A salt is an ionic compound formed when the cation from a base combine with the anion derived from an acid.
A salt is therefore formed when the hydrogen ions in an acid are replaced wholly/fully or partially/partly ,directly or indirectly by a metal or ammonium radical. (b) The number of ionizable/replaceable hydrogen in an acid is called basicity of an acid.
Some acids are therefore:
(i)monobasic acids generally denoted HX e.g.
HCl, HNO3,HCOOH,CH3COOH.
(ii)dibasic acids ; generally denoted H2X e.g.
H2SO4, H2SO3, H2CO3,HOOCOOH.
(iii)tribasic acids ; generally denoted H3X e.g.
H3PO4.
(c) Some salts are normal salts while other are acid salts.
(i)A normal salt is formed when all the ionizable /replaceable hydrogen in an acid is replaced by a metal or metallic /ammonium radical.
(ii)An acid salt is formed when part/portion the ionizable /replaceable hydrogen in an acid is replaced by a metal or metallic /ammonium radical.
Table showing normal and acid salts derived from common acids
| Acid name | Chemical formula | Basicity | Normal salt | Acid salt |
| Hydrochloric acid | HCl | Monobasic | Chloride(Cl–) | None |
| Nitric(V)acid | HNO3 | Monobasic | Nitrate(V)(NO3–) | None |
| Nitric(III)acid | HNO2 | Monobasic | Nitrate(III)(NO2–) | None |
| Sulphuric(VI)acid | H2SO4 | Dibasic | Sulphate(VI) (SO42-) | Hydrogen sulphate(VI) (HSO4–) |
| Sulphuric(IV)acid | H2SO3 | Dibasic | Sulphate(IV) (SO32-) | Hydrogen sulphate(IV) (HSO3–) |
| Carbonic(IV)acid | H2CO3 | Dibasic | Carbonate(IV)(CO32-) | Hydrogen carbonate(IV) (HCO3–) |
| Phosphoric(V) acid | H3PO4 | Tribasic | Phosphate(V)(PO43-) | Dihydrogen phosphate(V) (H2PO42-) Hydrogen diphosphate(V) (HP2O42-) |
The table below show shows some examples of salts.
| Base/alkali | Cation | Acid | Anion | Salt | Chemical name of salts |
| NaOH | Na+ | HCl | Cl– | NaCl | Sodium(I)chloride |
| Mg(OH)2 | Mg2+ | H2SO4 | SO42- | MgSO4 Mg(HSO4)2 | Magnesium sulphate(VI) Magnesium hydrogen sulphate(VI) |
| Pb(OH)2 | Pb2+ | HNO3 | NO3– | Pb(NO3)2 | Lead(II)nitrate(V) |
| Ba(OH)2 | Ba2+ | HNO3 | NO3– | Ba(NO3)2 | Barium(II)nitrate(V) |
| Ca(OH)2 | Ba2+ | H2SO4 | SO42- | MgSO4 | Calcium sulphate(VI) |
| NH4OH | NH4+ | H3PO4 | PO43- | (NH4 )3PO4 (NH4 )2HPO4 NH4 H2PO4 | Ammonium phosphate(V) Diammonium phosphate(V) Ammonium diphosphate(V) |
| KOH | K+ | H3PO4 | PO43- | K3PO4 | Potassium phosphate(V) |
| Al(OH)3 | Al3+ | H2SO4 | SO42- | Al2(SO4)2 | Aluminium(III)sulphate(VI) |
| Fe(OH)2 | Fe2+ | H2SO4 | SO42- | FeSO4 | Iron(II)sulphate(VI) |
| Fe(OH)3 | Fe3+ | H2SO4 | SO42- | Fe2(SO4)2 | Iron(III)sulphate(VI) |
(d) Some salts undergo hygroscopy, deliquescence and efflorescence.
(i) Hygroscopic salts /compounds are those that absorb water from the atmosphere but do not form a solution.
Some salts which are hygroscopic include anhydrous copper(II)sulphate(VI), anhydrous cobalt(II)chloride, potassium nitrate(V) common table salt.
(ii)Deliquescent salts /compounds are those that absorb water from the atmosphere and form a solution.
Some salts which are deliquescent include: Sodium nitrate(V),Calcium chloride, Sodium hydroxide, Iron(II)chloride, Magnesium chloride.
(iii)Efflorescent salts/compounds are those that lose their water of crystallization to the atmosphere.
Some salts which effloresces include: sodium carbonate decahydrate, Iron(II)sulphate(VI)heptahydrate, sodium sulphate (VI)decahydrate.
(e)Some salts contain water of crystallization.They are hydrated.Others do not contain water of crystallization. They are anhydrous.
