8b Qualitative Analysis of Salts

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QUALITATIVE ANALYSIS OF SALTS

1. Qualitative analysis – is a chemical technique used to determine the identities of chemical substances present in a mixture but not their quantities.

2.

Qualitative analysis of salt – scheme of tests carried out to identify the CATION AND ANION present in the salt.

3. The technique of qualitative analysis includes :

a.

Observe the colour of the SALT or the colour of the SALT SOLUTION

b.

Observe the EFFECT OF HEAT on the salt

c.

Identify the GAS FORMED when a test is performed on the salt

d.

Observe the SOLUBILITY in water

e.

Identify the PRECIPITATE formed, when a specific chemical reagent is added to the aqueous salt solution

f.

Carry out CONFIRMATORY TESTS (specific chemical test), to confirm the identity of cation or anion present in the salt

CONFIRMATORY TESTS

4. THE COLOUR OF SALT OR AQUEOUS SALT SOLUTION

a. Most salts are white, and when dissolved in water, will form colourless aqueous salt solution

COLOUR SOLID SOLUTION

White or colourless Salts of Na+_{, K}+_{, NH}

4+, Mg2+,

Ca2+_{, Ba}2+_{, Al}3+_{, Pb}2+_{, Zn}2+

(only if all the anions are colourless too)

Solution of salts of Na+_{, K}+_,

NH4+, Mg2+, Ca2+, Ba2+, Al3+,

Pb2+_{, Zn}2+

Yellow PbO, PbI2, PbCrO4, BaCrO4 Solution of salts of Fe3+,

CrO42-, chromate

Blue Hydrated Cu2+_salt

CuSO4, Cu(NO3)2 , Cu(OH)2

Cu2+

CuSO4, Cu(NO3)2

Green CuCO3, CuCl2

FeSO4 , Fe(NO3)2 , FeCl2 ,

Fe(OH)2 ,

Fe2+

FeSO4 , Fe(NO3)2 , FeCl2 ,

Black CuO, CuS, FeO, FeS

-Brown Fe2(SO4)3 , Fe(NO3)3 ,

FeCl3 , Fe(OH)3 ,

Fe3+_{, Cr}

2O72- dichromate

(orange) 5. SOLUBILITY OF SALTS IN WATER

Type of salts Solubility n water Salts of Na+_{, K}+_{, NH}

4+ All are soluble

Nitrate All are soluble

Sulphate All are soluble EXCEPT BaSO4, PbSO4, CaSO4

Chloride All are soluble EXCEPT AgCl, HgCl, PbCl2

Carbonate All are INSOLUBLE EXCEPT Na2CO3, K2CO3 ,

(NH4)2CO3

Oxide All are INSOLUBLE EXCEPT Na2O, K2O , CaO

(slightly soluble)

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Ba(OH)2

6. Test of gases Name of gas Colour of

gas Smell of gas Effect on damp litmus Confirmatory test

Oxygen, O2 Colourless No smell No effect - Test the gas with a glowing wooden splinter.

- The gas Will RElight the glowing wooden splinter

Hydrogen, H2 Colourless No smell No effect Test the gas with a lighted wooden splinter.

The gas will extinguish THE lighted wooden splinter with a ‘pop’ sound

Carbon dioxide, CO2

Colourless No smell Moist blue litmus change to red (ACID)

Will turn limewater chalky Ammonia, NH3 Colourless Pungent

smells like urine Moist red litmus change to blue (ALKALI)

When a glass rod dipped into concentrated HCl is placed near the mouth of the test tube with ammonia, white fumes are formed (NH4Cl)

Hydrogen Chloride, HCl

Colourless Pungent Moist blue litmus change to red (ACID)

When a glass rod dipped into concentrated ammonia is placed near the mouth of the test tube with HCl, white fumes are formed (NH4Cl)

Sulphur dioxide, SO2

Colourless Pungent Moist blue litmus change to red (ACID)

Will decolourises the purple colour of acidified potassium manganate (VII)

Or

Will change the colour of acidified potassium dichromate (VI) solution from orange to green

Name of gas Colour of gas Smell of gas Effect on damp litmus Confirmatory test Nitrogen dioxide, NO2

Brown Pungent Moist blue litmus change to red (ACID)

NO, ALREADY CONFIRMED BY ITS COLOUR Chlorine, Cl2 Yellowish green Choking Decolourises moist red or blue litmus Bleaching effect (ACIDIC)

