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+, NH4+, Mg2+, Solution of salts of Na+, K+, Ca2+, Ba2+, Al3+, Pb2+, Zn2+ NH4+, Mg2+, Ca2+, Ba2+, Al3+, (only if all the anions are Pb2+, Zn2+ colourless too) Yellow PbO, PbI2, PbCrO4, BaCrO4 Solution of salts of Fe3+, CrO42-, chromate 2+ Blue Hydrated Cu salt Cu2+ CuSO4, Cu(NO3)2 , Cu(OH)2 CuSO4, Cu(NO3)2 Green CuCO3, CuCl2 Fe2+ FeSO4 , Fe(NO3)2 , FeCl2 , FeSO4 , Fe(NO3)2 , FeCl2 , Fe(OH)2 , Black CuO, CuS, FeO, FeS Brown Fe2(SO4)3 , Fe(NO3)3 , Fe3+, Cr2O72- dichromate FeCl3 , Fe(OH)3 , (orange) 5. SOLUBILITY OF SALTS IN WATER Type of salts Salts of Na+, K+, NH4+ Nitrate Sulphate Chloride Carbonate Oxide Hydroxide
Solubility n water All are soluble All are soluble All are soluble EXCEPT BaSO4, PbSO4, CaSO4 All are soluble EXCEPT AgCl, HgCl, PbCl2 All are INSOLUBLE EXCEPT Na2CO3, K2CO3 , (NH4)2CO3 All are INSOLUBLE EXCEPT Na2O, K2O , CaO (slightly soluble) All are INSOLUBLE EXCEPT KOH, NaOH, Ca(OH)2,
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Ba(OH)2
6. Test of gases Name of gas Colour of gas Oxygen, O2 Colourless
Smell of gas No smell
Effect on damp litmus No effect
Hydrogen, H2
Colourless
No smell
No effect
Carbon dioxide, CO2
Colourless
No smell
Ammonia, NH3
Colourless
Pungent smells like urine
Hydrogen Chloride, HCl
Colourless
Pungent
Sulphur dioxide, SO2
Colourless
Pungent
Moist blue litmus change to red (ACID) Moist red litmus change to blue (ALKALI) Moist blue litmus change to red (ACID) Moist blue litmus change to red (ACID)
Name of gas
Colour of gas Brown
Smell of gas Pungent
Nitrogen dioxide, NO2 Chlorine, Cl2
Yellowish green
Choking
Effect on damp litmus Moist blue litmus change to red (ACID) Decolourises moist red or blue litmus
Confirmatory test - Test the gas with a glowing wooden splinter. - The gas Will RElight the glowing wooden splinter Test the gas with a lighted wooden splinter. The gas will extinguish THE lighted wooden splinter with a ‘pop’ sound Will turn limewater chalky
When a glass rod dipped into concentrated HCl is placed near the mouth of the test tube with ammonia, white fumes are formed (NH4Cl) When a glass rod dipped into concentrated ammonia is placed near the mouth of the test tube with HCl, white fumes are formed (NH4Cl) 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 Confirmatory test
NO, ALREADY CONFIRMED BY ITS COLOUR
NO, ALREADY CONFIRMED BY ITS COLOUR
Bleaching effect (ACIDIC)
7. HEATING TESTS ON SALTS CARBONATE SALTS Effect of heat Potassium carbonate Will not decomposes (nothing happen) Sodium carbonate Decompose to metal oxide & CO2 Calcium carbonate CaCO3 → CaO + CO2
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Magnesium carbonate Zinc carbonate Iron (III) carbonate Lead (II) carbonate Copper (II) carbonate
Ammonium carbonate
NITRATE SALTS
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 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
Ammonium nitrate
EFFECT OF HEAT Decomposes to metal nitrite & oxygen gas 2KNO3 (s) → 2KNO2 (s) + O2 (g) 2NaNO3 (s) → 2NaNO2 (s) + O2 (g) 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) Decomposes to water vapour & nitrous oxide gas without any residue NH4NO3 (s) → 2H2O (g) + N2O (g)
AMMONIUM SALTS All ammonium salts
EFFECT ON HEAT Will produce ammonia gas (NH3) except for ammonium nitrate (N2O)
SULPHATE SALTS Most sulphate salts Iron (II) sulphate Zinc sulphate Copper (II) sulphate
EFFECT ON HEAT Stable & do not decomposes when heated EXCEPT: 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 Most chloride salts Ammonium chloride
EFFECT ON HEAT Stable & do not decomposes when heated EXCEPT : NH4Cl (s) → NH3 (g) + HCl(g)
Potassium nitrate Sodium nitrate Calcium nitrate Magnesium nitrate Aluminium nitrate Zinc nitrate Iron (III) nitrate Lead (II) nitrate Copper (II) nitrate
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 CO2 O2
Type of ion Carbonate ion, CO32- ( except Na2CO3 & K2CO3) Only (NaNO3 & KNO3)
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NO2 & O2 SO2 NH3
