It is used as magnesia cement. It is a mixture of `MgO` and `MgCl_2`. It is also called Sorel's cement.

`text(Preparation)` : `MgO` can be prepared by the following reactions.

`2Mg + O_2 oversettext(burning)-> 2MgO`

`Mg(OH)_2 oversettext(heated)-> MgO + H_2O`

`2Mg(NO_3)_2 oversettext(heated)-> 2MgO + 4NO_2 + O_2`

`MgCO_3 oversettext(heated)-> MgO + CO_2`

`text(Properties)` :

(i) It is a light infusible white powder (m.pt. `2800-C`), slightly soluble in water and forms magnesium hydroxide.

`MgO + H_2O -> Mg(OH)_2`

(ii) It is basic in nature and thus reacts with acids to form corresponding salts.

`MgO + 2HCl -> MgCl_2 + H_2O`

(iii) It is reduced by carbon at very high temperature.

`MgO + C -> Mg + CO`

(iv) Magnesium oxide when mixed with a saturated solution of magnesium chloride sets to hard mass like cement known as magnesia cement or sorel's cement. The composition is `MgCl_2*5MgO*xH_2O`

Its aqueous suspension is used in Medicine as an antacid. Its medicinal name is milk of magnesia.

`text(Preparation)` : It is obtained by dissolving magnesium oxide in water or by treating magnesium salt with an alkali.

`MgO + H_2O -> Mg(OH)_2`

`MgCl_2 + Ca(OH)_2 -> Mg(OH)_2 + CaCl_2`

`MgCl_2 + 2NaOH -> Mg(OH)_2 + 2NaCl`

`text(Properties)` :

(i) Its white powder, sparingly soluble in water.

(ii) It is basic in nature and forms salts with acids.

(iii) It decomposes on heating.

(iv) It readily dissolves in strong solution of `NH_4Cl.`

`Mg(OH)_2+ 2NH_4Cl -> MgCl_2 + 2NH_4OH`

It is isomorphous with `(ZnSO_4*7H_2O)` It is used as a purgative in medicine, as a mordant in dyeing and as a stimulant to increase the secretion of bile.

It occurs in nature as kieserite, `MgSO_4*H_2O`; Epsom salt, `MgSO_4*7H_2O`; and kainite, `KCl*MgSO_4*3H_2O`.

`text(Preparation)` :

(i) From magnesite or dolomite

`undersettext(magnesite)(MgCO_3) +H_2SO_4 -> MgSO_4 +H_2O +CO_2`

`undersettext(dolomite)(MgCO_3*CaCO_3) + 2H_2SO_4 -> MgSO_4 + CaSO_4 downarrow + 2H_2O + 2CO_2`

(ii) From Kieserire (Commercial method) : By boiling kieserite in water and cooling the resulting solution when crystals of Epsom salt separate out.

`MgSO_4*H_2O + 6H_2O -> MgSO_4*7H_2O`

`text(Properties)` :

(i) It is a colourless, efflorescent crystalline substance with bitter taste and forms a number of hydrates.

(ii) When heated to `150-C`, it changes to monohydrate which on further heating changes to anhydrous state at `200-C`.

`MgSO_4*7H_2O underset(-6H_2O) overset(150^oC) -> MgSO_4*H_2O overset(200^oC)-> MgSO_4 +H_2O`

On further heating it decomposes to form `MgO`.

`4MgSO_4 overset(text(above) 200^oC)-> 4MgO +2SO_2 +2SO_3 +O_2`

(iii) It is reduced by lamp black at `300-C`.

`2MgSO_4 + C -> 2MgO + 2SO_2 + CO_2`

(iv) It forms double salts with alkali metal sulphates, e.g. `K_2SO_4*MgSO_4*6H_2O`

It is a deliquescent solid. Hydrated salt on heating in air undergoes partial hydrolysis.

`MgCl_2*6H_2O oversettext(Heat)-> Mg(OH)Cl + HCl + 5H_2O`

It occurs in nature as mineral carnallite, `KCl.MgCl_2*6H_2O` and bischofite, `MgCl_2*H_2O`. It is found in sea water, mineral springs, etc.