Table showing some hydrated salts.
| Name of hydrated salt | Chemical formula |
| Copper(II)sulphate(VI)pentahydrate | CuSO4.5H2O |
| Aluminium(III)sulphate(VI)hexahydrate | Al2 (SO4) 3.6H2O |
| Zinc(II)sulphate(VI)heptahydrate | ZnSO4.7H2O |
| Iron(II)sulphate(VI)heptahydrate | FeSO4.7H2O |
| Calcium(II)sulphate(VI)heptahydrate | CaSO4.7H2O |
| Magnesium(II)sulphate(VI)heptahydrate | MgSO4.7H2O |
| Sodium sulphate(VI)decahydrate | Na2SO4.10H2O |
| Sodium carbonate(IV)decahydrate | Na2CO3.10H2O |
| Potassium carbonate(IV)decahydrate | K2CO3.10H2O |
| Potassium sulphate(VI)decahydrate | K2SO4.10H2O |
(f)Some salts exist as a simple salt while some as complex salts. Below are some complex salts.
Table of some complex salts
| Name of complex salt | Chemical formula | Colour of the complex salt |
| Tetraamminecopper(II)sulphate(VI) | Cu(NH3) 4 SO4 H2O | Royal/deep blue solution |
| Tetraamminezinc(II)nitrate(V) | Zn(NH3) 4 (NO3 )2 | Colourless solution |
| Tetraamminecopper(II) nitrate(V) | Cu(NH3) 4 (NO3 )2 | Royal/deep blue solution |
| Tetraamminezinc(II)sulphate(VI) | Zn(NH3) 4 SO4 | Colourless solution |
(g)Some salts exist as two salts in one. They are called double salts.
Table of some double salts
| Name of double salts | Chemical formula |
| Trona(sodium sesquicarbonate) | Na2CO3 NaHCO3.2H2O |
| Ammonium iron(II)sulphate(VI) | FeSO4(NH4) 2SO4.2H2O |
| Ammonium aluminium(III)sulphate(VI) | Al2(SO4) 3(NH4) 2SO4.H2O |
(h)Some salts dissolve in water to form a solution. They are said to be soluble. Others do not dissolve in water. They form a suspension/precipitate in water.
Table of solubility of salts
| Soluble salts | Insoluble salts |
| All nitrate(V)salts | |
| All sulphate(VI)/SO42- salts except | Barium(II) sulphate(VI)/BaSO4 Calcium(II) sulphate(VI)/CaSO4 Lead(II) sulphate(VI)/PbSO4 |
| All sulphate(IV)/SO32- salts except | Barium(II) sulphate(IV)/BaSO3 Calcium(II) sulphate(IV)/CaSO3 Lead(II) sulphate(IV)/PbSO3 |
| All chlorides/Cl– except | Silver chloride/AgCl Lead(II)chloride/PbCl2(dissolves in hot water) |
| All phosphate(V)/PO43- | |
| All sodium,potassium and ammonium salts | |
| All hydrogen carbonates/HCO3– | |
| All hydrogen sulphate(VI)/ HSO4– | |
| Sodium carbonate/Na2CO3, potassium carbonate/ K2CO3, ammonium carbonate (NH4) 2CO3 | except All carbonates |
| All alkalis(KOH,NaOH, NH4OH) | except All bases |
13 Salts can be prepared in a school laboratory by a method that uses its solubility in water.
- Soluble salts may be prepared by using any of the following methods:
(i)Direct displacement/reaction of a metal with an acid.
By reacting a metal higher in the reactivity series than hydrogen with a dilute acid,a salt is formed and hydrogen gas is evolved.
Excess of the metal must be used to ensure all the acid has reacted.
When effervescence/bubbling /fizzing has stopped ,excess metal is filtered.
The filtrate is heated to concentrate then allowed to crystallize.
Washing with distilled water then drying between filter papers produces a sample crystal of the salt. i.e.
M(s) + H2X -> MX(aq) + H2(g)
Examples
Mg(s) + H2SO4(aq) -> MgSO4 (aq) + H2(g)
Zn(s) + H2SO4(aq) -> ZnSO4 (aq) + H2(g)
Pb(s) + 2HNO3(aq) -> Pb(NO3) 2(aq) + H2(g)
Ca(s) + 2HNO3(aq) -> Ca(NO3) 2(aq) + H2(g)
Mg(s) + 2HNO3(aq) -> Mg(NO3) 2(aq) + H2(g)
Mg(s) + 2HCl(aq) -> MgCl 2(aq) + H2(g)
Zn(s) + 2HCl(aq) -> ZnCl 2(aq) + H2(g)
(ii)Reaction of an insoluble base with an acid
By adding an insoluble base (oxide/hydroxide )to a dilute acid until no more dissolves, in the acid,a salt and water are formed. Excess of the base is filtered off. The filtrate is heated to concentrate ,allowed to crystallize then washed with distilled water before drying between filter papers e.g.
