Question

what preliminary test results are you looking for to identify conclusively your unknown solutions as this set : AgNO3(aq), Ba(NO)3(aq), HCl(aq), H2SO4(aq), NaOH(aq) ?

Answer 1

Hi,

A)Add HNO₃ dropwise until solution is acidic (unless of course it was dissolved in nitric acid), then add a few drops of AgNO₃ and observe any reaction. A precipitate will form with certain cations that form insoluble silver compounds, but because of the acidic environment, some insoluble silver salts (e.g. salts containing CO₃^2-, S^2-, and PO₄^3- ions) are "destroyed." Cl^-, Br^-, and I^- form insoluble compounds, while SO₄^2- forms a moderately insoluble compound.

Ag⁺ + Cl^- ? AgCl(s)

The standard qualitative test for chlorides in solution is to add silver nitrate:
NaCl(aq) + AgNO3(aq) ? AgCl(s) + NaNO3(aq)
The silver chloride precipitaes as a white solid.

B) Add HNO₃ dropwise until solution is acidic, boil the solution for two minutes, then test with litmus paper. Continue adding and boiling until solution remains acidic after boiling. Cool the solution and add a few drops of Ba(NO₃)₂ and observe any reaction. A precipitate will form with anions that form an insoluble barium compound

C) Add HCl dropwise until solution tests acidic to litmus paper and observe any reaction. A precipitate will form with any cation that forms an insoluble chloride (refer to the solubility rules). For example:

Pb²⁺ + 2Cl- ? PbCl₂(s)

D)Add H₂SO₄ dropwise until solution is acidic and observe any reaction. A precipitate will form with any cation that forms an insoluble sulfate (refer to the solubility rules). For example:

Ba²⁺ + SO₄^2- ? BaSO₄(s)

E)

Add NaOH dropwise to the solution, stir or shake the solution, and observe any reaction (if the compound was dissolved in nitric acid, the first several drops will neutralize the acid so be sure to check the pH with litmus paper). Look for a precipitate (refer to the solubility rules for hydroxides). If a precipitate forms, continue adding NaOH. Some metal hydroxides are amphoteric and will form a complex ion and redissolve. See Figures 1, 2, and 3 for an example of this reaction. The following ions are amphoteric:

Species	Acidic Solution	Slightly Basic Solution	Basic Solution
Al3+	Al3+(aq)	Al(OH)3(s)	Al(OH)4-(aq)
Cr3+	Cr3+(aq)	Cr(OH)3(s)	Cr(OH)4-(aq)
Pb2+	Pb2+(aq)	Pb(OH)2(s)	Pb(OH)42-(aq)
Zn2+	Zn2+(aq)	Zn(OH)2(s)	Zn(OH)42-(aq)
Sn4+	Sn4+(aq)	Sn(OH)4(s)	Sn(OH)62-(aq)

Figure 1. Al(NO₃)₃ in solution.

Figure 2. Al(OH)₃ precipitates with the addition of NaOH.

Figure 3. When excess NaOH is added, the precipitate redissolves as the Al(OH)₄^- complex ion is formed.