`text(Preparation)` :

(a) The mineral carnallite is fused and cooled to `176-C` when whole of KCl is deposited while `MgCl_2*6H_2O` remains in the fused state.

(b) It can also be obtained by dissolving `Mg`, `MgO`, `Mg(OH)_2` or `MgCO_3` in dilute hydrochloric acid. The preparation of anhydrous magnesium chloride has already been described in the extraction of magnesium.

`text(Properties)` : It is colourless crystalline solid, highly deliquescent and highly soluble in water. It is the starting material for various magnesium compounds.

It occurs in nature as magnesite `(MgCO_3)` and dolomite `(MgCO_3*CaCO_3)`.

`text(Preparation)` : By adding sodium bicarbonate to a hot solution of magnesium salt, `MgCO_3` is formed.

`MgSO_4 + 2NaHCO_3 -> MgCO_3 + Na_2SO_4 + H_2O + CO_2`

The magnesium carbonate cannot be obtained by the addition of sodium carbonate to the solution of magnesium salt. A white precipitate of a basic carbonate of composition `3MgCO_3*Mg(OH)_2*3H_2O` is obtained which is known as magnesia alya. It is suspended in water and `CO_2` is passed when magnesium bicarbonate known as fluid magnesia is formed. The solution is boiled when normal magnesium carbonate separates out.

`2MgSO_4 + 2Na_2CO_3 + H_2O -> MgCO_3*Mg(OH)_2 + 2Na_2SO_4 + CO_2`

`MgCO_3*Mg(OH)_2 + 3CO_2 + H_2O -> 2Mg(HCO_3)_2`

`text(Properties)` :

(i) It is a white powder insoluble in water. It dissolves readily in water containing excess of carbon dioxide.

`MgCO_3 + CO_2 + H_2O -> Mg(HCO_3)_2`

(ii) It dissolves in acids forming salts with evolution of `CO_2`.

`MgCO_3 + 2HCl -> MgCl_2 + H_2O + CO_2`

`MgCO_3 + H_2SO_4 -> MgSO_4 + H_2O + CO_2`

(iii) On heating it decomposes with evolution of `CO_2`.

`MgCO_3 -> MgO + CO_2`

(iv) It forms double carbonates with alkali carbonates.

`MgCO_3 + Na_2CO_3 -> undersettext(soluble)[Na_2Mg(CO_3)_2]`

(i) `text(Ores of Calcium)` : Lime stone or marble or chalk `(CaCO_3)`, Gypsum `(CaSO_4*2H_2O)`, Dolomite `(CaCO_3*MgCO_3)`, fluorspar `(CaF_2)` phosphorite `(Ca_3(PO_4)_2)` Calcium phosphate is a constituent of bones and teeth.

(ii) Manufacture : It is manufactured by the electrolysis of a molten mixture of calcium chloride containing some calcium fluoride. Calcium chloride in turn is obtained as a by product of the solvay process.

It's aqueous suspension is known as slaked lime.

`CaO + H_2O oversettext(hissing sound)-> Ca(OH)_2 + text(Heat)`,

`text(Preparation)` : `CaO` is formed by the decomposition of `CaCO_3` at `800^oC`

`CaCO_3 overset(800^oC)-> CaO + CO_2`

(i) The reactions being reversible and thus to assure the complete decomposition of `CaCO_3`, carbon dioxide formed must be swept away by a current of air.

(ii) At high temperature, clay present as impurity in lime stone reacts with lime to form fusible silicates and thus temperature should not be high.

`text(Properties)` :

(i) Calcitun oxide is a white amorphous substance which on heating in oxy-hydrogen flame, gives an intense white light called lime light.

(ii) `text(Action of water :)` On adding water, it gives a hissing sound and forms calcium hydroxide, commonly known as slaked lime. The reaction is exothermic and known as slaking as slaking of lime.

`CaO + H_2O -> Ca(OH)_2` `DeltaH = -15000` calories.