PbO(s) + 2HNO3(aq) -> Pb(NO3) 2(aq) + H2O (l)
Pb(OH)2(s) + 2HNO3(aq) -> Pb(NO3) 2(aq) + 2H2O (l)
CaO (s) + 2HNO3(aq) -> Ca(NO3) 2(aq) + H2O (l)
MgO (s) + 2HNO3(aq) -> Mg(NO3) 2(aq) + H2O (l)
MgO (s) + 2HCl(aq) -> MgCl 2(aq) + H2O (l)
ZnO (s) + 2HCl(aq) -> ZnCl 2(aq) + H2O (l)
Zn(OH)2(s) + 2HNO3(aq) -> Zn(NO3) 2(aq) + 2H2O (l)
CuO (s) + 2HCl(aq) -> CuCl 2(aq) + H2O (l)
CuO (s) + H2SO4(aq) -> CuSO4(aq) + H2O (l)
Ag2O(s) + 2HNO3(aq) -> 2AgNO3(aq) + H2O (l)
Na2O(s) + 2HNO3(aq) -> 2NaNO3(aq) + H2O (l)
(iii)reaction of insoluble /soluble carbonate /hydrogen carbonate with an acid.
By adding an excess of a soluble /insoluble carbonate or hydrogen carbonate to adilute acid, effervescence /fizzing/bubbling out of carbon(IV)oxide gas shows the reaction is taking place. When effervescence /fizzing/bubbling out of the gas is over, excess of the insoluble carbonate is filtered off. The filtrate is heated to concentrate ,allowed to crystallize then washed with distilled water before drying between filter paper papers e.g.
PbCO3 (s) + 2HNO3(aq) -> Pb(NO3) 2(aq) + H2O (l)+ CO2(g)
ZnCO3 (s) + 2HNO3(aq) -> Zn(NO3) 2(aq) + H2O (l)+ CO2(g)
CaCO3 (s) + 2HNO3(aq) -> Ca(NO3) 2(aq) + H2O (l)+ CO2(g)
MgCO3 (s) + H2SO4(aq) -> MgSO4(aq) + H2O (l)+ CO2(g)
Cu CO3 (s) + H2SO4(aq) -> CuSO4(aq) + H2O (l) + CO2(g)
Ag2CO3 (s) + 2HNO3(aq) -> 2AgNO3(aq) + H2O (l) + CO2(g)
Na2CO3 (s) + 2HNO3(aq) -> 2NaNO3(aq) + H2O (l) + CO2(g)
K2CO3 (s) + 2HCl(aq) -> 2KCl(aq) + H2O (l) + CO2(g)
NaHCO3 (s) + HNO3(aq) -> NaNO3(aq) + H2O (l) + CO2(g)
KHCO3 (s) + HCl(aq) -> KCl(aq) + H2O (l) + CO2(g)
(iv)neutralization/reaction of soluble base/alkali with dilute acid
By adding an acid to a burette into a known volume of an alkali with 2-3 drops of an indicator, the colour of the indicator changes when the acid has completely reacted with an alkali at the end point. The procedure is then repeated without the indicator .The solution mixture is then heated to concentrate , allowed to crystallize ,washed with distilled water before drying with filter papers. e.g.
NaOH (aq) + HNO3(aq) -> NaNO3(aq) + H2O (l)
KOH (aq) + HNO3(aq) -> KNO3(aq) + H2O (l)
KOH (aq) + HCl(aq) -> KCl(aq) + H2O (l)
2KOH (aq) + H2SO4(aq) -> K2SO4(aq) + 2H2O (l)
2 NH4OH (aq) + H2SO4(aq) -> (NH4)2SO4(aq) + 2H2O (l)
NH4OH (aq) + HNO3(aq) -> NH4NO3(aq) + H2O (l)
(iv)direct synthesis/combination.
When a metal burn in a gas jar containing a non metal , the two directly combine to form a salt. e.g.
2Na(s) + Cl2(g) -> 2NaCl(s)
2K(s) + Cl2(g) -> 2KCl(s)
Mg(s) + Cl2(g) -> Mg Cl2 (s)
Ca(s) + Cl2(g) -> Ca Cl2 (s)
Some salts once formed undergo sublimation and hydrolysis. Care should be taken to avoid water/moisture into the reaction flask during their preparation.Such salts include aluminium(III)chloride(AlCl3) and iron (III)chloride(FeCl3)
1. Heated aluminium foil reacts with chlorine to form aluminium(III)chloride that sublimes away from the source of heating then deposited as solid again
2Al(s) + 3Cl2(g) -> 2AlCl3 (s/g)
Once formed aluminium(III)chloride hydrolyses/reacts with water vapour / moisture present to form aluminium hydroxide solution and highly acidic fumes of hydrogen chloride gas.
AlCl3(s)+ 3H2 O(g) -> Al(OH)3 (aq) + 3HCl(g)
2. Heated iron filings reacts with chlorine to form iron(III)chloride that sublimes away from the source of heating then deposited as solid again
2Fe(s) + 3Cl2(g) -> 2FeCl3 (s/g)
Once formed , aluminium(III)chloride hydrolyses/reacts with water vapour / moisture present to form aluminium hydroxide solution and highly acidic fumes of hydrogen chloride gas.
