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Ok here's a question:For the GCE or even GCSE 'O' levels examinations (Cambridge), under qualitative analysis:-to test for the presence of anions such as chlorides, iodides and sulphates, it is mentioned that we should acidify it with dilute nitric acid, before we proceed to add silver nitrate, lead(II) nitrate and barium nitrate respectively.chloride : add dil. nitric acid, then add silver nitrate --> white ppt. AgCl formediodide : add dil. nitric acid, then add lead(II) nitrate --> yellow ppt. PbCl2 formed
sulphate : add dil. nitric acid, then add barium nitrate --> white ppt., BaSO4 formedmy colleague said it is to remove other anions such as carbonates and hydroxides that might interfere with the reaction, is that true?Is there any other underlying reason?
Thanks peeps...ok for my next question. Aluminium, lead (II) and zinc, when they form a precipitate with sodium hydroxide, it will dissolve in excess. My question here is, is it because they form sodium aluminate, sodium leadate (not sure of the naming) and lastly sodium zincate? Is it because of their nature as amphoteric hydroxides? Al(OH)3 + NaOH --> NaAl(OH)4 Is this equation correct?
Or am I entirely wrong about their nature being amphoteric as the reason for these reactions?
Do we call these salts complexes?
How do you define a complex?
My next question here is aluminium and lead(II) hydroxides ppts. are insoluble in excess aqueous ammonia but zinc is soluble. Why is that so? Here copper (II) hydroxide ppt. also dissolves in excess aqueous ammonia to give a dark blue solution. Are these two reactions of complexes and less similar to the one above?
Oh... and for ammonium compounds, when we heat it, ammonia gas will be given out also right? So what is the purpose of adding sodium hydroxide? as a catalyst?
Since I'm at it, I might as well ask this last question. How does the mechanism of testing for nitrates work? Add sodium hydroxide, then aluminium foil and warm, ammonia will be formed. hmmm... I'm looking for the answers in the internet as well but I can't find it... Hope you guys can help me out... Thanks a mil
chloride : add dil. nitric acid, then add silver nitrate --> white ppt. AgCl formediodide : add dil. nitric acid, then add lead(II) nitrate --> yellow ppt. PbCl2 formedsulphate : add dil. nitric acid, then add barium nitrate --> white ppt., BaSO4 formed