chemistry chp ter 8

7/30/2019 Chemistry Chp Ter 8

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SaltsWhat is salt?

2 types of salts :

Soluble salt salts that can be dissolve in water at room temperature

Insoluble salt salts cannot be dissolve in water at room temperature

Type of salt Solubility in water

Sodium salts

Potassium salts

Ammonium salts

All dissolves in water

Nitrate salts All dissolves in water

Chloride saltsAll dissolves in water, Except: Lead(II) chloride, PbCl2

Silver chloride, AgCl

Mercury chloride, HgCl

Sulphate salts

All dissolves in water Except: Lead(II) sulphate, PbSO4Barium sulphate, BaSO4Calcium sulphate, CaSO4

Carbonate salts

All did not dissolves in water, Except: Sodium carbonate, Na2CO3

Potassium carbonate, K2CO3Ammonium carbonate, (NH4)2CO3

Special Properties of lead(II) chloride and lead(II) iodide

Lead halide such as lead(II) chloride (PbCl2), lead(II) bromide (PbBr2) & lead(II) iodide (PbI2) did

not dissolve in cold water but dissolve in hot water.

1

White precipitate of

PbCl2

White precipitate

dissolves in hotwater

White precipitate formed

when the water is cooleddown.

Yellow precipitateof PbI

2

Yellow crystals formedwhen the water is cooled

down.

Yellow precipitate

dissolves in hot water

Salt is an ionic compound formed when the hydrogen ion, H+ from

acid is replaced by a metal ion or ammonium ion, NH4

+

PbI2

soluble in hot

water.

PbCl2

are soluble in

hot water.


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Uses of salts

Item Use Example

Food preparation

FlavorMonosodium glutamate (MSG)

Sodium chloride

PreservativesSodium chloride - salted fishSodium benzoate - sauce

Sodium nitrite - processed meat, burger

Baking powder Sodium hydrogen carbonate

AgricultureNitrogen fertilizers

Potassium nitrate

Sodium nitrate

PesticideCopper(II) sulphate

Iron(II) sulphate

Medicine

Reduce stomach acidic

(gastric)

Calcium carbonate

Calcium hydrogen carbonate

Sniff salt (fainted) Ammonium carbonate

Plaster of Paris (cement tosupport broken bone)

Calcium sulphate

Preparation of Salt

Insoluble salt is prepared through precipitation reaction.

Soluble salt is prepared by one of these reactions;i. Acid and alkali

ii. Acid and metal oxide

iii. Acid and metal carbonate

iv. Acid and reactive metal

Preparing Insoluble Salts

Through precipitation /double decomposition reactions.

Involves :

2[aq] solutions/soluble salts mix together

1 of the solutions contains the cations & 1 anions of the insoluble salt.

The ions of the 2 [aq] solutions above interchange to produce 2 new compoundwhich is insolublesalt/precipitate & [aq] solution.

The precipitate produced is obtained by filtration. The residue left in the filter paper is theinsoluble salt. The filtrate is soluble salt.

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The residue/precipitate then rinsed with distilled water to remove other ions as impurities.

Chemical and ionic equations

Chemical equation : MX(aq) + NY(aq) MY(s) + NX(aq)

solution solution precipitate solution

Ionic equation : M+(aq) + Y-(aq) MY(s)

3

Na+ Na+

NO3

-NO3

-

PbCl2

Pb2+ ions combined with Cl-

ions to form white

precipitate

Na+ ions and NO3

- ions do not take

part in the reaction and are free to

move in the solution

Ionic equation: Pb2+ + 2Cl- PbCl2

Glass rod

Residue/precipitate

(Insoluble salt)

Filtrate (Soluble salt)

Mixture of solutions

Filter funnel

Filter paper

Retort stand


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ii. Preparation & purification of insoluble salts

Preparation of Plumbum(II) iodide

Chemical equation : Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) + 2KNO3 (aq)

Ionic equation : Pb2+ (aq) + 2I- (aq) PbI2 (s)

Step 1: Preparation

1. 20 cm3 lead(II) nitrate 0.1 mol dm-3 solution is measured with measuring cylinder 50 ml,

and poured into a beaker.