FeCl3(s)+ 3H2 O(g) -> Fe(OH)3 (aq) + 3HCl(g)
(b)Insoluble salts can be prepared by reacting two suitable soluble salts to form one soluble and one insoluble. This is called double decomposition or precipitation. The mixture is filtered and the residue is washed with distilled water then dried.
CuSO4(aq) + Na2CO3 (aq) -> CuCO3 (s) + Na2 SO4(aq)
BaCl2(aq) + K2SO4 (aq) -> BaSO4 (s) + 2KCl (aq)
Pb(NO3)2(aq) + K2SO4 (aq) -> PbSO4 (s) + 2KNO3 (aq)
2AgNO3(aq) + MgCl2 (aq) -> 2AgCl(s) + Mg(NO3)2 (aq)
Pb(NO3)2(aq) + (NH4) 2SO4 (aq) -> PbSO4 (s) + 2NH4NO 3(aq)
BaCl2(aq) + K2SO3 (aq) -> BaSO3 (s) + 2KCl (aq)
14. Salts may lose their water of crystallization , decompose ,melt or sublime on heating on a Bunsen burner flame.
The following shows the behavior of some salts on heating gently /or strongly in a laboratory school burner:
(a)effect of heat on chlorides
All chlorides have very high melting and boiling points and therefore are not affected by laboratory heating except ammonium chloride. Ammonium chloride sublimes on gentle heating. It dissociate into the constituent ammonia and hydrogen chloride gases on strong heating.
NH4Cl(s) NH4Cl(g) NH3(g) + HCl(g)
(sublimation) (dissociation)
(b)effect of heat on nitrate(V)
(i) Potassium nitrate(V)/KNO3 and sodium nitrate(V)/NaNO3 decompose on heating to form Potassium nitrate(III)/KNO2 and sodium nitrate(III)/NaNO2 and producing Oxygen gas in each case.
2KNO3 (s) -> 2KNO2(s) + O2(g)
2NaNO3 (s) -> 2NaNO2(s) + O2(g)
(ii)Heavy metal nitrates(V) salts decompose on heating to form the oxide and a mixture of brown acidic nitrogen(IV)oxide and oxygen gases. e.g.
2Ca(NO3)2 (s) -> 2CaO(s) + 4NO2(g) + O2(g)
2Mg(NO3)2(s) -> 2MgO(s) + 4NO2(g) + O2(g)
2Zn(NO3)2(s) -> 2ZnO(s) + 4NO2(g) + O2(g)
2Pb(NO3)2(s) -> 2PbO(s) + 4NO2(g) + O2(g)
2Cu(NO3)2(s) -> 2CuO(s) + 4NO2(g) + O2(g)
2Fe(NO3)2(s) -> 2FeO(s) + 4NO2(g) + O2(g)
(iii)Silver(I)nitrate(V) and mercury(II) nitrate(V) are lowest in the reactivity series. They decompose on heating to form the metal(silver and mercury)and the Nitrogen(IV)oxide and oxygen gas. i.e.
2AgNO3(s) -> 2Ag (s) + 2NO2(g) + O2(g)
2Hg(NO3)2 (s) -> 2Hg (s) + 4NO2(g) + O2(g)
(iv)Ammonium nitrate(V) and Ammonium nitrate(III) decompose on heating to Nitrogen(I)oxide(relights/rekindles glowing splint) and nitrogen gas respectively.Water is also formed.i.e.
NH4NO3(s) -> N2O (g) + H2O(l)
NH4NO2(s) -> N2 (g) + H2O(l)
(c) effect of heat on nitrate(V)
Only Iron(II)sulphate(VI), Iron(III)sulphate(VI) and copper(II)sulphate(VI) decompose on heating. They form the oxide, and produce highly acidic fumes of acidic sulphur(IV)oxide gas.
2FeSO4 (s) -> Fe2O3(s) + SO3(g) + SO2(g)
Fe2(SO4) 3(s) -> Fe2O3(s) + SO3(g)
CuSO4 (s) -> CuO(s) + SO3(g)
(d) effect of heat on carbonates(IV) and hydrogen carbonate(IV).
(i)Sodium carbonate(IV)and potassium carbonate(IV)do not decompose on heating.
(ii)Heavy metal nitrate(IV)salts decompose on heating to form the oxide and produce carbon(IV)oxide gas.Carbon (IV)oxide gas forms a white precipitate when bubbled in lime water. The white precipitate dissolves if the gas is in excess. e.g. CuCO3 (s) -> CuO(s) + CO2(g)
CaCO3 (s) -> CaO(s) + CO2(g)
PbCO3 (s) -> PbO(s) + CO2(g)
FeCO3 (s) -> FeO(s) + CO2(g)
ZnCO3 (s) -> ZnO(s) + CO2(g)
(iii)Sodium hydrogen carbonate(IV) and Potassium hydrogen carbonate(IV)decompose on heating to give the corresponding carbonate (IV) and form water and carbon(IV)oxide gas. i.e.
