It wasn't a question. This flowchart is an excerpt from Jacaranda HSC Chemistry Textbook. I was confused as to why solubility rules state that BaSO4 is a precipitate but we required alkalinity to test for it.That depends on your experimental procedure, you might well have changed the pH or used another ion to precipitate out any phosphate ions before you used barium nitrate to test for sulfate ions, without additional context to the question, it is extremely difficult for us to give you a specific rationalisation why that is the case.
That makes sense. I didn't realise that the equilibrium with phosphate and water lied so far to the products side. I really hope we aren't meant to just know that.It is to do with Le Chat.
In an acidified solution, Ba produces ppt with sulfate, but not phosphate due to the relative weakness of the hydrogen phosphate ion.
This is because hydrogen sulfate is able to react with water to produce hydronium ions and sulfate ions. Enough sulfate ions are produced to form a ppt with Ba.
Whereas, because hydrogen phosphate when it reacts with water to produce hydronium ions and phosphate ions, the equilibrium lies too far to the left to produce enough phosphate ions to form ppt.
When ammonia is introduced, [H3O+] decreases, shifting equilibrium right to produce more PO43- ions which can then form ppt with Ba2+