Paste of lime in water is called milk of lime white its clear filtrate is knowm as lime water. Chemically either of these is calcium hydroxide.

(iii) `CaO` reacts with `SiO_2` and `P_2O_5` at high temperatures forming calcium silicate, `CaSiO_3` and calcium phosphate, `Ca_3(PO_4)_2` respectively.

`CaO + SiO_2 -> CaSiO_3`

`6CaO + 2P_2O_5 -> 2Ca_3(PO_4)_2`

(iv) It reacts with moist chlorine to form bleaching powder, `CaOCl_2`; with moist `CO_2` it forms `CaCO_3,` with moist `SO_2` it forms `CaSO_3`, with moist `HCl` gas it forms `CaCl_2`. However, these gases do not react when perfectly dried.

On heating with carbon at `2000-C`, it forms calcium carbide.

`CaO + 3C overset(2000^oC)-> CaC_2 + CO`

Calcium carbide is an important compound used for preparing acetylene and calcium cyanamide. Mixture of calcium cyanamide and carbon is known as nitrolim and is used as nitrogenous fertilizer.

`CaC_2 + 2H_2O -> undersettext(Cal.carbide)[Ca(OH)_2] + undersettext( Acetylene)(C_2H_2) uparrow`



`CaC_2 + N_2 overset(1000^oC)-> undersettext(Calcium cyanamide)(CaCN_2) + C`

`CaCN_2 + 5H_2O -> CaCO_3 + 2NH_4OH`

When exposed to oxy-hydrogen flame, it starts emitting light called lime light.

`text(Note)` : `CaO` is used as basic flux, for removing hardness of water, as a drying agent (for `NH_3` gas) for preparing mortar ( `CaO +` sand +water).

Fused `(CaCl_2)` is a good dessicant (drying agent). It can't be used to dry alcohol or ammonia as it forms additional products with them.

`Ca(OH)_2 +CO_2 -> CaCO_3 +H_2O`

`text(Preparation)` :

It obtained by passing carbon dioxide through lime water or by adding sodium carbonate solution to `CaCl_2`.

`Ca(OH)_2 + CO_2 -> CaCO_3 +H_2O`

`CaCl_2 + Na_2CO_3 -> CaCO_3 + 2NaCl`

`text(Properties)` :

(i) It is a white powder, insoluble in water.

(ii) It dissolves in water in presence of `CO_2` due to formation of calcium bicarbonate, otherwise insoluble.

`CaCO_3 + H_2O + CO_2 -> Ca(HCO_3)_2`

`text(Note)` : It is insoluble in water but dissolves in the presence of `CO_2`, due to the formation of calcium bicarbonate.

`CaCO_3+ H_2O + CO_2 -> Ca(HCO_3)_2`

It is a constituent of protective shells of marine animals.

On partially dehydrates to produce plaster of paris.

`CaSO_4 +2 H_2O overset(120^oC)-> CaSO_4*1/2H_2O + 1 1/2 H_2O`

`text(Plaster of Paris)` :

`CaSO_4*1/2H_2O undersettext(setting) overset(H_2O)-> CaSO_4*2H_2O oversettext(hardening)-> undersettext[Monoclinic(gypsum)][CaSO_4*2H_2O]`

`text(Gypsum) overset(200^oC)-> undersettext[(dead burnt plaster)](CaSO_4)(text[anhydrous])`

`text(Preparations)` :

It is prepared by reacting any calcium salt with either sulphuric acid or a soluble sulphate (e.g. `Na_2SO_4`)

`CaCl_2 +H_2SO_4 -> CaSO_4 +2HCl`

`CaCl_2 +Na_2SO_4 -> CaSO_4 +2NaCl`

`text(Properties)` :

(i) It is a white crystalline solid, sparingly soluble in water and solubility decreases as the temperature increases.

(ii) It dissolves in dilute acids. It also dissolves in ammonium sulphate due to the formation of double sulphate, `(NH_4)_2SO_4*CaSO_4*H_2O`.