2. 20 cm3 potassium iodide 0.1 mol dm-3 solution is measured with measuring cylinder 50 ml

and poured into a beaker contains lead(II) nitrate solution.

3. The mixture is stirred with a glass rod. A yellow precipitate is formed.

4. The mixture is filtered to obtain the yellow solids of lead(II) iodide as the residue.

Step 2: Purification

4

+

20 cm3 Lead(II) nitrat 0.1 mol dm-320 cm3 potassium iodide 0.1 mol dm-3

Glass rod

Precipitate of lead(II) iodide

(yellow)

Sodium nitrate solution

Mixture of solutions

Filter funnel

Filter paper

Retort standBeaker


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5. The residue is rinsed with distilled water to remove other ions in it.

6. The yellow solid is dried by pressing between two pieces of filter paper.

b. Soluble Salt

5

Glass rodDistilled water

Precipitate of lead(II)

iodide

Precipitate of lead(II)

iodideFilter paper


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i. Preparaing Soluble Salt

- Sodium salts

- Potassium salts Acid + alkali salts + water

- Ammonium salts

Soluble Salts

Acid + metal oxide salts + water

- Others salts Acid + reactive metal salts + hydrogen gas

Acid + metal carbonate salt + water + carbon dioxide

Notes: Reactive metal is magnesium, aluminium, and zincUnreactive metal is iron, lead, silver

a. Sodium, potassium or ammonium salts prepared from acid and alkali reaction.

Salt Alkali Acid Chemical equation

NaCl NaOH HCl NaOH + HCl NaCl + H2O

K2SO4 KOH H2SO4 2KOH + H2SO4 K2SO4 + 2H2O

NH4NO3 NH3/NH4OH HNO3 NH3 + HNO3 NH4NO3 + H2O

CH3COONa NaOH CH3COOH NaOH + CH3COOH CH3COONa + H2O

Note: To prepare the above salts, titration technique is use.

b. Soluble salt (except sodium, potassium and ammonium salt) is prepared using these methods

- Acid and metal- Acid and metal oxide

- Acid and metal carbonate

Name of SaltAcid that must be

used

Substance that can be use to react with acid

Metal Metal oxide Metal carbonate

ZnCl2 HCl Zn ZnO ZnCO3

Mg(NO)3 HNO3 Mg MgO MgCO3

CuSO4 H2SO4 CuO CuCO3

Pb(NO3)2 HNO3 PbO PbCO3

Write a chemical equation for each experiment above.

Remember this notes ok

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1. Metal that is lessreactive from hydrogen such as copper, lead and

silver/argentum didnot react with dilute acid.

2. Metal, metal oxide and metal carbonate above is a solid that cannot dissolves in

water, hence during reaction that solid must be added excessively to make sure

all hydrogen ions in acid is completely reacted. Excess solid can be expelling

through filtration.

3. Impure soluble salt can be purified through crystallization process.

ii. Preparation and purification of soluble salts

A. Preparing soluble salt through reaction between acid and alkali.

Preparation of Soluble Sodium, Potassium and ammonium Salts

Soluble salts of sodium, potassium and ammonium can be prepared by the reaction between an

acid and alkali.

Acid (aq) + alkali (aq) Salt (aq) + Water (l)

Procedure : Using pipette, 25 cm3 of alkali solution is measured and

transferred into a conical flask.

Two drops of phenolphthalein are added to the alkali solution.

Dilute acid is place in a burette. The initial reading is recorded.

Acid is added slowly into the alkali solution while shaking

the conical flaks, until the pink solution turn colourless.

The final reading of the burette is recorded.

The volume of acid added, V cm3 is calculated.