2NaHCO 3(s) -> Na2CO3(s) + CO2(g) + H2O(l)
2KHCO 3(s) -> K2CO3(s) + CO2(g) + H2O(l)
(iii) Calcium hydrogen carbonate (IV) and Magnesium hydrogen carbonate(IV) decompose on heating to give the corresponding carbonate (IV) and form water and carbon(IV)oxide gas. i. e.
Ca(HCO3) 2(aq) -> CaCO3(s) + CO2(g) + H2O(l)
Mg(HCO3) 2(aq) -> MgCO3(s) + CO2(g) + H2O(l)
15. Salts contain cation(positively charged ion) and anions(negatively charged ion).When dissolved in polar solvents/water.
The cation and anion in a salt is determined/known usually by precipitation of the salt using a precipitating reagent.
The colour of the precipitate is a basis of qualitative analysis of a compound.
16.Qualitative analysis is the process of identifying an unknown compound /salt by identifying the unique qualities of the salt/compound.
It involves some of the following processes.
(a)Reaction of cation with sodium/potassium hydroxide solution.
Both sodium/potassium hydroxide solutions are precipitating reagents.
The alkalis produce unique colour of a precipitate/suspension when a few/three drops is added and then excess alkali is added to unknown salt/compound solution.
NB: Potassium hydroxide is not commonly used because it is more expensive than sodium hydroxide.
The table below shows the observations, inferences / deductions and explanations from the following test tube experiments:
Procedure
Put about 2cm3 of MgCl2, CaCl2, AlCl3, NaCl, KCl, FeSO4, Fe2(SO4) 3, CuSO4, ZnSO4NH4NO3, Pb(NO3) 2, Ba(NO3) 2 each into separate test tubes. Add three drops of 2M sodium hydroxide solution then excess (2/3 the length of a standard test tube).
| Observation | Inference | Explanation |
| No white precipitate | Na+ and K+ | Both Na+ and K+ ions react with OH– from 2M sodium hydroxide solution to form soluble colourless solutions Na+(aq) + OH–(aq) -> NaOH(aq) K+(aq) + OH–(aq) -> KOH(aq) |
| No white precipitate then pungent smell of ammonia /urine | NH4+ ions | NH4+ ions react with 2M sodium hydroxide solution to produce pungent smelling ammonia gas NH4+ (aq) + OH–(aq) -> NH3 (g) + H2O(l) |
| White precipitate insoluble in excess | Ba2+ ,Ca2+, Mg2+ ions | Ba2+ ,Ca2+ and Mg2+ ions react with OH– from 2M sodium hydroxide solution to form insoluble white precipitate of their hydroxides. Ba2+(aq) + 2OH–(aq) -> Ba(OH) 2(s) Ca2+(aq) + 2OH–(aq) -> Ca(OH) 2(s) Mg2+(aq) + 2OH–(aq) -> Mg(OH) 2(s) |
| White precipitate soluble in excess | Zn2+ ,Pb2+, Al3+ ions | Pb2+ ,Zn2+ and Al3+ ions react with OH– from 2M sodium hydroxide solution to form insoluble white precipitate of their hydroxides. Zn2+(aq) + 2OH–(aq) -> Zn(OH) 2(s) Pb2+(aq) + 2OH–(aq) -> Pb(OH) 2(s) Al3+(aq) + 3OH–(aq) -> Al(OH) 3(s) The hydroxides formed react with more OH– ions to form complex salts/ions. Zn(OH) 2(s) + 2OH(aq) -> [ Zn(OH) 4]2-(aq) Pb(OH) 2(s) + 2OH(aq) -> [ Pb(OH) 4]2-(aq) Al(OH) 3(s) + OH(aq) -> [ Al(OH) 4]–(aq) |
| Blue precipitate insoluble in excess | Cu2+ | Cu2+ ions react with OH– from 2M sodium hydroxide solution to form insoluble blue precipitate of copper(II) hydroxide. Cu2+(aq) + 2OH–(aq) -> Cu(OH) 2(s) |
| Green precipitate insoluble in excess On adding 3cm3 of hydrogen peroxide, brown/yellow solution formed | Fe2+ Fe2+ oxidized to Fe3+ | Fe2+ ions react with OH– from 2M sodium hydroxide solution to form insoluble green precipitate of Iron(II) hydroxide. Fe2+(aq) + 2OH–(aq) -> Fe(OH) 2(s) Hydrogen peroxide is an oxidizing agent that oxidizes green Fe2+ oxidized to brown Fe3+ Fe(OH) 2(s) + 2H+ -> Fe(OH) 3(aq) |
| Brown precipitate insoluble in excess | Fe3+ | Fe3+ ions react with OH– from 2M sodium hydroxide solution to form insoluble brown precipitate of Iron(II) hydroxide. Fe3+(aq) + 3OH–(aq) -> Fe(OH) 3(s) |
(b)Reaction of cation with aqueous ammonia
Aqueous ammonia precipitating reagent that can be used to identify the cations present in a salt.
