Acids and Bases

– At a school laboratory:
– An acid may be defined as a substance that turn litmus red.
– A base may be defined as a substance that turn litmus blue.
– Litmus is a lichen found mainly in West Africa.
– It changes its colour depending on whether the solution it is in, is basic/alkaline or acidic.
– It is thus able to identify/show whether.

1. An acid is a substance that dissolves in water to form H⁺/H₃O⁺ as the only positive ion/cation.

– This is called the Arrhenius definition of an acid.
– From this definition, an acid dissociate/ionize in water releasing H⁺ thus:

HCl_(aq) → H⁺ _(aq) + Cl^– _(aq)
HNO_3(aq) → H⁺ _(aq) + NO₃^–_(aq)
CH₃COOH_(aq) → H⁺ _(aq) + CH₃COO^–_(aq)
H₂SO_4(aq) → 2H⁺ _(aq) + SO₄^2-_(aq)
H₂CO_3(aq) → 2H⁺ _(aq) + CO₃^2-_(aq)
H₃PO_4(aq) → 3H⁺ _(aq) + PO₄^3-_(aq)

2. A base is a substance which dissolves in water to form OH- as the only negatively charged ion/anion.

– This is called Arrhenius definition of a base. – From this definition, a base dissociate/ionize in water releasing OH^– thus:

KOH_(aq) → K⁺_(aq) + OH^–_(aq)
NaOH_(aq) → Na⁺_(aq) + OH^–_(aq)
NH₄OH_(aq) → NH₄⁺_(aq) + OH^–_(aq)
Ca(OH)_2(aq) → Ca₂⁺_(aq) + 2OH^–_(aq)
Mg(OH)_2(aq) → Mg₂⁺_(aq) + 2OH^–_(aq)

3. An acid is a proton donor.

– A base is a proton acceptor. – This is called Bronsted-Lowry definition of acids and bases. – From this definition, an acid donates H⁺. – H⁺ has no electrons and neutrons .It contains only a proton.

Examples

I. From the equation:

Every base /acid from Bronsted-Lowry definition thus must have a conjugate product/reactant.

II. From the equation:

4. Acids and bases show acidic and alkaline properties/characteristics only in water but not in other solvents e.g.

(a)Hydrogen chloride gas dissolves in water to form hydrochloric acid Hydrochloric acid dissociates/ionizes in water to free H+(aq)/H3O+(aq) ions. The free H₃O+_(aq) / H⁺_(aq) ions are responsible for:

1. Turning blue litmus paper/solution red.
2. Show pH value 1/2/3/4/5/6.
3. Are good electrolytes/conductors of electricity/undergo electrolysis.
4. React with metals to produce / evolve hydrogen gas and a salt. i.e.

Ionically:

For a monovalent metal: 2M_(s) + 2H⁺_(aq) → 2M⁺_(aq) + H_2(g)
For a divalent metal: M_(s) + 2H⁺_(aq) → M₂⁺_(aq) + H_2(g)
For a trivalent metal: 2M_(s) + 6H⁺_(aq) → 2M₃⁺_(aq) + 3H_2(g)
Examples:
For a monovalent metal: 2Na_(s) + 2H⁺_(aq) → 2Na+_(aq) + H_2(g)
For a divalent metal: Ca_(s) + 2H⁺_(aq) → Ca₂⁺_(aq) + H_2(g)
For a trivalent metal: 2Al_(s) + 6H⁺_(aq) → 2Al₃⁺_(aq) + 3H_2(g)

5. React with metal carbonates and hydrogen carbonates to produce /evolve carbon(IV)oxide gas ,water and a salt. i.e.

Ionically:

For a monovalent metal:
M₂CO_3(s)+ 2H⁺_(aq) → 2M+_(aq) + H₂O _(l)+ CO_2(g)
MHCO_3(s)+ H⁺_(aq) → M+_(aq) + H₂O _(l)+ CO_2(g)
For a divalent metal: MCO_3(s)+ 2H⁺_(aq) → M2+_(aq) + H₂O _(l)+ CO_2(g)
M(HCO₃)_2(aq)+2H⁺_(aq) →M2+_(aq)+2H₂O_(l)+2CO_2(g)
Examples:
For a monovalent metal: K2CO_3(s)+ 2H⁺_(aq) → 2K+_(aq) + H₂O _(l)+ CO_2(g)
NH4HCO_3(s)+ H⁺_(aq) → NH4+_(aq) + H₂O _(l)+ CO_2(g)
For a divalent metal: ZnCO_3(s)+ 2H⁺_(aq) → Zn2+_(aq) + H₂O _(l)+ CO_2(g)
Mg(HCO₃) 2_(aq)+2H⁺_(aq) →Mg2+_(aq)+2H₂O_(l)+2CO_2(g)
(vi)neutralize metal oxides/hydroxides to salt and water only. i.e.