The experiment is repeated by adding V cm3 of acid to

25 cm3 of alkali solution in a beaker without using

phenolphthalein as an indicator. The mixture is transferred into a evaporating dish.

The mixture is heated until saturated and the saturated solution

is allowed to cool at room temperature.

Salt crystals formed are filtered and rinsed with a little of cold distilled water.

Salt crystals are dried by pressing it between filter papers (or in oven)

The reaction between acid and alkali is known as what process?

Refer to acids and bases notes ok. Dont worry Ill help you.!

Kita bukan along kita cuma nak tolong.

Example: Preparing sodium chloride

Step 1:Preparation (Titration)

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1. 25.0 cm3 sodium hydroxide solutions is pipette into conical flask.2. Two drops of phenolphthalein indicator are added into conical flask. The colour of solution is recorded.

3. A 50 cm3 burette is filled with hydrochloric acid. The initial burette reading is recorded.

4. Hydrochloric acid is added gradually from a burette into conical flask and swirling the conical flask.

5. Titration is stopped when phenolphthalein changes from pink to colourless. The final burette reading isrecorded.

6. The volume of hydrochloric acid used is calculated.

7. The experiment is repeated by adding hydrochloric acid (known volume) to 25.0 cm3 sodium hydroxidein a beaker without using phenolphthalein.

Step 2: Preparation (Crystallization)

8. The mixture is transferred into a evaporating dish.

9. The colourless solution is slowly heated/evaporated until its saturated or to about one-third (1/3) of theoriginal volume.

10. The saturated solution is then cooled to allow crystallization to occur.


8

Retortstand

Burette

Hydrochloric acid

Conical flask

25 cm3 NaOH + phenolphthalein

indicator

Bunsen burner

Evaporating dishSalt solution


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10. The white crystals formed are then filtered, rinsed with a little distilled

water and dried by pressing between filter paper.

Note: Phenolphthalein indicator is used at the beginning of the experiment to determine the volume

of hydrochloric acid that is required to react completely with 25 cm3 of sodium hydroxide.

However experiment is repeated without using phenolphthalein so that the salt prepared will

not contaminated by the indicator.

B. Preparing soluble salt through reaction between acid

i. Metal oxide. ii. Metal iii. Metal carbonate

Procedure To Prepare a Soluble Salt (not Na, K or NH4+)

50 cm3 of acid is measured using a measuring cylinder and poured into a beaker. The acid is heatedslowly.

9

Glass rodDistilled water

Copper(II) sulphate

Filter funnel


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Using a spatula, metal / metal oxide / metal carbonate powder is added a little at a time while stirring

the mixture with a glass rod.

The addition of the solid powder is stopped when some solids no longer dissolve anymore. (the solid is

excess and all the acid is completely neutralised by the solid)

The mixture is filtered to remove the excess solid powder.

The filtrate is transferred to an evaporating dish.

The filtrate is heated until saturated. (The filtrate is evaporated to about one-third (1/3) of the original

volume)

The saturated solution is then allowed to cool to room temperature and the salt crystals are formed.

The crystals are filtered and rinsed with a little cold distilled water.

Salt crystals are then dried by pressing it between filter papers.

Example: Preparing copper(II) sulphate

(Sulphuric acid and copper(II) oxide powder)

Step 1: Preparation

10

Heat ing

AcidP o w d e r

of :

M eta l ox ide

M etal carbon

M e t a l

Excess of solid powd

Filtrate (Salt solution)

H ea t ing

S a t u r a t e d s o l u t io n

C ry sta

F i l tra te


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1. 50 cm3 sulphuric acid 0.1 mol dm-3 is put in a beaker and is heated.

2. Using spatula copper(II) oxide powder is added a little at a time to the hot sulphuric acid while stirringcontinuously with glass rod.