Nitrate ion, NO3- (except NaNO3, NH4NO3 & KNO3) Sulphate ion, SO42- (only zinc, iron, copper) 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 Colour of residue after heating Metal oxide Cations present in of salt produced 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
CO32-
HCl (aq) or HNO3 (aq)
Gas evolved turns lime water milky
HNO3 (aq) followed by AgNO3 (aq)
HNO3 (aq) & Ba(NO3)2 or HCl (aq) & BaCl2 (aq)
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NO3-
-
-
SO42-
-
-
Pb(NO3) (aq)
White precipitate soluble in acid
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White precipitate
Other test
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Salt solution + dilute H2SO4 + freshly made FeSO4 + concentrated H2SO4 : Brown ring is formed
White precipitate soluble in acid
(i) Cl-
-
White precipitate
-
White precipitate soluble in acid or when heated
(ii)
Heat solid salt + concentrated H2SO4 : HCl gas is produced Heat solid salt + MnO2 + concentrated H2SO4 : Cl2 gas is produced
11. TEST FOR CATIONS a. The cations usually tested : Al3+, Pb2+, Zn2+, Mg2+, Ca2+, Fe2+, Fe3+, Cu2+, & 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- + PbSO4 + HNO3 = H2SO4 + Pb(NO3)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 Blue precipitate Dirty green precipitate Brown precipitate White precipitate No precipitate
Formula of metal hydroxide Cu(OH)2 Fe(OH)2
Cation present Cu2+ Fe2+
Fe(OH)3 Al(OH)3, Pb(OH)2, Zn(OH)2, Mg(OH)2, Ca(OH)2, -
Fe3+ Al3+, Pb2+, Zn2+, Mg2+, Ca2+ Na+, K+, NH4+
13. HYDROXIDE TEST FOR CATIONS Cation NH4+
Excess NaOH (aq) No precipitate formed, NH3 gas evolved when heated White precipitate soluble in excess NaOH
A little NH3(aq) No precipitate formed
Excess NH3(aq) No precipitate formed
Pb2+
A little NaOH (aq) No precipitate formed, NH3 gas evolved when heated White precipitate
White precipitate
Zn2+
White precipitate
White precipitate soluble in excess NaOH
White precipitate
Al3+
White precipitate
White precipitate soluble in excess NaOH
White precipitate
Cation Mg2+
A little NaOH (aq) White precipitate
Excess NaOH (aq) White precipitate insoluble in excess NaOH
A little NH3(aq) White precipitate
Ca2+
White precipitate
No precipitate formed
Cation Cu2+
A little NaOH (aq) Blue precipitate
White precipitate insoluble in excess NaOH Excess NaOH (aq) Blue precipitate insoluble in excess NaOH
White precipitate insoluble in excess NH3 White precipitate soluble in excess NH3 White precipitate insoluble in excess NH3 Excess NH3(aq) White precipitate insoluble in excess NH3 No precipitate formed
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A little NH3(aq) Blue precipitate
Excess NH3(aq) Blue precipitate soluble in excess NH3 to form
Fe2+
Dirty green precipitate
Dirty green precipitate insoluble in excess NaOH
Dirty green precipitate
Fe3+
Brown precipitate
Brown precipitate insoluble in excess NaOH
Brown precipitate
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DARK BLUE solution Dirty green precipitate insoluble in excess NH3 Brown precipitate insoluble in excess NH3
14. CONFIRMATORY TESTS FOR Pb2+, NH4+, Fe2+, Fe3+ IONS CATIONS
SPECIFIC REAGENT KI, NaI,
KCl, NaCl, HCl Pb2+ Al
NH4+
Fe2+
Fe3+
OBSERVATION Pb – Yellow precipitate, soluble in hot water & recrystallises when cooled Al – white precipitate 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
Nesslar reagent Potassium hexacyanoferrate (II), K4Fe(CN)6 Potassium hexacyanoferrate (III), K3Fe(CN)6 Acidified KMnO4 Potassium thiocyanate, KSCN Potassium hexacyanoferrate (II), K4Fe(CN)6 Potassium hexacyanoferrate (III), K3Fe(CN)6
Brown precipitate Light blue precipitate
Prussian blue (dark blue) precipitate Purple colour decolourises Blood red colour Turnbull’s blue (dark blue) precipitate Greenish-brown solution
Fe(NO3)3 Iron (III) nitrate
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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 Slowly add concentrated Sulphuric acid to the solution
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A brown ring can be seen
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