Ionically:

For a monovalent metal: M2O_(s) + 2H⁺_(aq) → 2M+_(aq) + H₂O _(l)
MOH_(aq) + H⁺_(aq) → M+_(aq) + H₂O _(l)
For a divalent metal: MO_(s) + 2H⁺_(aq) → M2+_(aq) + H₂O _(l)
M(OH) 2_(s) + 2H⁺_(aq) → M2+_(aq) + 2H₂O_(l)
For a trivalent metal: M2O3_(s) + 6H⁺_(aq) → 2M3+_(aq) + 3H₂O _(l)
M(OH) 3_(s) + 3H⁺_(aq) → M3+_(aq) + 3H₂O_(l)
Examples:
For a monovalent metal: K2O_(s) + 2H⁺_(aq) → 2K+_(aq) + H₂O _(l)
NH4OH_(aq) + H⁺_(aq) → NH4+_(aq) + H₂O _(l)
For a divalent metal: ZnO _(s) + 2H⁺_(aq) → Zn2+_(aq) + H₂O _(l)
Pb(OH) 2_(s) + 2H⁺_(aq) → Pb2+_(aq) + 2H₂O_(l)
(b) Hydrogen chloride gas dissolves in methylbenzene /benzene but does not dissociate /ionize into free ions.
It exists in molecular state showing none of the above properties.
(c) Ammonia gas dissolves in water to form aqueous ammonia which dissociate/ionize to free NH4+ _(aq) and OH-_(aq) ions.
This dissociation/ionization makes aqueous ammonia to:

(d) Ammonia gas dissolves in methylbenzene/benzene /kerosene but does not dissociate into free ions therefore existing as molecules.

6. Solvents are either polar or non-polar.

A polar solvent is one which dissolves ionic compounds and other polar solvents.
Water is polar solvent that dissolves ionic and polar substance by surrounding the free ions as below:

Note:Water is polar .It is made up of :
Oxygen atom is partially negative and two hydrogen atoms which are partially positive.
They surround the free H⁺ and Cl^– ions.
A non polar solvent is one which dissolved non-polar substances and covalent compounds.
If a polar ionic compound is dissolved in non-polar solvent ,it does not ionize/dissociate into free ions as below:

7. Some acids and bases are strong while others are weak.

(a) A strong acid/base is one which is fully/wholly/completely dissociated / ionized into many free H⁺ /OH- ions i.e. I. Strong acids exists more as free H⁺ ions than molecules. e.g.

II. Strong bases/alkalis exists more as free OH- ions than molecules. e.g.

(b) A weak base/acid is one which is partially /partly dissociated /ionized in water into free OH- _(aq) and H⁺_(aq) ions. I. Weak acids exists more as molecules than as free H⁺ ions. e.g.

II. Weak bases/alkalis exists more as molecules than free OH- ions. e.g.

8. The concentration of an acid/base/alkali is based on the number of moles of acid/bases dissolved in a decimeter(litre)of the solution.

An acid/base/alkali with more acid/base/alkali in a decimeter(litre) of solution is said to be concentrated while that with less is said to be dilute.

9. (a) (i)strong acids have pH 1/2/3 while weak acids have high pH 4/5/6.

(ii) a neutral solution have pH 7.
(iii) strong alkalis/bases have pH 12/13/14 while weak bases/alkalis have pH 11/10 /9 / 8.
(b) pH is a measure of H⁺_(aq) concentration in a solution.
The higher the H+(aq)ions concentration ;

– At pH 7 , a solution has equal concentration of H⁺_(aq) and OH-_(aq).
– Beyond pH 7,the concentration of the OH-_(aq) increases as the H⁺_(aq) ions decreases.

10. (a) When acids /bases dissolve in water, the ions present in the solution conduct electricity.

The more the dissociation the higher the yield of ions and the greater the electrical conductivity of the solution. A compound that conducts electricity in an electrolyte and thus a compound showing high electrical conductivity is a strong electrolyte while a compound showing low electrical conductivity is a weak electrolyte. (b) Practically, a bright light on a bulb ,a high voltage reading from a voltmeter high ammeter reading from an ammeter,
a big deflection on a galvanometer is an indicator of strong electrolyte(acid/base) and the opposite for weak electrolytes(acids/base).

