Preparation of Salt Mr.
Cheng
Preparation of Salt 3. Preparation of insoluble salt:
Preparation of
Salt
Soluble Salt Insoluble Salt
Double decomposition reaction (also known
Does not
as precipitation reaction)
Contains Na+,
contain Na+,
K+ or NH4+ salt solution A + salt solution B → insoluble
K+ or NH4+
salt (precipitate)
Insoluble salt (precipitate) produced is then
+ +
1. Preparation of soluble salt that contains Na , K or filtered out
+
NH4 : 4. Protip: preparation of soluble salts involve acids.
Preparation of insoluble salt does not involve acid.
(A) Preparation of Soluble Salt that Contains
Na+, K+ or NH4+
1. Neutralisation between exact amount of acid and
Neutralisation between acid and alkali:
alkali is carried out.
acid + alkali → salt + water
acid(aq) + alkali(aq) → salt(aq) + water(𝑙)
Must use exact amount of acid and alkali (no
2. Choosing reactants for preparation of salt:
excess), titration method is used to determine
the exact volume of acid an alkali
HCl + NaOH → NaCl + H2O
2. Preparation of soluble salt that does not contain
Na+, K+ or NH4+ :
To prepare potassium chloride, KCl
__________________ + _________________
To prepare sodium sulphate, Na2SO4
__________________ + _________________
To prepare ammonium nitrate, NH4NO3
Use one of the following reaction
__________________ + _________________
acid + metal oxide → salt + H2O
3. Steps during preparation of salt:
acid + metal carbonate → salt + H2O + CO2
1) Titration between acid and alkali (to obtain
acid + reactive metal → salt + H2
the exact volume needed)
Metal oxide/metal carbonate/reactive metal
2) Neutralisation between acid and alkali is
are solid which are added in excess, later can
carried out again using the exact volume
be filtered out easily
without phenolphthalein
3) Crystallisation is carried out to obtain solid
salt crystals
Preparation of Salt Mr. Cheng
Activity: Preparation of Potassium Chloride, KCl 10. Salt crystals formed are filtered and rinsed with a
little amount of distilled water, then dried by
Material: 1.0 mol dm-3 hydrochloric acid HCl, 1.0 mol
pressing between sheets of filter papers.
dm-3 potassium hydroxide KOH, phenolphthalein
indicator, filter paper, distilled water Discussion:
1. Chemical equation involved:
Procedure:
HCl + KOH → KCl + H2O
2. Why is the salt solution obtained at (4) discarded
away and not used to obtain salt crystal?
3. Why is it important to use the exact volume of
acid and alkali? What will happen if either one is
in excess?
4. Why salt crystal obtained can only be rinsed with
a little amount of distilled water?
3 -3
1. 25 cm of 1.0 mol dm potassium hydroxide
solution is added into a conical flask using a 5. During crystallisation, why not heat it until all the
pipette. water dry out?
2. Two drops of phenolphthalein indicator are added
into the conical flask.
3. A burette is filled with 1.0 mol dm-3 hydrochloric
acid. Initial reading is recorded.
4. Hydrochloric acid is added slowly into conical flask
until pink solution changes to colourless.
5. Final reading on burette is recorded. Volume of
acid used V cm3 is calculated. (exact volume is
obtained)
6. 25 cm3 of 1.0 mol dm-3 potassium hydroxide
solution is added into a conical flask using a
pipette. (without phenolphthalein)
7. V cm3 of 1.0 mol dm-3 hydrochloric acid is added
slowly from burette into conical flask.
8. Solution formed is transferred to an evaporating
dish and heated slowly until saturated solution is
obtained.
9. Saturated solution is left to cool for crystallisation
to occur.
Preparation of Salt Mr. Cheng
(B) Preparation of Soluble Salt that Does Not 4. Important: Copper Cu is not a reactive metal, it
Contain Na+, K+ or NH4+ cannot react with acid to produce salt and H2
5. Steps during preparation of salt:
1. Firstly, can you state 5 soluble salts that does not
contain Na+, K+ or NH4+?
2. Soluble salts that does not contain Na+, K+ or NH4+
can be prepared using one of the reactions below:
acid(aq) + metal oxide(s) → salt(aq) + water(𝑙) 1) Acid is added with excess solid (metal oxide /
metal carbonate / reactive metal)
2HCl + MgO → MgCl2 + H2O 2) Unreacted excess solid is filtered out
3) Crystallisation is carried out to obtain solid
acid(aq) + metal carbonate(s) → salt(aq) + salt crystals
water(𝑙) + carbon dioxide(g)
Activity: Preparation of Copper(II) Nitrate, Cu(NO3)2
2HNO3 + CaCO3 → Ca(NO3)2 + H2O + CO2 Material: 1.0 mol dm-3 nitric acid HNO3 , copper(II)
oxide powder CuO, filter paper, distilled water
acid(aq) + reactive metal(s) → salt(aq) + H2(g)
Procedure:
H2SO4 + Zn → ZnSO4 + H2
3. Choosing reactants for preparation of salt:
To prepare aluminium nitrate, Al(NO3)3
__________________
________________ + __________________
__________________
To prepare zinc chloride, ZnCl2
__________________
________________ + __________________ 1. 25 cm3 of 1.0 mol dm-3 nitric acid is poured into a
__________________ beaker. Acid is heated slowly.
To prepare magnesium sulphate, MgSO4 2. Copper(II) oxide powder is added a little at a time
__________________ into the beaker and stirred.
________________ + __________________ 3. Continue adding copper(II) oxide until the powder
__________________ no longer dissolves. (in excess)
To prepare copper(II) chloride, CuCl2 4. Mixture is filtered to remove the excess copper(II)
__________________ oxide powder.
________________ + __________________ 5. Filtrate (salt solution) formed is transferred to an
__________________ evaporating dish and heated slowly until
saturated solution is obtained.
Preparation of Salt Mr. Cheng
6. Saturated solution is left to cool for crystallisation (C) Preparation of Insoluble Salts
to occur.
1. Firstly, can you state 5 insoluble salts?
7. Salt crystals formed are filtered and rinsed with a
2. Insoluble salt is prepared using precipitation
little amount of distilled water, then dried by
reaction (also known as double decomposition
pressing between sheets of filter papers.
reaction)
Discussion: salt solution A (aq) + salt solution B (aq) →
1. Chemical equation involved: insoluble salt (s) + salt solution C (aq)
2HNO3 (aq) + CuO (s) → Cu(NO3)2 (aq) + H2O (𝑙) 3. Example: preparation of lead(II) chloride PbCl2
2. Why is copper(II) oxide added in excess? What Choosing salt solutions (reactants) to be used,
happens if HNO3 is in excess instead of CuO? must be soluble salt solutions
PbCl2
3. Why is the acid heated during chemical reaction?
Pb2+ Cl-
Purification of Soluble Salts by Recrystallisation
Pb(NO3)2 (aq) NaCl (aq) // KCl (aq)
Method
Chemical equation involved:
1. If soluble salts contain impurities, it can be Pb(NO3)2(aq) + 2NaCl(aq) → PbCl2(s) + 2NaNO3(aq)
purified using recrystallisation method.
2. Steps involved in recrystallisation:
1) Dissolve the impure salt in distilled water
2) Filter the mixture to remove impurities
(insoluble) Ionic equation involved:
3) Carry out crystallisation to obtain pure salt Pb(NO3)2(aq) + 2NaCl(aq) → PbCl2(s) + 2NaNO3(aq)
crystals
3. Example: Recrystallisation of copper(II) sulphate
CuSO4
4. Choosing reactants for preparation of salt:
Protip: always choose soluble salt solutions
with NO3- ion and Na+/K+ ion
To prepare lead(II) sulphate, PbSO4
Reactants: ____________ + ____________
Chemical Eq: __________________________
Ionic equation: ________________________
To prepare calcium carbonate, CaCO3
Can recrystallisation method be used to purify
Reactants: ____________ + ____________
insoluble salts? Why?
Chemical Eq: __________________________
Ionic equation: ________________________
Preparation of Salt Mr. Cheng
5. Steps during preparation of salt: Exercise:
1. Below is a list of salts:
NaNO3 BaSO4 CaCO3
NaCl Pb(NO3)2 MgSO4
1) Add salt solution A and salt solution B into a
K2CO3 AgCl (NH4)2SO4
beaker
PbI2 BaCrO4 ZnCl2
2) Filter out the precipitate (insoluble salt)
a. Classify the salts into soluble salt and
formed
insoluble salt
Activity: Preparation of Barium Sulphate BaSO4 b. Identify the reactants used to prepare each of
the salts above
Material: 1.0 mol dm-3 sodium sulphate solution
Na2SO4 , 1.0 mol dm-3 barium chloride solution BaCl2 ,
distilled water, filter paper
Procedure:
1. 25 cm3 of 1.0 mol dm-3 sodium sulphate solution
and 25 cm3 of 1.0 mol dm-3 barium chloride
solution are poured into a beaker and stirred.
2. Mixture is filtered to obtain the precipitate
(insoluble salt).
3. Residue of filtration (insoluble salt) is rinsed with
distilled water, then pressed between filter
papers to dry it.
Discussion:
1. Chemical eq: BaCl2 + Na2SO4 → BaSO4 + 2NaCl
2. Ionic equation: Ba2+ + SO42- → BaSO4
3. Is it ok if either BaCl2 solution or Na2SO4 solution is
in excess? Discuss.
4. Can we substitute Na2SO4 (salt solution) with
H2SO4 (acid)? Discuss.
