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Carbonates and Bicarbonates of Magnesium and Calcium
Calcium Carbonate (CaCO3)
It occurs in nature as marble, limestone, chalk, coral, calcite, etc. It is prepared as a white powder, known as precipitated chalk, by dissolving marble or limestone in hydrochloric acid and removing iron and aluminium present by precipitating with NH3, and then adding ammonium carbonate to the solution; the precipitate is filtered, washed and dried.
CaCl2 + (NH4)2CO3 →CaCO3 + 2NH4Cl
Properties
It dissolves in water containing CO2, forming Ca(HCO3)2 but is precipitated from solution by boiling.
CaCO3 + H2O + CO2 
Ca(HCO3)2
Illustration:
Thermal decomposition of a compound 'X' yields, a basic oxide ( Y ) and acidic oxide( Z ) simultaneously. The acidic oxide(Z) can be absorbed by alkaline KOH. What is X, Y, Z.
Solution:
CaCO3 ———→ CaO + CO2
'X' Basic Acidic
(Y) (Z)
Magnesium Carbonate (MgCO3)
It is obtained as magnesite in nature. It can be prepared as a white precipitate by adding sodium bicarbonate to a solution of a magnesium salt.
MgCl2 + NaHCO3 → MgCO3 + NaCl + HCl
Properties
(i) It is very much more soluble in water.
(ii) It dissolves in water containing CO2 due to formation of soluble bicarbonate.
MgCO3 + H2O + CO2 → Mg(HCO3)2
Bicarbonates of Mg & Ca
Calcium bicarbonate [Ca(HCO3)2]
It is obtained when CaCO3 is dissolved in water containing CO2 but it remains in the solution form CaCO3 + H2O + CO2 Ca(HCO3)2.
Magnesium bicarbonate [Mg(HCO3)2]
It is obtained when MgCO3 is dissolved in water containing CO2 but it remains in the solution form MgCO3 + H2O + CO2 Mg(HCO3)2.
Illustration:
NaHCO3 and NaOH cannot exist together in solution- Why?
Solution:
NaHCO3 is an acid salt which must react with NaOH which is strong base. The reaction is as follows:
NaHCO3 + NaOH → Na2CO3 + H2O
Halides of Mg & Ca
Calcium Chloride (CaCl2×6H2O)
It separates out as deliquescent crystals when a solution of lime or calcium carbonate in HCl is evaporated.
CaCO3 + 2HCl → CaCl2 + H2CO3
But it separates out from the reaction mixture as CaCl2×6H2O. The anhydrous salt is obtained on heating above 200°C.
Properties
It is a colourless, deliquescent salt, highly soluble in water. The anhydrous salt is an excellent drying agent.
Exercise:
What is being used as a laboratory dessicant?
Magnesium chloride (MgCl2×6H2O)
It is prepared in the laboratory by crystallizing a solution of the oxide, hydroxide or carbonate in dilute hydrochloric acid.
MgO + 2HCl → MgCl2 + H2O
Properties
It is colourless, crystalline salt, deliquescent in nature and exceedingly soluble in water.
Illustration:
Complete the following reactions:
heat
(i) MgCI2. 6H2O ——→
HCl(g)
(ii) MgCI2. 6H2O ——→
Δ
Solution:
heat
(i) MgCI2. 6H2O ——→ MgO + 2HCI + 5H2O
HCl(g)
(ii) MgCI2. 6H2O ——→ MgCI2 + 6H2O
Δ
Exercise:
Which magnesium compounds are the constituents of toothpaste?