Chemistry Some important compounds of calcium

### Topics to be covered

=> Some important compounds of calcium
=> Calcium oxide
=> Calcium hydroxide
=> Calcium carbonate
=> Calcium sulphate
=> Cement
=> Biological importance of magnesium and calcium

### SOME IMPORTANT COMPOUNDS OF CALCIUM

color{green}(★) Important compounds of calcium are calcium oxide, calcium hydroxide, calcium sulphate, calcium carbonate and cement.

color{green}(★) These are industrially important compounds. The large scale preparation of these compounds and their uses are described below.

### Calcium Oxide or Quick Lime, CaO

color{green}(★) It is prepared on a commercial scale by heating limestone color{red}((CaCO_3)) in a rotary kiln at color{red}("1070-1270K.")

color{red}(CaCO_3 oversettext(heat)⇌ CaO + CO_2)

color{green}(★) Calcium oxide is a white amorphous solid. It has a melting point of color{red}(2870 K). On exposure to atmosphere, it absorbs moisture and carbon dioxide.

color{red}(CaO + H_2O → Ca (OH)_2)

color{red}(CaO + CO_2 → CaCO_3)

color{green}(★) The addition of limited amount of water breaks the lump of lime. This process is called slaking of lime.

color{green}(★) Quick lime slaked with soda gives solid sodalime. Being a basic oxide, it combines with acidic oxides at high temperature.

color{red}(CaO + SiO_2 → CaSiO_3)

color{red}(6 CaO + P_4O_(10) → 2Ca_3 (PO_4)_2)

### Uses

(i) It is an important primary material for manufacturing cement and is the cheapest form of alkali.

(ii) It is used in the manufacture of sodium carbonate from caustic soda.

(iii) It is employed in the purification of sugar and in the manufacture of dye stuffs.

### Calcium Hydroxide (Slaked lime), Ca(OH)_2

color{green}(★) Calcium hydroxide is prepared by adding water to quick lime, color{red}(CaO)

color{green}(★) It is a white amorphous powder. It is sparingly soluble in water. The aqueous solution is known as lime water and a suspension of slaked lime in water is known as milk of lime.

color{green}(★) When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.

color{red}(Ca(OH)_2 + CO_2 → CaCO_3 + H_2O)

color{green}(★) On passing excess of carbon dioxide, the precipitate dissolves to form calcium hydrogen carbonate.

color{red}(CaCO_3 + CO_2 + H_2O → Ca (HCO_3)_2)

color{green}(★) Milk of lime reacts with chlorine to form hypochlorite, a constituent of bleaching powder.

color{red}(2Ca(OH)_2 +2Cl_2 → undersettext(Bleaching powder)(CaCl_2 +Ca(OCl)_2) + 2H_2O)

### Uses

(i) It is used in the preparation of mortar, a building material.

(ii) It is used in white wash due to its disinfectant nature.

(iii) It is used in glass making, in tanning industry, for the preparation of bleaching powder and for purification of sugar.

### Calcium Carbonate , CaCO_3

color{green}(★) Calcium carbonate occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride.

color{red}(Ca(OH)_2 + CO_2 → CaCO_3 + H_2O)

color{red}(CaCl_2 + Na_2CO_3 → CaCO_3 + 2NaCl)

color{green}(★) Excess of carbon dioxide should be avoided since this leads to the formation of water soluble calcium hydrogencarbonate.

color{green}(★) Calcium carbonate is a white fluffy powder. It is almost insoluble in water. When heated to color{red}(1200 K), it decomposes to evolve carbon dioxide.

color{red}(CaCO_3 overset(1200K)→ CaO + CO_2)

color{green}(★) It reacts with dilute acid to liberate carbon dioxide.

color{red}(CaCO_3 + 2HCl → CaCl_2 + H_2O + CO_2)

color{red}(CaCO_3 + H_2SO_4 → CaSO_4 + H_2O + CO_2)

### Uses

color{green}(★) It is used as a building material in the form of marble and in the manufacture of quick lime.

color{green}(★) Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron.

color{green}(★) Specially precipitated color{red}(CaCO_3) is extensively used in the manufacture of high quality paper.

color{green}(★) It is also used as an antacid, mild abrasive in tooth paste, a constituent of chewing gum, and a filler in cosmetics.