3. The addition of copper(II) oxide is stopped when solids powder remain undissolved.

4. The mixture is filtered to remove the excess copper(II) oxide.

5. The filtrate is transferred to an evaporating dish.

11

xxxxxxxxxxxxxxxx

Copper(II) oxide

Glass rod

Beaker

Wire gauze

50 cm3 sulphuric acid 0.1 mol dm-3

Tripod

Bunsen

burner

Spatula

Stir

Glass rod

Reactant mixture

Excess copper(II) oxide

Copper(II) sulphate solution


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6. The filtrate is slowly heated/evaporated until its saturated, or to about one-third (1/3) of the originalvolume.

7. The saturated solution is then allowed to cool to room temperature.


8. The crystals are filtered and rinsed with a little cold distilled water.

9. Salt crystals are then dried by pressing it between filter papers.

REMEMBER. THIS NOTES OK

Unreactive metal such as lead (Pb), copper (Cu), and silver (Ag) cannot react with dilute asid. So to

prepare salt contains lead ions (Pb2+), copper ions (Cu2+)

or silver ions (Ag+), we must use eitheroxide powder orcarbonate powder only.

Example: CuO + H2SO4 CuSO4 + H2O (ok)

CuCO3 + H2SO4 CuSO4 + H2O + CO2 (ok)

Cu + H2SO4 no reaction (not ok)

B. Physical Characteristics of Crystals.

12

Bunsen burner

Evaporating dish

Copper(II) sulphate

solution

Glass rod

Distilled water

Copper(II) sulphate


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A salt is made upof positive and negative ions. When these ions are packed closely with a regular and repeated

arrangement in an orderly manner, a solid with definite geometry known as crystal lattice is formed.

All crystals have these physical characteristics:

a) Reqular geometry shapes, such as cubic or hexagonal.

b) Flat faces, straight edges and sharp angles.c) Same angle between adjacent faces.

d) All crystals of the same salt have the same shape although the sizes may be different.

The solubility of a salt in water depends on the types ofcations and anions present.

C. Qualitative Analysis

of Salts

What is Qualitativeanalysis?

Inthe

qualitative analysis ofsalts, we need to identifythe ions that are present in

salts. This can be done by analysing theirphysical and chemical properties.

Observations on the physical properties of salts

1. Colour and solubility in water

Certain physical properties of salts such colour and solubitity in water are observed to help us infer certain cations andanions that are present in salts.The table shows the colour of salts in solid , in aqueous solution and the solubility of salts in water

Salt Colour in solidSolubility inwater

Colour in Aqueous solution

1. Ammonium chloride NH4Cl white soluble colourless

2. Ammonium nitrateNH4(NO3)3 white soluble colourless

3. Calcium carbonate CaCO3 white insoluble -

4. Calcium nitrate Ca(NO3)2 white soluble colourless

13

Salt Solubility in water

Sodium, potassium andammonium salts(Na+, K+, NH4

+) All are soluble

Nitrate salt (NO3-) All are soluble

Chloride salt (Cl-) All chloride salts are soluble in waterexcept PbCl2, AgCl

and HgCl2

Sulphate salt (SO42-) All sulphate salts are soluble in waterexcept PbSO4,

BaSO4and CaSO4

Carbonate salt (CO32-

) All carbonate salts are insolubleexcept Na2CO3, K2CO3 and

(NH4)2CO3

Qualitative analysis is a chemical technique used to determine

substances are present in a mixture but not their quantities


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5. Magnesium sulphate MgSO4 white soluble colourless

