`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)`

`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)`

`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)`

`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.

`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.