Like NaOH/KOH the OH– ion in NH4OH react with the cation to form a characteristic hydroxide .
Below are the observations ,inferences and explanations of the reactions of aqueous ammonia with salts from the following test tube reactions.
Procedure
Put about 2cm3 of MgCl2, CaCl2, AlCl3, NaCl, KCl, FeSO4, Fe2(SO4) 3, CuSO4, ZnSO4NH4NO3, Pb(NO3) 2, Ba(NO3) 2 each into separate test tubes.
Add three drops of 2M aqueous ammonia then excess (2/3 the length of a standard test tube).
| Observation | Inference | Explanation |
| No white precipitate | Na+ and K+ | NH4+,Na+ and K+ ions react with OH– from 2M aqueous ammonia to form soluble colourless solutions NH4+ (aq) + OH–(aq) -> NH4+OH(aq) Na+(aq) + OH–(aq) -> NaOH(aq) K+(aq) + OH–(aq) -> KOH(aq) |
| White precipitate insoluble in excess | Ba2+ ,Ca2+, Mg2+ ,Pb2+, Al3+, ions | Ba2+ ,Ca2+,Mg2+ ,Pb2+ and Al3+, ions react with OH– from 2M aqueous ammonia to form insoluble white precipitate of their hydroxides. Pb2+ (aq) + 2OH–(aq) -> Pb(OH) 2(s) Al3+ (aq) + 3OH–(aq) -> Al(OH) 3(s) Ba2+ (aq) + 2OH–(aq) -> Ba(OH) 2(s) Ca2+ (aq) + 2OH–(aq) -> Ca(OH) 2(s) Mg2+ (aq) + 2OH–(aq) -> Mg(OH) 2(s) |
| White precipitate soluble in excess | Zn2+ ions | Zn2+ ions react with OH– from 2M aqueous ammonia to form insoluble white precipitate of Zinc hydroxide. Zn2+(aq) + 2OH–(aq) -> Zn(OH) 2(s) The Zinc hydroxides formed react NH3(aq) to form a complex salts/ions. Zn(OH) 2(s) + 4NH3(aq) ->[ Zn(NH3) 4]2+(aq)+ 2OH–(aq) |
| Blue precipitate that dissolves in excess ammonia solution to form a deep/royal blue solution | Cu2+ | Cu2+ ions react with OH– from 2M aqueous ammonia to form blue precipitate of copper(II) hydroxide. Cu2+(aq) + 2OH–(aq) -> Cu(OH) 2(s) The copper(II) hydroxide formed react NH3(aq) to form a complex salts/ions. Cu(OH) 2 (s) + 4NH3(aq) ->[ Cu(NH3) 4]2+(aq)+ 2OH–(aq) |
| Green precipitate insoluble in excess. On adding 3cm3 of hydrogen peroxide, brown/yellow solution formed | Fe2+ Fe2+ oxidized to Fe3+ | Fe2+ ions react with OH– from 2M aqueous ammonia to form insoluble green precipitate of Iron(II) hydroxide. Fe2+(aq) + 2OH–(aq) -> Fe(OH) 2(s) Hydrogen peroxide is an oxidizing agent that oxidizes green Fe2+ oxidized to brown Fe3+ Fe(OH) 2(s) + 2H+ -> Fe(OH) 3(aq) |
| Brown precipitate insoluble in excess | Fe3+ | Fe3+ ions react with OH– from 2M aqueous ammonia to form insoluble brown precipitate of Iron(II) hydroxide. Fe3+(aq) + 3OH–(aq) -> Fe(OH) 3(s) |
Note
(i) Only Zn2+ ions/salts form a white precipitate that dissolve in excess of both 2M sodium hydroxide and 2M aqueous ammonia.
(ii) Pb2+ and Al3+ ions/salts form a white precipitate that dissolve in excess of 2M sodium hydroxide but not in 2M aqueous ammonia.
(iii) Cu2+ ions/salts form a blue precipitate that dissolve to form a deep/royal blue solution in excess of 2M aqueous ammonia but only blue insoluble precipitate in 2M sodium hydroxide
(c)Reaction of cation with Chloride (Cl–)ions
All chlorides are soluble in water except Silver chloride and Lead (II)chloride (That dissolve in hot water).When a soluble chloride like NaCl, KCl, NH4Cl is added to about 2cm3 of a salt containing Ag+ or Pb2+ions a white precipitate of AgCl or PbCl2 is formed. The following test tube reactions illustrate the above.