### Calcium Sulphate (Plaster of Paris), CaSO_4 1/2 H_2O

color{green}(★) It is a hemihydrate of calcium sulphate. It is obtained when gypsum, color{red}(CaSO_4 · 2H_2O), is heated to color{red}(393 K).

color{red}(2 ( CaSO_4 . 2H_2O ) → 2 ( CaSO_4 ) . H_2O + 3H_2O)

color{green}(★) Above color{red}(393 K), no water of crystallisation is left and anhydrous calcium sulphate, color{red}(CaSO_4) is formed. This is known as ‘dead burnt plaster

color{green}(★) It has a remarkable property of setting with water. On mixing with an adequate quantity of water it forms a plastic mass that gets into a hard solid in 5 to 15 minutes.

### Uses:

color{green}(★) The largest use of Plaster of Paris is in the building industry as well as plasters.

color{green}(★) It is used for immoblising the affected part of organ where there is a bone fracture or sprain.

color{green}(★) It is also employed in dentistry, in ornamental work and for making casts of statues and busts.

### Cement

color{green}(★) Cement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin.

color{green}(★) It is also called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.
color{green}(★) Cement is a product obtained by combining a material rich in lime, color{red}(CaO) with other material such as clay which contains silica, color{red}(SiO_2) along with the oxides of aluminium, iron and magnesium.

color{green}(★) The average composition of Portland cement is : color{red}(CaO, \ \ 50- 60%; \ \ SiO_2 \ \ , 20-25%; \ \ \ Al_2O_3, \ \ \ 5-10%; \ \ MgO , \ \ 2- 3%; \ \ Fe_2O_3, \ \ 1-2%) and color{red}(SO_3, \ \ 1-2%).

color{green}(★) For a good quality cement, the ratio of silica color{red}((SiO_2)) to alumina color{red}((Al_2O_3)) should be between 2.5 and 4 and the ratio of lime color{red}((CaO)) to the total of the oxides of silicon color{red}((SiO_2)) aluminium color{red}((Al_2O_3)) and iron color{red}((Fe_2O_3)) should be as close as possible to 2.

color{green}(★) The raw materials for the manufacture of cement are limestone and clay.

color{green}(★) When clay and lime are strongly heated together they fuse and react to form ‘cement clinker’.

color{green}(★) This clinker is mixed with 2-3% by weight of gypsum color{red}((CaSO_4 · 2H_2O)) to form cement.

color{green}(★) Thus important ingredients present in Portland cement are dicalcium silicate color{red}((Ca_2SiO_4)) 26%, tricalcium silicate color{red}((Ca_3SiO_5)) 51% and tricalcium aluminate color{red}((Ca_3Al_2O_6)) 11%.

color{green}(★) Setting of Cement : When mixed with water, the setting of cement takes place to give a hard mass.

color{green}(•) This is due to the hydration of the molecules of the constituents and their rearrangement.

color{green}(•) The purpose of adding gypsum is only to slow down the process of setting of the cement so that it gets sufficiently hardened.

color{green}(★) Uses: Cement has become a commodity of national necessity for any country next to iron and steel.

It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.

### BIOLOGICAL IMPORTANCE OF MAGNESIUM AND CALCIUM

color{green}(★) An adult body contains about color{red}(25 g) of color{red}(Mg) and color{red}(1200 g) of color{red}(Ca) compared with only color{red}(5 g) of iron and color{red}(0.06 g) of copper.

color{green}(★) The daily requirement in the human body has been estimated to be color{red}(200 – 300 mg).

color{green}(★) All enzymes that utilise ATP in phosphate transfer require magnesium as the cofactor.

color{green}(★) The main pigment for the absorption of light in plants is chlorophyll which contains magnesium.

color{green}(★) About color{red}(99 %) of body calcium is present in bones and teeth. It also plays important roles in neuromuscular function, interneuronal transmission, cell membrane integrity and blood coagulation.

color{green}(★) The calcium concentration in plasma is regulated at about color{red}(100 mgL^(–1)).

color{green}(★) It is maintained by two hormones: calcitonin and parathyroid hormone.

color{green}(★) Do you know that bone is not an inert and unchanging substance but is continuously being solubilised and redeposited to the extent of color{red}(400 mg) per day in man? All this calcium passes through the plasma.