Dilute sulfuric acid reacts with a colourless solution to form a
white precipitate. May I know what the colourless solution is?
I wrote calcium hydroxide. Are there any other answers?
and by the way, to test for (SO4)2- ions, you add dilute
nitric/hydrochloric acid before adding barium nitrate/chloride. But
there is no reaction between sulfuric acid and barium sulphate, is
there?
The reason you add protons (ie. H+ ions), is to remove any
carbonate ions present, and/or hydroxide ions present, that might
give false positives (ie. another ppt which may mislead you into
thinking the test result is positive).
Go to my thread "Collection of Questions" here (http://www.sgforums.com/forums/2297/topics/320107),
and then find (control-F) "acidify silver nitrate".
As for the colourless solution, one possible answer (and the one
most likely required at 'O' levels) is barium nitrate solution.
Ba2+(aq) + SO4 2-(aq) --> BaSO4(s)