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Qualitative Analysis
Question | Answer |
---|---|
What is qualitative analysis? | The identification of constituents of single substances or mixtures of substances. |
Give the usual colour of oxides and sulphides. | Black |
Give the usual colour of copper(II) salts. | Blue or blue-green |
Give the usual colour of iron(II) salts. | Pale green |
Give the usual colour of iron(III) salts. | Yellow-brown |
Which ion gives a very pungent smell similar to urine? | Ammonium (NH4+) salts. These release ammonia gas (NH3). |
Which ions are usually present in deliquescent salts (which absorb water, eventually dissolving)? | Chlorides (Cl-) and nitrates (NO3-) |
A flame colour which is lilac indicates the presence of | K+ |
A flame colour which is a persistent orange-yellow indicates the presence of | Na+ |
A flame colour which is brick red indicates the presence of | Ca2+ |
A flame colour which is bluish indicates the presence of | Pb2+ |
A flame which is blue-green indicates the presence of | Cu2+ |
When dilute NaOH is added, a white ppt is formed which is insoluble in excess. Identify the cation. | Ca2+ |
When dilute NaOH is added, a white ppt is formed which dissolves in excess. Identify the possible cations. | Zn2+. Al3+ and Pb2+ |
When dilute NaOH is added, a blue ppt is formed which is insoluble in excess. Identify the cation. | Cu2+ |
When dilute NaOH is added, a grey-green (dirty green) ppt is formed which is insoluble in excess. Identify the cation. | Fe2+ |
When dilute NaOH is added, red-brown (rusty brown) ppt is formed which is insoluble in excess. Identify the cation. | Fe3+ |
When dilute NaOH is added, no ppt is formed but a NH3 (g) gas is evolved on warming. NH3 has a pungent, urine-like odour and turns red litmus blue since it is a basic gas. Identify the cation. | NH4+ ions present. |
T or F. Metal ppts which are basic remain insoluble in excess NaOH but those which are amphoteric (Zn, Al and Pb) form soluble salts so their ppts dissolve in excess. | T |
Cu2+ (aq) + 2OH- (aq) -> | Cu(OH)2(s) |
NH4+ (aq) + OH- (aq) -> | NH3 (g) + H2O (l) |
When ammonia solution (NH4OH - ammonium hydroxide) is added, NO ppt is seen. Identify the cation. | Ca2+ |
When ammonia solution (NH4OH - ammonium hydroxide) is added, a white ppt is formed which is insoluble. Identify the possible cations. | Al3+ or Pb2+ |
When ammonia solution (NH4OH - ammonium hydroxide) is added, a white ppt is formed which dissolves in excess to form a colourless solution. Identify the cation. | Zn2+ |
When ammonia solution (NH4OH - ammonium hydroxide) is added, a blue ppt is formed which dissolves in excess to form a deep blue solution. Identify the cation. | Cu2+ |
When ammonia solution (NH4OH - ammonium hydroxide) is added, a grey-green ppt is formed which is insoluble in excess. Identify the cation. | Fe2+ |
When ammonia solution (NH4OH - ammonium hydroxide) is added, a red-brown ppt is formed which is insoluble in excess. Identify the cation. | Fe3+ |
T or F. Zn(OH)2 and Cu(OH)2 react with excess ammonia solution to form soluble complex hydroxides while the other hydroxides do not. | T |
Describe the results of the confirmatory test with potassium iodide solution (KI(aq)) used to distinguish between Pb2+ and Al3+. | Bright yellow ppt (PbI2) formed if Pb2+ is present. No ppt formed if Al3+ is present. |
Describe the results of the confirmatory test with dilute hydrochloric acid (HCl) used to distinguish between Pb2+ and Al3+. | White ppt which dissolves on heating an reforms ppt on cooling (PbCl2) formed if Pb2+ present. No ppt if Al3+ present. |
Describe the results of the confirmatory test with potassium hexacyanoferrate (III) to distinguish between Fe 2+ and Fe3+ . | Dark blue ppt formed (prussian blue) if Fe 2+ present. No blue ppt with Fe3+. |
Describe the results of the confirmatory test with potassium hexacyanoferrate (II) to distinguish between Fe2+ and Fe 3+. | Dark blue ppt formed with Fe3+. No blue ppt with Fe2+. |
What does CO2(g) evolved upon heating a substance indicate about the anion present? | CO3^2- or HCO3- present. |
What does O2(g) and NO2(g) evolved upon heating a substance indicate about the anion present? | NO3- ions may be present. |
What does SO2 (g) evolved upon heating a substance indicate about he anion present? | SO3^2- ions may be present. |
Describe the wooden splint test for O2(g). | O2(g) relights a glowing splint. |
Describe the wooden splint test for H2(g). | Extinguishes a lighted splint with a pop sound. |
Describe the wooden splint test for CO2(g). | Extinguishes the lighted splint completely. The lime water test is a more common test for CO2(g) though. |
Describe the lime water test for CO2 (g). | The gas is bubbled into calcium hydroxide (lime water - Ca(OH)2(aq). A white ppt of CaCO3(s) is formed when they react. Ca(OH)s (s) + CO2 (g) -> CaCO3 (s) + H2O (l) |
This is a black solid which turns into a purple vapour readily. Identify the cation present. | iodide (I-) |
This is a brown gas. | NO2 (g) |
When silver nitrate is added to the solution, white ppt is formed which turns purple in sunlight and dissolves in aqueous ammonia. Identify the anion. | Cl- |
When silver nitrate is added to the solution, cream ppt is formed which turns yellow-green in sunlight and is slightly soluble in aqueous ammonia. Identify the anion. | Br- |
When silver nitrate is added to the solution, a pale yellow ppt is formed which is insoluble in aqueous ammonia. Identify the anion. | I- |
When barium nitrate or barium chloride is added to the solution, a white ppt is formed. Identify the possible cations. | SO4^2-, SO3^2- and CO3^2- possible |
Give the test which can be used to distinguish between SO4^2- and SO3^2- ions using barium nitrate or barium chloride followed by HCl(aq) or HNO3(aq). | SO4^2- forms a white ppt which is insoluble. SO3^2- forms a white ppt which is soluble. SO2 (g) also evolved on warming. |
Describe a test which can be used to identify SO2(g) gas. | Pass a piece of filter paper soaked in a coloured oxidising agent such as potassium dichromate. It will decolourize it from orange to green. |
Describe the test and results expected for nitrate ions using H2SO4 and copper turnings. | Test: Add conc. H2SO4 and Cu turnings to the solid then warm gently. Result: Blue solution forms and NO2(g) is evolved. |
Describe the test and results expected in the brown ring test for nitrates. | Test: To the unknown solution, add saturated iron (II) sulphate solution followed by sulphuric acid down the side of the test tube. Result: Sulphuric acid sinks, brown ring forms between the two liquid layers. |