NO, ALREADY CONFIRMED BY ITS COLOUR

7. HEATING TESTS ON SALTS

CARBONATE SALTS Effect of heat

Potassium carbonate Sodium carbonate

Will not decomposes (nothing happen) Calcium carbonate

Decompose to metal oxide & CO2

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Magnesium carbonate Zinc carbonate Iron (III) carbonate Lead (II) carbonate Copper (II) carbonate

MgCO3 (s) → MgO (s) + CO2 ZnCO3 (s) → ZnO (s) + CO2 Fe2(CO3)3 (s) → Fe2O3 (s) + 3CO2 PbCO3 (s) → PbO (s) + CO2 CuCO3 (s) → CuO (s) + CO2 Ammonium carbonate

Decomposes to CO2, NH3 & water vapour without any residue

(NH4)2CO3(s) → 2NH3(g) + H2O(g) + CO2(g)

H2O can change dry blue cobalt chloride paper to pink

NITRATE SALTS EFFECT OF HEAT

Potassium nitrate Sodium nitrate

Decomposes to metal nitrite & oxygen gas 2KNO3 (s) → 2KNO2 (s) + O2 (g) 2NaNO3 (s) → 2NaNO2 (s) + O2 (g) Calcium nitrate Magnesium nitrate Aluminium nitrate Zinc nitrate Iron (III) nitrate Lead (II) nitrate Copper (II) nitrate

Decomposes to metal oxide, oxygen gas & nitrogen dioxide gas 2Ca(NO3)2 (s) → 2CaO(s) + O2 (g) + 4NO2 (g)

2Mg(NO3)2 (s) → 2MgO(s) + O2 (g) + 4NO2 (g)

4Al(NO3)3 (s) → 2Al2O3(s) + 3O2 (g) + 12NO2 (g)

2Zn(NO3)2 (s) → 2ZnO(s) + O2 (g) + 4NO2 (g)

4Fe(NO3)3 (s) → 2Fe2O3 (s) + 3O2 (g) + 12NO2 (g)

2Pb(NO3)2 (s) → 2PbO(s) + O2 (g) + 4NO2 (g)

2Cu(NO3)2 (s) → 2CuO(s) + O2 (g) + 4NO2 (g)

Ammonium nitrate

Decomposes to water vapour & nitrous oxide gas without any residue NH4NO3 (s) → 2H2O (g) + N2O (g)

AMMONIUM SALTS EFFECT ON HEAT

All ammonium salts Will produce ammonia gas (NH3) except for ammonium nitrate (N2O)

SULPHATE SALTS EFFECT ON HEAT

Most sulphate salts Stable & do not decomposes when heated EXCEPT: Iron (II) sulphate

Zinc sulphate Copper (II) sulphate

Will decomposes to sulphur dioxide OR sulphur trioxide 2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)

ZnSO4 (s) → ZnO(s) + SO3 (g)

CuSO4 (s) → CuO(s) + SO3 (g)

CHLORIDE SALTS EFFECT ON HEAT

Most chloride salts Stable & do not decomposes when heated EXCEPT : Ammonium chloride NH4Cl (s) → NH3 (g) + HCl(g)

8. So, when a salt is heated

a.

The types of gas evolved has to be identified –-- This will give the information on the ANION (or CATION , NH4+) present

b.

The colour change of the solid in the test tube must be recorded. ---This will give information regarding the type of CATION present.

Type of gas produced Type of ion

CO2 Carbonate ion, CO32- ( except Na2CO3 & K2CO3)

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NO2 & O2 Nitrate ion, NO3- (except NaNO3, NH4NO3 & KNO3)

SO2 Sulphate ion, SO42- (only zinc, iron, copper)

NH3 Ammonium ion, NH4+

9.

Most salts that decomposes produced METAL OXIDES as residue. The change of colour during heating gives a good indication towards the types of metal oxide formed.

Original colour of salt

Colour of residue after heating Metal oxide produced

Cations present in salt

White Yellow when hot, white when cold ZnO Zn2+

White Brown when hot, yellow when cold PbO Pb2+

Blue / green Black CuO Cu2+

Green / brown Black Fe2O3 Fe3+ / Fe2+

10. TEST FOR THE PRESENTS OF ANIONS IN AQUEOUS SOLUTIONS

Reagent / Anion HCl (aq) or HNO3 (aq) HNO3 (aq) followed by AgNO3 (aq)

HNO3 (aq) &

Ba(NO3)2

or

HCl (aq) & BaCl2 (aq)

Pb(NO3) (aq) Other test

CO3 2-Gas evolved turns lime water milky - - White precipitate soluble in acid -NO3- - - - -Salt solution + dilute H2SO4 +

freshly made FeSO4 +

concentrated H2SO4 :

Brown ring is formed SO42- - - White _precipitate White precipitate _{soluble in acid}

Cl- _- White

precipitate

-White precipitate soluble in acid or when heated

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Heat solid salt + concentrated H2SO4 :

HCl gas is produced

(ii)

Heat solid salt + MnO2

+ concentrated H2SO4 : Cl2 gas is

produced

11. TEST FOR CATIONS

a.