Experiment
Put about 2cm3 of silver nitrate(V) andLead(II)nitrate(V)solution into separate test tubes. Add five drops of NaCl /KCl / NH4Cl/HCl. Heat to boil.
| Observation | Inference | Explanation |
| (i)White precipitate does not dissolve on heating | Ag+ ions | Ag+ ionsreacts with Cl– ions from a soluble chloride salt to form a white precipitate of AgCl |
| (ii)White precipitate dissolve on heating | Pb2+ ions | Pb2+ ionsreacts with Cl– ions from a soluble chloride salt to form a white precipitate of PbCl2. PbCl2 dissolves on heating. |
Note
Both Pb2+ and Al3+ ions forms an insoluble white precipitate in excess aqueous ammonia. A white precipitate on adding Cl– ions/salts shows Pb2+.
No white precipitate on adding Cl– ions/salts shows Al3+.
Adding a chloride/ Cl– ions/salts can thus be used to separate the identity of Al3+ and Pb2+.
(d)Reaction of cation with sulphate(VI)/SO42- and sulphate(IV)/SO32- ions
All sulphate(VI) and sulphate(IV)/SO32- ions/salts are soluble/dissolve in water except Calcium sulphate(VI)/CaSO4, Calcium sulphate(IV)/CaSO3, Barium sulphate(VI)/BaSO4, Barium sulphate(IV)/BaSO3, Lead(II) sulphate(VI)/PbSO4and Lead(II) sulphate(IV)/PbSO3.When a soluble sulphate(VI)/SO42- salt like Na2SO4, H2SO4, (NH4)2SO4 or Na2SO3 is added to a salt containing Ca2+, Pb2+, Ba2+ ions, a white precipitate is formed.
The following test tube experiments illustrate the above.
Procedure
Place about 2cm3 of Ca(NO3)2, Ba(NO3)2, BaCl2 and Pb(NO3)2, in separate boiling tubes. Add six drops of sulphuric(VI)acid /sodium sulphate(VI)/ammonium sulphate(VI)solution. Repeat with six drops of sodium sulphate(IV).
| Observation | Inference | Explanation |
| White precipitate | Ca2+, Ba2+, Pb2+ ions | CaSO3 and CaSO4 do not form a thick precipitate as they are sparingly soluble. Ca2+(aq)+ SO32-(aq) -> CaSO3(s) Ca2+(aq)+ SO42-(aq) -> CaSO4(s) Ba2+(aq)+ SO32-(aq) -> BaSO3(s) Ba2+(aq)+ SO42-(aq) -> BaSO4(s) Pb2+(aq)+ SO32-(aq) -> PbSO3(s) Pb2+(aq)+ SO42-(aq) -> PbSO4(s) |
(e)Reaction of cation with carbonate(IV)/CO32- ions
All carbonate salts are insoluble except sodium/potassium carbonate(IV) and ammonium carbonate(IV).
They dissociate /ionize to releaseCO32- ions. CO32- ions produce a white precipitate when the soluble carbonate salts is added to any metallic cation.
Procedure
Place about 2cm3 of Ca(NO3)2, Ba(NO3)2, MgCl2 ,Pb(NO3)2 andZnSO4 in separate boiling tubes.
Add six drops of Potassium /sodium carbonate(IV)/ ammonium carbonate (IV)solution.
| Observation | Inference | Explanation |
| Green precipitate | Cu2+ ,Fe2+,ions CO32-(aq) | Copper(II)carbonate(IV) and Iron(II) carbonate (IV) are precipitated as insoluble green precipitates. Cu2+(aq)+ CO32-(aq) -> CuCO3(s) Fe2+(aq)+ CO32-(aq) -> FeCO3(s) When sodium carbonate(IV)is added to CuCO3(s) the CO32-(aq) ions are first hydrolysed to produce CO2(g) and OH–(aq)ions. CO32-(aq) + H2O (l) -> CO2 (g) + 2OH– (aq) The OH–(aq) ions further react to form basic copper(II) carbonate(IV). Basic copper(II) carbonate(IV) is the only green salt of copper. Cu2+(aq)+ CO32-(aq)+2OH– (aq) ->CuCO3.Cu(OH)2 (s) |
| White precipitate | CO32- | White ppt of the carbonate(IV)salt is precipitated Ca2+(aq) + CO32- (aq) -> CaCO3(s) Mg2+(aq) + CO32- (aq) -> MgCO3(s) Pb2+(aq) + CO32- (aq) -> PbCO3(s) Zn2+(aq) + CO32- (aq) -> ZnCO3(s) |
Note
(i)Iron(III)carbonate(IV) does not exist.
(ii)Copper(II)Carbonate(IV) exist only as the basic CuCO3.Cu(OH) 2
(iii)Both BaCO3 and BaSO3 are insoluble white precipitate. If hydrochloric acid is added to the white precipitate;
I. BaCO3 produces CO2 gas. When bubbled/directed into lime water solution,a white precipitate is formed.
II. I. BaSO3 produces SO2 gas. When bubbled/directed into orange acidified potassium dichromate(VI) solution, it turns to green/decolorizes acidified potassium manganate(VII).
