Qualitative Analysis

CSEC Chemistry

What is qualitative analysis?

The identification of constituents of single substances or mixtures of substances.

Give the usual colour of oxides and sulfides.

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.