The cations usually tested : Al3+_{, Pb}2+_{, Zn}2+_{, Mg}2+_{, Ca}2+_{, Fe}2+_{, Fe}3+_{, Cu}2+_{, &}

NH4+ ions

b. An aqueous solution of the cation is prepared by

i. Dissolving the salt in water (if the salt soluble in water) ii. Dissolving the salt in dilute acid & then filter (if the salt is

insoluble in water) – the filtrate contains the cation 2+ 2 +

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c. The aqueous solution is then tested with i. NaOH

ii.

NH3 (aq)

iii. A specific reagent as a confirmatory test

iv.

NaOH & NH3 will supply OH- to produce metal hydroxides

12. COLOURS OF METAL HYDROXIDES

Observation Formula of metal hydroxide Cation present

Blue precipitate Cu(OH)2 Cu2+

Dirty green precipitate

Fe(OH)2 Fe2+

Brown precipitate Fe(OH)3 Fe3+

White precipitate Al(OH)3, Pb(OH)2, Zn(OH)2,

Mg(OH)2, Ca(OH)2,

Al3+_{, Pb}2+_{, Zn}2+_{, Mg}2+_{, Ca}2+

No precipitate - Na+_{, K}+_{, NH}

4+

13. HYDROXIDE TEST FOR CATIONS

Cation A little NaOH (aq) Excess NaOH (aq) A little NH3(aq) Excess NH3(aq)

NH4+ No precipitate formed, NH3 gas evolved when heated No precipitate formed, NH3 gas evolved when heated No precipitate

formed No precipitate formed Pb2+ _{White precipitate} _{White precipitate}

soluble in excess NaOH White precipitate White precipitate insoluble in excess NH3

Zn2+ _{White precipitate} _{White precipitate}

soluble in excess

NaOH

White

precipitate White precipitate

NH3

Al3+ _{White precipitate} _{White precipitate}

soluble in excess NaOH White precipitate White precipitate insoluble in excess NH3

Mg2+ _{White precipitate} _{White precipitate}

insoluble in excess NaOH White precipitate White precipitate insoluble in excess NH3

Ca2+ _{White precipitate} _{White precipitate}

insoluble in excess NaOH No precipitate formed No precipitate formed

Cu2+ _{Blue precipitate} _{Blue precipitate}

insoluble in excess

NaOH

Blue precipitate Blue precipitate

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DARK BLUE solution Fe2+ _{Dirty green} precipitate Dirty green precipitate insoluble in excess NaOH Dirty green precipitate Dirty green precipitate insoluble in excess NH3

Fe3+ _{Brown precipitate} _{Brown precipitate}

insoluble in excess NaOH Brown precipitate Brown precipitate insoluble in excess NH3

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14.

CONFIRMATORY TESTS FOR Pb2+_{, NH}

4+ , Fe 2+ , Fe 3+ IONS

CATIONS SPECIFIC REAGENT OBSERVATION

Pb2+

Al

KI, NaI, Pb – Yellow precipitate, soluble in hot water & recrystallises when cooled Al – white precipitate KCl, NaCl, HCl Pb – White precipitate,

soluble in hot water & recrystallises when cooled Al – white precipitate – soluble in water

K2SO4, Na2SO4,

H2SO4,

Pb – White precipitate, insoluble in hot water

Al – white precipitate soluble in water

NH4+ Nesslar reagent Brown precipitate

Fe2+

Potassium

hexacyanoferrate (II), K4Fe(CN)6

Light blue precipitate Potassium

hexacyanoferrate (III), K3Fe(CN)6

Prussian blue (dark blue) precipitate

Acidified KMnO4 Purple colour decolourises

Fe3+

Potassium thiocyanate, KSCN

Blood red colour Potassium

hexacyanoferrate (II), K4Fe(CN)6

Turnbull’s blue (dark blue) precipitate

Potassium

hexacyanoferrate (III), K3Fe(CN)6

Greenish-brown solution

Fe(NO3)3 Iron (III) nitrate

TO DESCRIBE A TEST - u must write

i. name of the test (if any) ii. “mini” procedure of the test iii. Observable result

- brown ring test

- Add dilute sulphuric acid to salt P solution - Add freshly made Iron (II) sulphate solution

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