6. Magnesium carbonate MgCO3 white insoluble -

7. Zinc sulphate Zn SO4 white soluble colourless

8. Zinc nitrate Zn(NO3)2 white soluble colourless

9. Lead(II) chloride , PbCl2 white insoluble -

10. Lead(II) sulphate , PbSO4 white insoluble -11. Lead(II) carbonate , PbCO3 white insoluble -

12. Copper(II) chloride , CuCl2 Blue soluble Blue

13 Copper(II) sulphate , PbSO4 Blue soluble Blue

14. Copper(II) carbonate , PbCO3 Green insoluble -

15. Iron(II) sulphate , FeSO4 Green soluble Pale green

16. Iron(III) chloride , FeCl3 Brown / Yellow soluble Brown/Yellow/ Yellowish brown

17. Sodium nitrate , NaNO3 white soluble colourless

18, Sodium carbonate , Na2CO3 white soluble colourless

19. Potassium nitrate , KNO3 white soluble colourless

20. Potassium carbonate , K2CO3 white soluble colourless

The table shows the colour of different cations in the solid form or in aqueous solution

Observation Inference

Blue solution Ion copper (Cu2+ ) present

Pale green solution Ion Iron(II) Fe2+ present

Yellow/Yellowish-brown/brown solution Ion Iron (III) Fe3+ present

Green solid Hydrated Fe 2+, CuCO3

Brown solid Hydrated Fe 3+ salt

White solidSalts of Na+ , K+ ,NH4+, Mg 2+, Ca 2+ Al 3+ , Zn 2+, Pb 2+ (If the anions arecolourless

Colourless solution Na+ , K+ ,NH4+, Mg 2+, Ca 2+ , Al 3+ , Zn 2+, Pb 2+

The table shows the solubility of different types of salts in water

Compounds Solubility in water

Sodium saltsPotassium salts

Ammonium saltsAll are soluble

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Nitrate saltsAll are soluble

Chloride saltsAll are soluble except AgCl, HgCl and PbCl2 (soluble in hot water)

Sulphate saltsAll are soluble except BaSO4, PbSO4 and CaSO4

Carbonate salts All are insoluble except sodium carbonate, potassium carbonate andammonium carbonate

2. Tests for gases

Gases are often produced from reactions carried out during laboratory tests on salts. By identifying the gases evolved,it ispossible to infer the types of cations and anions that are present in a salt.

The table shows the test and the result of different gases

Gas Test Result

Oxygen gas, O2Test with a glowing woodensplinter

Wooden splinter is rekindled /lighted

Hydrogen gas , H2Test with a lighted woodensplinter

Gas explodes with a pop sound

Carbon dioxide gas , CO2Bubble the gas produced intolime water

Lime water turns milky

Ammonia gas, NH3 Test with moist red litmus paper Moist red litmus paper turns blue

Chlorine gas, Cl2 Test with moist blue litmus paperMoist blue litmus paper turns red and then turnswhite

Hydrogen chlorine gas , HClTest with a drop of concentratedammonia NH3 solution Dense white fumes

Sulphur dioxide gas , SO2

Bubble the gas produced intopurple acidified potassiummanganate (VII), KMnO4 solution

Purple acidified potassium manganate(VII),KMnO4 solution decolourises

Nitrogen dioxide gas , NO2 Test with moist blue litmus paper moist blue litmus paper turns red

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3. Action of heat on salts

Effect of heat on carbonate salts

Carbonate saltColour of salt beforeheating

Colour of residueEffect on lime water

Hot cold

Copper (II) carbonate,CuCO3

Green powder Black powder Black powder The gas liberated turns limewater milky/chalky

Zinc carbonate ,ZnCO3

White solid Yelow solid White solidThe gas liberated turns limewater milky/chalky

Lead(II) carbonate,PbCO3

White solid Brown sold Yelow solidThe gas liberated turns limewater milky/chalky

Sodium carbonate,Na2CO3

White solid White solid White solid No change

Calcium carbonate,CaCO3

White solid White solid White solidThe gas liberated turns limewater milky/chalky

Potassium carbonate,K2CO3

White solid White solid White solid No change

Magnesiumcarbonate, MgCO3

White solid White solid White solidThe gas liberated turns limewater milky/chalky

Effect of heat on nitrate salts

Nitrate SaltColour of saltbefore heating

Colour of residueTest on gases liberated

Hot cold

Copper (II) nitrate,Cu(NO3)2

Blue solidBlackpowder

Blackpowder

A brown gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter

Zinc nitrate, Zn(NO3)2 White solidYellowsolid

White solid

A browan gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter

Lead(II) nitrate,

Pb(NO3)2 White solid Brown solid

Yellow

solid

A browan gas that turns blue litmus paperred is liberated.