(iii) Gypsum on heating first changes from monoclinic form to orthorhombic form without loss of water. At `120-C`, it loses three-fourth of its water of crystallation and forms hemihydrate `(2CaSO_4)H_2O` or `CaSO_4.(1/2)H_2O` which is commonly known as plaster of paris. It becomes anhydrous at `200-C`. The anhydrous form is known as burnt plaster or dead plaster. On strongly heating, it decomposes to give calcium oxide.

`2CaSO_4 *2H_2O overset(120^oC)-> [2CaSO_4*H_2O] +3H_2O`

`[2CaSO_4*H_2O] overset(200^oC)-> 2CaSO_4 +H_2O`

`2CaSO_4 undersettext(heated)oversettext(strongly)-> 2CaO +2SO_2 + O_2`

(iv) A suspension of gypsum on saturated with ammonia and carbon dioxide forms ammonium sulphate, a nitrogenous fertilizer.

`2NH_3 + CaSO_4 + CO_2 + H_2O -> (NH_4)_2SO_4 + CaCO_3`

(v) It forms calcium sulphide on heating strongly with carbon, `CaSO_4 + 4C -> CaS + 4CO`

Gypsum when heated to about `200^oC` is converted into anhydrous calcium sulphate. The anhydrous form (anhydrite) is known as dead burnt plaster because it does not set like plaster of paris when moistened with water.

`CaO + H_2O -> Ca(OH)_2`

`Ca(OH)_2 +CO_2 -> CaCO_3 + Ca(HCO_3)_2`


Suspension of `Ca(OH)_2` in water is called milk of lime.

`Ca(OH)_2 +Cl_2 -> CaOCl_2 + H_2O`

`text(Preparation)` :

(i) By treating time (quick lime) with water.

`CaO + H_2 O -> Ca(OH)_2`

(ii) By the action of caustic alkalies on a soluble calcium salt

`CaCl_2 + 2NaOH -> Ca(OH)_2 + 2NaCl`

`text(Properties)` :

(i) It is a white amorphous powder, sparingly soluble in water.

(ii) When dried and heated to redness, it loses a molecule of water and get converted into calcium oxide (lime).

(iii) Action of `CO_2`: Lime water [Lime `+ H_2O -> Ca(OH)_2`) is frequently used for the detection of `CO_2` and `SO_2` gas. These gases turn lime water milky due to formation of `CaCO_3` or `CaSO_3`.

`Ca(OH)_2 + CO_2 -> CaCO_3 downarrow + H_2O`

However, the precipitate disappears on prolonged action of `CO_2` because of the conversion of `CaCO_3` (insoluble) to calcitun bicarbonate (soluble).

`undersettext(Insoluble) (CaCO_3) + H_2O + CO_2 -> undersettext(Soluble)(Ca(HCO_3)_2)`

`CaSO_3 +H_2O +SO_2 -> Ca(HCO_3)_2`

The clear solution, if heated, again gives turbidity, due to decomposition of calcium bicarbonate to calcium carbonate.

`text(Preparation)` : It is obtained when gypsum, is heated at `120-C`.

`undersettext(Gypsum)(2[CaSO_4*2H_2O]) -> undersettext[(Calcium sulphate hemihdrate)](undersettext(Plaster of Paris)[(2CaSO_4)*H_2O]) + 3H_2O`

`text(Properties)` :

(i) Plaster of Paris is a white powder.

(ii) Plaster of Paris has the property of setting to a hard mass when a paste with water is allowed to stand aside for sometime. Slight expansion occurs during the setting as water is absorbed to reform `CaSO_4*2H_2O` (gypsum). The setting process is exothermic. The process of setting involves two stages. In the first stage, there is conversion of plaster of Paris into orthorhombic form of gypsum (setting step) and in the second stage orthorhombic form changes into monoclinic form (hardening step).

`undersettext(Plaster of Paris)[(2CaSO_4)*H_2O] underset(H_2O)oversettext(stirring)-> undersettext(Orthorhombic)[CaSO_4*2H_2O] -> undersettext(Monoclinic)[CaSO_4*2H_2O]`


The setting of plaster of Paris is catalysed by sodium chloride and is retarded by borax or alum.