11. Some compounds exhibit/show both properties of acids and bases/alkalis.

– A substance that reacts with both acids and bases is said to be amphotellic.
– The examples below show the amphotellic properties of:

(a) Zinc (II)oxide(ZnO) and Zinc hydroxide(Zn(OH)2)

(i)When ½ spatula full of Zinc(II)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.

Examples:
Chemical equation

Ionic equation

(ii) When reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.

Examples:
Chemical equation
1. When Zinc oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxozincate(II) complex salt.

2. When Zinc oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxozincate(II) complex salt.

Ionic equation

(ii) When Zinc(II)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.

Examples:
Chemical equation

Ionic equation

(ii) When reacting with sodium hydroxide, the hydroxide showsacidic properties by reacting with a base to form a complex salt.

Examples:
Chemical equation
1. When Zinc hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxozincate(II) complex salt.

2. When Zinc hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxozincate(II) complex salt.

Ionic equation

(b) Lead (II)oxide(PbO) and Lead(II) hydroxide (Pb(OH)2)

(i) When ½ spatula full of Lead(II)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) When reacting with nitric(V)acid, the oxide shows basic properties by reacting with an acid to form a simple salt and water only. All other Lead salts are insoluble.
Chemical equation

Ionic equation

(ii) When reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.
Chemical equation
1. When Lead(II) oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoplumbate(II) complex salt.

2. When Lead(II) oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoplumbate(II) complex salt.

Ionic equation

(ii) When Lead(II)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the hydroxide shows basic properties. It reacts with the acid to form a simple salt and water only.
Chemical equation

Ionic equation

(ii) when reacting with sodium hydroxide, the hydroxide shows acidic properties. It reacts with a base to form a complex salt.
Chemical equation
1. When Lead(II) hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoplumbate(II) complex salt.

2. When Lead(II) hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoplumbate(II) complex salt.

Ionic equation

(c) Aluminium(III)oxide(Al₂O₃) and Aluminium(III)hydroxide(Al(OH)3)

(i)When ½ spatula full of Aluminium(III)oxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the oxide shows basic properties by reacting with an acid to form a simple salt and water only.
Chemical equation

Ionic equation

(ii) When reacting with sodium hydroxide, the oxide shows acidic properties by reacting with a base to form a complex salt.
Chemical equation
1. When Aluminium(III) oxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoaluminate(III) complex salt.

2. When Aluminium(III) oxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoaluminate(II) complex salt.

Ionic equation

(ii) When Aluminium(III)hydroxide is placed in a boiling tube containing 10cm3 of either 2M nitric(V)acid or 2M sodium hydroxide hydroxide solution, it dissolves on both the acid and the alkali/base to form a colourless solution. i.e.
(i) when reacting with nitric(V)acid, the hydroxide shows basic properties. It reacts with the acid to form a simple salt and water only.
Chemical equation

Al(OH)_3 (s) + 3HNO_3(aq) → Al(NO₃)3 _(aq) + 3H₂O_(l)
Al(OH)_3 (s) + 3HCl_(aq) → AlCl_3 (aq) + 3H₂O_(l)
2Al(OH)_3 (s) + 3H₂SO_4(aq) → Al₂(SO₄)_3 (aq) + 3H₂O_(l)

Ionic equation

(ii) when reacting with sodium hydroxide, the hydroxide shows acidic properties. It reacts with a base to form a complex salt.
Chemical equation
1. When aluminium(III) hydroxide is reacted with sodium hydroxide the complex salt is sodium tetrahydroxoaluminate(III) complex salt.

2.When aluminium(III) hydroxide is reacted with potassium hydroxide the complex salt is potassium tetrahydroxoaluminate(III) complex salt.

Ionic equation

12. (a) 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:

(c) Some salts are normal salts while other are acid salts.

(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.

(f)Some salts exist as a simple salt while some as complex salts. Below are some complex salts.

_(g)Some salts exist as two salts in one. They are called double salts.

(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.

13. Salts can be prepared in a school laboratory by a method that uses its solubility in water.

(a) Soluble salts may be prepared by using any of the following methods:

(i) Direct displacement/reaction of a metal with an acid.

– 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.