The gas liberated also ignites a glowingsplinter

Sodium nitrate,NaNO3

White solid White solid White solidA colourless gas that rekindles a glowingsplinter is liberated

Calcium nitrate,Ca(NO3)2

White solid White solid White solid


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Potassium nitrate,KNO3

White solid White solid White solidA colourless gas that rekindles a glowingsplinter is liberated

Magnesium nitrate,Mg(NO3)2

White solid White solid White solid


Iron(II) nitrate,Fe(NO3)2

Pale Green solidPale Greensolid

Pale Greensolid


Iron(III) nitrate,Fe(NO3)3

Brown solidReddish-Brown solid

Reddish-Brown solid


The table shows the comparison of the effect of heat on carbonate and nitrate salts

Metal Effect of heat on carbonate salt Effect of heat on nitrate salt

Potassium

Sodium Are not decomposed by heat Decompose to nitrite salt and oxygen gas.CalciumMagnesium

AluminiumZincIronTinLeadCopper

Decompose to metal oxide andcarbon dioxide gas.

Decompose to metal oxide, nitrogen dioxide gasand oxygen gas.

MercurySilverGold

Decompose to metal, carbon dioxidegas and oxygen gas.

Decompose to metal , nitrogen dioxide gas andoxygen gas.

Most sulphate salts are not decomposed by heat. Only a few sulphate such as iron(II) sulphate,zinc sulphate and coppersulphate decompose to sulphur dioxide or sulphur trioxide gas when heated.

All chloride salts are stable when heated except ammonium chloride. Ammonium chloride sublimes and decomposes toproduce ammonia gas and hydrogen chloride gas.

The table shows the deduction of the types of ion present based on the gas produced

Type of gas produced Type of ion present(anion)

CO2 Carbonate ion (CO32- ) present except Na2CO3 and K2CO3

O2 Nitrate ion (NO3-) present

NO2 Nitrate ion (NO3-) present except NaNO3 and KNO3

SO2 Sulphate ion (SO42- ) present

NH3 Ammonim ion (NH4+) present

Tests for anions

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Reagent / Condition Observation Anion Ionic Equation (if any)

2 cm3 the unknown solution +dilute hydrochloric acid / nitric acid

/ sulphuric acid pour into a test

tube gas liberated isimmediately bubbled through limewater.

Effervescence.Colourless gasturns lime watermilky.

CO32- ion CO3

2- + 2H+ CO2 +H2O

2 cm3 of nitric acid + 2 cm3 of the

unknown solution pour into a

test tube + 2 cm3 silver nitratesolution

White precipitate isformed.

Cl- ion Ag+ + Cl- AgCl

2 cm3 of dilute hydrochloric acid /nitric acid + 2 cm3 of the unknown

solution pour into a test tube+ 2 cm3 of barium chloride /

barium nitrate solution shakewell

White precipitate isformed.

SO42- ion Ba2+ + SO4

2 - BaSO4

2 cm3 of the unknown solution

pour into a test tube 2 cm3 of dilute sulphuric acid + 2

cm3 of iron(II) sulphate solutionshake well.Then drop carefully and slowly afew drops of concentratedsulphuric acid along the side of aslanting test tube into the mixturewithout shaking it.

