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Hydroxides of Mg & Ca
Magnesium Hydroxide [Mg(OH)2]
It is obtained by adding caustic soda solution to a solution of magnesium sulphate or chloride.
MgSO4 + 2NaOH → Na2SO4 + Mg(OH)2
(i) It is converted into its oxide on heating.
(ii)It dissolves in NH4Cl solution easily.
Mg(OH)2 + 2NH4Cl → MgCl2 + 2NH4OH
Calcium hydroxide, Slaked lime [Ca(OH)2]
(i)From Quick lime
Calcium hydroxide is prepared on commercial scale by adding water to quick lime (Slaking of lime)
CaO + H2O → Ca (OH)2
During the process of slaking, lumps of quick lime crumble to a fine power.
(ii) From calcium chloride
It is obtained by treating calcium chloride with caustic soda.
CaCI2 + 2NaOH → Ca (OH)2 + 2NaCI
It is a white amorphous powder sparingly soluble in water, the solubility decreasing further with rise in temperature. An aqueous solution is known as lime water and a suspension of slaked lime in water is called milk of lime.
it loses water only at temperature above 700 K.
Ca(OH)2 ———→ CaO + H2O
(ii) Reaction with chlorine
It forms calcium hypochlorite a constituent of bleaching power.
2Ca (OH)2 + 2CI2 ——→ CaCI2 + Ca (OCI)2 + 2H2O
(iii) Reaction with carbon dioxide
When CO2 is passed through lime water, it turns milky due to formation of insoluble calcium carbonate
Ca(OH)2 + CO2 ——→ CaCO3 ↓ + 2H2O
If excess of Co2 is passed CaCO3 (ppt) dissolves to form soluble calcium bicarbonate due to which milkiness disappears.
CaCO3 + CO2 + H2O ——→ Ca (HCO3)2
If this clear solution of calcium bicarbonate is heated, the solution again turns milky due to the decomposition of ca(HCO3)2 back to CaCO3.
Ca ( HCO3)2 (aq) ———→ CaCO3 (s) + CO2 (g) + H2O (L)
Slaked lime being a strong base reacts with acids and acidic gases forming salts.
Ca (OH)2 + 2HCI ———→ CaCI2 + H2O
Ca (OH)2 + SO3 ——→ CaSO4 + H2O
However, Ca(OH)2 does not dissolve in dil. H2SO4 because the CaSO4 formed is sparingly soluble in water.
(i) Calcium hydroxide is used for absorbing acidic gases such as CO2, NO2, SO2, SO3etc.
(ii) For preparing ammonia from ammonium salts.
(ii) For softening of hard water.
(iv)In the laboratory, as lime water for detection of CO2.
(v) In white washing due to its disinfectant properties.
(vi)In the production of mortar which is used as a building material.
Which is the weakest base among NaOH, Ca(OH)2, KOH and Be(OH)2
Be(OH)2 is weakest base , because alkali metal hydroxides are more stronger base than alkaline earth metal hydroxides. Also basic character of hydroxides of alkaline earth metals increases down the group. So Be(OH)2 is the weakest one.