Examples

Mg_(s) + H₂SO_4(aq) → MgSO_4 (aq) + H_2(g)
Zn_(s) + H₂SO_4(aq) → ZnSO_4 (aq) + H_2(g)
Pb_(s) + 2HNO_3(aq) → Pb(NO₃) 2_(aq) + H_2(g)
Ca_(s) + 2HNO_3(aq) → Ca(NO₃) 2_(aq) + H_2(g)
Mg_(s) + 2HNO_3(aq) → Mg(NO₃) 2_(aq) + H_2(g)
Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H_2(g)
Zn_(s) + 2HCl_(aq) → ZnCl_2(aq) + H_2(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) + 2HNO_3(aq) → Pb(NO₃) 2_(aq) + H₂O _(l)
Pb(OH)_2(s) + 2HNO_3(aq) → Pb(NO₃) 2_(aq) + 2H₂O _(l)
CaO _(s) + 2HNO_3(aq) → Ca(NO₃) 2_(aq) + H₂O _(l)
MgO _(s) + 2HNO_3(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) + 2HNO_3(aq) → Zn(NO₃) 2_(aq) + 2H₂O _(l)
CuO _(s) + 2HCl_(aq) → CuCl 2_(aq) + H₂O _(l)
CuO _(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O _(l)
Ag₂O_(s) + 2HNO_3(aq) → 2AgNO_3(aq) + H₂O _(l)
Na₂O_(s) + 2HNO_3(aq) → 2NaNO_3(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 a dilute 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.

PbCO_3 (s) + 2HNO_3(aq) → Pb(NO₃) 2_(aq) + H₂O _(l)+ CO_2(g)
ZnCO_3 (s) + 2HNO_3(aq) → Zn(NO₃) 2_(aq) + H₂O _(l)+ CO_2(g)
CaCO_3 (s) + 2HNO_3(aq) → Ca(NO₃) 2_(aq) + H₂O _(l)+ CO_2(g)
MgCO_3 (s) + H₂SO_4(aq) → MgSO_4(aq) + H₂O _(l)+ CO_2(g)
CuCO_3 (s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O _(l) + CO_2(g)
Ag₂CO_3 (s) + 2HNO_3(aq) → 2AgNO_3(aq) + H₂O _(l) + CO_2(g)
Na₂CO_3 (s) + 2HNO_3(aq) → 2NaNO_3(aq) + H₂O _(l) + CO_2(g)
K₂CO_3 (s) + 2HCl_(aq) → 2KCl_(aq) + H₂O _(l) + CO_2(g)
NaHCO_3 (s) + HNO_3(aq) → NaNO_3(aq) + H₂O _(l) + CO_2(g)
KHCO_3 (s) + HCl_(aq) → KCl_(aq) + H₂O _(l) + CO_2(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.

(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.

– 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.

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.

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.

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.

(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.

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.

(b) Effect of heat on nitrate .

(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.

(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.

(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.

(c) Effect of heat on Sulphate

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.

(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.

(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.

(iv) 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.

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.

(f) Reaction of cation with sulphide / S^2- 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.

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.

20. Solubility and solubility curves are therefore used

21.Natural fractional crystallization takes place in Kenya/East Africa at:

22.Extraction of soda ash from Lake Magadi in Kenya
Rain water drains underground in the great rift valley and percolate underground where it is heated geothermically.
The hot water dissolves underground soluble sodium compounds and comes out on the surface as alkaline springs which are found around the edges of Lake Magadi in Kenya.
Temperatures around the lake are very high (30-40oC) during the day.
The solubility of trona decrease with increase in temperature therefore solid crystals of trona grows on top of the lake (upto or more than 30metres thick).
A bucket dredger mines the trona which is then crushed ,mixed with lake liquor and pumped to washery plant where it is further refined to a green granular product called CRS.
The CRS is then heated to chemically decompose trona to soda ash(Sodium carbonate).
Chemical equation

Soda ash(Sodium carbonate) is then stored .It is called Magadi Soda. Magadi Soda is used :

Summary flow diagram showing the extraction of Soda ash from Trona

23. Extraction of common salt from Indian Ocean at Ngomeni in Kenya.
Oceans are salty.They contain a variety of dissolved salts (about 77% being sodium chloride).
During high tide ,water is collected into shallow pods and allowed to crystallize as evaporation takes place.The pods are constructed in series to increase the rate of evaporation.
At the final pod ,the crystals are scapped together,piled in a heap and washed with brine (concentrated sodium chloride).
It contains MgCl₂ and CaCl₂. MgCl₂ and CaCl2are hygroscopic. They absorb water from the atmosphere and form a solution.
This makes table salt damp/wet on exposure to the atmosphere.
24. Some water form lather easily with soap while others do not.
Water which form lather easily with soap is said to be soft.
Water which do not form lather easily with soap is said to be hard.
Hardness of water is caused by the presence of Ca²⁺ and Mg²⁺ ions.
Ca2+ and Mg2+ ions react with soap to form an insoluble grey /white suspension/precipitate called Scum/ curd. Ca2+ and Mg2+ ions in water come from the water sources passing through rocks containing soluble salts of Ca2+ and Mg2+ e.g. Limestone or gypsum.
There are two types of water hardness:

(a) Temporary hardness of water

Temporary hardness of water is caused by the presence of dissolved calcium hydrogen carbonate/Ca(HCO₃)₂ and magnesium hydrogen carbonate/Mg(HCO₃)₂ .
When rain water dissolve carbon(IV) oxide from the air it forms weak carbonic(IV) acid i.e.