Brown ring isformed at theboundary betweenthe concentratedH2SO4 (top layer)and aqueoussolution of themixture (bottomlayer)

NO3- ion -

Tests for cationsConfirmatory Test for Fe2+, Fe3+, Pb2+, NH4

+ Ions

Confirmatory Test for Fe2+

Reagent Observation Conclusion

Potassium hexacyanoferrate(II)solution

Pale blue precipitate Fe2+ ion is present

Dark blue precipitate Fe3+ ion is present

Potassium hexacyanoferrate(III)solution

Dark blue precipitate Fe2+ ion is present

Greenish-brown solution Fe3+ ion is present

Potassium thiocyanate solution Pale red colouration Fe2+ ion is present

Blood red colouration Fe3+ ion is present

Confirmatory Test for Pb2+

Method Observation Ionic Equation

Using aqueous solution of chloride- 2 cm3 of any solution of Cl-+

2 cm3 of any solution of Pb2+

dilute with 5 cm3 of distilled water

- A white precipitate isformed

When heated dissolve inwater to form colourless

Pb2+ + 2Cl- PbCl2

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19/21

heat until no further change occurs allow the content to cool to roomtemperature using running water fromthe tap

solution

When cooled whiteprecipitate reappear

Using aqueous solution of iodide- 2 cm3 of any solution of I- +

2 cm3 of any solution of Pb2+

dilute with 5 cm3 of distilled water

heat until no further change occurs allow the content to cool to roomtemperature using running water fromthe tap

- A yellow precipitate isformed

When heated dissolve inwater to form colourlesssolution

When cooled yellowprecipitate reappear

Pb2+ + 2I- PbI2

Confirmatory Test for NH4+

Method Observation

2 cm3 of any solution of NH4+ + 2 cm3 of NaOH /

KOH / Ca(OH)2 heat put a piece of moistred litmus paper at the mouth of the test tube

- Moist red litmus paper turns blue

Reaction with Nesslers Reagent

2 cm3 of any solution of NH4+ + 2 cm3 of

Nesslers Reagent shake well

- A brown precipitate is formed

Reaction of Cations With NaOH

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Pb2+ Zn2+ Al3+ : White precipitate dissolves in excess NaOHCa2+ Mg2+ : White precipitate insoluble in excess NaOH

Reaction of Cations With NH3

20

Cations

+ NaOH (aq)

Precipitate producedNo precipitate

White precipitate Coloured precipitate

Green Blue Brown

Fe2+ Cu2+ Fe2+NH

4

+K+ Na+

Dissolve in excessNaOH (aq) to formcolourless solution

Sodium hydroxide solution is pouredslowly into 2 cm3 of the solution to betested in a test tube, until in excess.

Insoluble inexcessNaOH (aq)

NH3

gas produced

warm

Zn2+

Al3+

Pb2+

Ca2+

Mg2+


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Zn2+ : White precipitate dissolves in excess NH3Pb2+ Al3+ Mg2+ : White precipitate insoluble in excess NH3

Zn2+ ion is the only cation that form white precipitate & dissolves in both excess NaOH & NH3.

Mg2+ ion is the only cation that form white precipitate & insoluble in both excess NaOH & NH3.

Ca2+ ion in the only cation that form white precipitate in NaOH solutions, but no precipitate in

NH3 solution.

Fe2+ , Fe3+ and Cu2+ ions is easy to spot because the ions shows coloured precipitate.

Pb2+ ion and Al3+ ion form white precipitate and dissolves in excess NaOH solution, but insoluble

in excess NH3 solutions.

21

Cations

Precipitate producedNoprecipitate

White precipitate Coloured precipitate

Green Blue Brown

Fe2+ Cu2+ Fe2+

NH4

+K+ Na+

Aqueous ammonia solution ispoured slowly into 2 cm3 of thesolution to be tested in a test tubeuntil in excess.

NH3

gas produced

warm

Zn2+ Al3+Pb2+

Ca2+

Mg2+

+ NH3 (aq)

+ excess

NH3

(aq)

Darkblue

solution

Dissolve in excess NH3

(aq) to form colourlesssolution

Insoluble inexcess NH

3(aq)

chemistry chp ter 8

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