Chemistry Chp Ter 8
Transcript of 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)
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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
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+
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
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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.
Step 3: Purification
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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.
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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
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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.
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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.
Step 3: Purification
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.
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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
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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
A browan gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter
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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
A browan gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter
Iron(II) nitrate,Fe(NO3)2
Pale Green solidPale Greensolid
Pale Greensolid
A browan gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter
Iron(III) nitrate,Fe(NO3)3
Brown solidReddish-Brown solid
Reddish-Brown solid
A browan gas that turns blue litmus paperred is liberated.The gas liberated also ignites a glowingsplinter
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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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
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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.
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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)