(f) Reaction of cation with sulphide / S2- ions
All sulphides are insoluble black solids/precipitates except sodium sulphide/ Na2S/ potassium sulphide/K2S.When a few/3drops of the soluble sulphide is added to a metal cation/salt, a black precipitate is formed.
Procedure
Place about 2cm3 of Cu(NO3)2, FeSO4, MgCl2,Pb(NO3)2 and ZnSO4 in separate boiling tubes.
Add six drops of Potassium /sodium sulphide solution.
| Observation | Inference | Explanation |
| Black ppt | S2- ions | CuS, FeS,MgS,PbS, ZnS are black insoluble precipitates Cu2+(aq) + S2-(aq) -> CuS(s) Pb2+(aq) + S2-(aq) -> PbS(s) Fe2+(aq) + S2-(aq) -> FeS(s) Zn2+(aq) + S2-(aq) -> ZnS(s) |
Sample qualitative analysis guide
You are provided with solid Y(aluminium (III)sulphate(VI)hexahydrate).Carry out the following tests and record your observations and inferences in the space provided.
1(a) Appearance
Observations inference (1mark)
White crystalline solid Coloured ions Cu2+ , Fe2+ ,Fe3+ absent
(b)Place about a half spatula full of the solid into a clean dry boiling tube. Heat gently then strongly.
Observations inference (1mark)
Colourless droplets formed on the cooler Hydrated compound/compound
part of the test tube containing water of crystallization
Solid remains a white residue
(c)Place all the remaining portion of the solid in a test tube .Add about 10cm3 of distilled water. Shake thoroughly. Divide the mixture into five portions.
Observation Inference (1mark)
Solid dissolves to form Polar soluble compound
a colourless solution Cu2+ , Fe2+ ,Fe3+ absent
(i)To the first portion, add three drops of sodium hydroxide then add excess of the alkali.
Observation Inference (1mark)
White ppt, soluble in excess Zn2+ , Pb2+ , Al3+
(ii)To the second portion, add three drops of aqueous ammonia then add excess of the alkali.
Observation Inference (1mark)
White ppt, insoluble in excess Pb2+ , Al3+
(iii)To the third portion, add three drops of sodium sulphate(VI)solution.
Observation Inference (1mark)
No white ppt Al3+
(iv)I.To the fourth portion, add three drops of Lead(II)nitrate(IV)solution. Preserve
Observation Inference (1mark)
White ppt CO32-, SO42-, SO32-, Cl–,
II.To the portion in (iv) I above , add five drops of dilute hydrochloric acid.
Observation Inference (1mark)
White ppt persist/remains SO42-, Cl–,
III.To the portion in (iv) II above, heat to boil.
Observation Inference (1mark)
White ppt persist/remains SO42-,
Note that: (i)From test above, it can be deduced that solid Y i
aluminium(III)sulphate(VI) solid
(ii)Any ion inferred from an observation below must be derived from previous correct observation and inferences above. e.g.
Al3+ in c(iii) must be correctly inferred in either/or in c(ii) or c(i)above
SO42- in c(iv)III must be correctly inferred in either/or in c(iv)II or c(iv)I above
(iii)Contradiction in observations and inferences should be avoided.e.g.
“White ppt soluble in excess” to infer presence of Al3+ ,Ba2+ ,Pb3+
(iv)Symbols of elements/ions should be correctly capitalized. e.g.
“SO4-2” is wrong, “sO42-” is wrong, “cu2+” is wrong.
Sample solutions of salt were labeled as I,II, III and IV. The actual solutions, not in that order are lead nitrate, zinc sulphate potassium chloride and calcium chloride.
a)When aqueous sodium carbonate was added to each sample separately, a white precipitate was formed in I, III and IV only. Identify solution II.
b)When excess sodium hydroxide was added to each sample separately, a white precipitate was formed in solutions III and I only.
Identify solution I
17.When solids/salts /solutes are added to a solvent ,some dissolve to form a solution.
Solute + Solvent -> Solvent
If a solution has a lot of solute dissolved in a solvent ,it is said to be concentrated.
If a solution has little solute dissolved in a solvent ,it is said to be dilute.
There is a limit to how much solute can dissolve in a given /specified amount of solvent/water at a given /specified temperature.
The maximum mass of salt/solid/solute that dissolve in 100g of solvent/water at a specified temperature is called solubility of a salt.
When no more solute can dissolve in a given amount of solvent at a specified temperature, a saturated solution is formed.
For some salts, on heating, more of the salt/solid/solute dissolve in the saturated solution to form a super saturated solution.
The solubility of a salt is thus calculated from the formula
Solubility = Mass of solute/salt/solid x 100
Mass/volume of water/solvent