When carbonic(IV) acid passes through limestone/dolomite rocks it reacts to form soluble salts i.e.

(b) Permanent hardness of water

Permanent hardness of water is caused by the presence of dissolved calcium sulphate(VI)/CaSO₄ and magnesium sulphate(VI)/MgSO₄ Permanent hardness of water is caused by water dissolving CaSO₄ and MgSO₄ from ground rocks.
Hardness of water can be removed by the following methods:

(a)Removing temporary hardness of water

(i) Boiling/heating.

Boiling decomposes insoluble calcium hydrogen carbonate/Ca(HCO₃)2 and magnesium hydrogen carbonate/Mg(HCO₃)2 to insoluble CaCO₃ and MgCO₃ that precipitate away. i.e
Chemical equation

(ii) Adding sodium carbonate (IV) /Washing soda.

Since boiling is expensive on a large scale ,a calculated amount of sodium carbonate decahydrate /Na₂CO₃.10H₂O precipitates insoluble Ca2+_(aq) and Mg2+_(aq) ions as carbonates to remove both temporary and permanent hardness of water . This a double decomposition reaction where two soluble salts form an insoluble and soluble salt. i.e.
(i) With temporary hard water
Chemical equation

Ionic equation

(ii) With permanent hard water
Chemical equation

Ionic equation

(iii) Adding calcium (II)hydroxide/Lime water

– Lime water/calcium hydroxide removes only temporary hardness of water from by precipitating insoluble calcium carbonate(IV).
Chemical equation
Ca(OH)_2 (aq) + Ca(HCO₃)_2 (aq) → 2H₂O_(l) + 2CaCO_3 (s)
– Excess of Lime water/calcium hydroxide should not be used because it dissolves again to form soluble calcium hydrogen carbonate(IV) causing the hardness again.

(iv) Adding aqueous ammonia

– Aqueous ammonia removes temporary hardness of water by precipitating insoluble calcium carbonate(IV) and magnesium carbonate(IV)
Chemical equation

(v) Use of ion-exchange permutit

– This method involves packing a chamber with a resin made of insoluble complex of sodium salt called sodium permutit.
– The sodium permutit releases sodium ions that are exchanged with Mg2+ and Ca2+ ions in hard water making the water to be soft. i.e.

– Hard water containing Mg and Ca²⁺
– Na⁺ ions replace Mg²⁺ and Ca²⁺ to make the water soft.
– When all the Na⁺ ions in the resin is fully exchanged with Ca²⁺ and Ng²⁺ ions in the permutit column, it is said to be exhausted.
– Brine /concentrated sodium chloride solution is passed through the permutit column to regenerated /recharge the column again.

(vi) Deionization /demineralization

– This is an advanced ion exchange method of producing deionized water.
– Deionized water is extremely pure water made only of hydrogen and oxygen only without any dissolved substances.
– Deionization involve using the resins that remove all the cations by using:

• A cation exchanger which remove /absorb all the cations present in water and leave only H⁺ ions.
• An anion exchanger which remove /absorb all the anions present in water and leave only OH^– ions.

The H⁺_(aq) and OH^– _(aq) neutralize each other to form pure water.
Chemical equation

When exhausted the cation exchanger is regenerated by adding H⁺_(aq) from sulphuric(VI)acid/hydrochloric acid.
When exhausted the anion exchanger is regenerated by adding OH^–_(aq) from sodium hydroxide.

Advantages of hard water

Hard water has the following advantages:

• Ca²⁺_(aq) in hard water are useful in bone and teeth formation.
• Is good for brewing beer.
• Contains minerals that cause it to have better /sweet taste.
• Animals like snails and coral polyps use calcium to make their shells and coral reefs respectively.
• Processing mineral water.

Disadvantages of hard water

• Waste a lot of soap during washing before lather is formed.
• Causes stains/blemishes/marks on clothes/garments.
• Causes fur on electric appliances like kettle ,boilers and pipes form decomposition of carbonates on heating .This reduces their efficiency hence more/higher cost of power/electricity.