Introduction to Salts
- 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.
- The number of ionizable/replaceable hydrogen in an acid is called basicity of an acid.
- Some acids are therefore:
- monobasic acids generally denoted HX e.g.
HCl, HNO3,HCOOH,CH3COOH. - dibasic acids ; generally denoted H2X e.g.
H2SO4, H2SO3, H2CO3,HOOCOOH. - tribasic acids ; generally denoted H3X e.g.
H3PO4.
- monobasic acids generally denoted HX e.g.
- Some salts are normal salts while other are acid salts.
- A normal salt is formed when all the ionizable /replaceable hydrogen in an acid is replaced by a metal or metallic /ammonium radical.
- 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-) |
Table Showing 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 | Ca2+ | H2SO4 | SO42- | CaSO4 | Calcium sulphate(VI) |
NH4OH | NH4+ | H3PO4 | PO43- | (NH4)3PO4 (NH4)2HPO4 NH4H2PO4 |
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) |
- Some salts undergo hygroscopy, deliquescence and efflorescence.
- 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.
- 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.
- 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.
- 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 |
- 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)4SO4H2O | 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)4SO4 | Colourless solution |
- 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) | Na2CO3NaHCO3.2H2O |
Ammonium iron(II)sulphate(VI) | FeSO4(NH4)2SO4.2H2O |
Ammonium aluminium(III)sulphate(VI) | Al2(SO4)3(NH4)2SO4.H2O |
- 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 the ones on the right | Barium(II) sulphate(VI)/BaSO4, Calcium(II) sulphate(VI)/CaSO4, Lead(II) sulphate(VI)/PbSO4 |
All sulphate(IV)/SO32- salts except the ones on the right | Barium(II) sulphate(IV)/BaSO3, Calcium(II) sulphate(IV)/CaSO3, Lead(II) sulphate(IV)/PbSO3 |
All chlorides/Cl- salts except the ones on the right | 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 | All carbonates/CO32- except the ones on the left |
All alkalis(KOH,NaOH, NH4OH) | All bases |
Preparation of Salts
Salts can be prepared in a school laboratory by a method that uses its solubility in water.
Soluble Salts
- 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) → MgCl2(aq) + H2(g)
Zn(s) + 2HCl(aq) → ZnCl2(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.
Examples
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) → MgCl2(aq) + H2O(l)
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
Zn(OH)2(s) + 2HNO3(aq) → Zn(NO3)2(aq) + 2H2O(l)
CuO(s) + 2HCl(aq) → CuCl2(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
Examples
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)
CuCO3(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.
Examples
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)
2NH4OH(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) → MgCl2(s)
Ca(s) + Cl2(g) → CaCl2(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)
- 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)+ 3H2O(g) → Al(OH)3(aq) + 3HCl(g) - 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 , iron(III)chloride hydrolyses/reacts with water vapour / moisture present to form iron hydroxide solution and highly acidic fumes of hydrogen chloride gas.
FeCl3(s)+ 3H2O(g) → Fe(OH)3(aq) + 3HCl(g)
Insoluble Salts
- 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.
Examples
CuSO4(aq) + Na2CO3(aq) → CuCO3(s) + Na2SO4(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) + 2NH4NO3(aq)
BaCl2(aq) + K2SO3(aq) → BaSO3(s) + 2KCl(aq)
Effect of Heat on Salts
- 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:
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 Dissciation
Effect of Heat on Nitrate(V)
- Potassium nitrate(V)/KNO3 and sodium nitrate(V)/NaNO3 decompose on heating to form Potassium nitrate(III)/KNO2 and sodium nitrate(III)/NaNO2 respectively and producing Oxygen gas in each case.
2KNO3(s) → 2KNO2(s) + O2(g)
2NaNO3(s) → 2NaNO2(s) + O2(g) - 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) - )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) - 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)
Effect of Heat on Sulphate(VI)
- 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)
Effect of Heat on Carbonates(IV) and Hydrogen Carbonate(IV).
- Sodium carbonate(IV)and potassium carbonate(IV)do not decompose on heating.
- 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) - 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.
2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(l)
2KHCO3(s) → K2CO3(s) + CO2(g) + H2O(l) - 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)
Download SALTS - Chemistry Notes Form 2.
Tap Here to Download for 50/-
Get on WhatsApp for 50/-
Why download?
- ✔ To read offline at any time.
- ✔ To Print at your convenience
- ✔ Share Easily with Friends / Students