: Identification of ions and gases
Post by: Twigg on January 01, 2011, 04:57:21 PM
Vmelkon: point taken. I think you're on to the right answer. Mass spectrometry would give the identity of iodide ions by their unique mass-to-charge ratio.
Title: Re: Identification of ions and gases
Post by: sci994 on January 01, 2011, 05:18:48 PM
After adding HNO3 and Pb(No3)3 a yellow ptt is formed (PbI2)
Reaction:
Pb(NO3)2 (aq) + 2KI (aq)  :rarrow: PbI2 (s) + 2KNO3

Pb2++2I- :rarrow: PbI2
Title: Re: Identification of ions and gases
Post by: cliverlong on January 02, 2011, 04:36:57 AM
Iodine / thiosulphate clock reaction, Hydrogen peroxide variation?

http://en.wikipedia.org/wiki/Iodine_clock_reaction
Title: Re: Identification of ions and gases
Post by: Evaldas on January 02, 2011, 06:05:10 AM
After adding HNO3 and Pb(No3)3 a yellow ptt is formed (PbI2)
Reaction:
Pb(NO3)2 (aq) + 2KI (aq)  :rarrow: PbI2 (s) + 2KNO3

Pb2++2I- :rarrow: PbI2
Other than displacement!
Plus there are other halogen compounds in sea water, like NaCl which would also react with the nitrate and form a precipitate.
Title: Re: Identification of ions and gases
Post by: alicia_tan2113 on January 02, 2011, 10:59:29 PM
 ;D you guys get my pain xD