Gypsum and Plaster of paris
Mortar
Gypsum and Plaster of paris :
In nature , calcium sulphate occurs as gypsum and also as anhydrite . Alabaster is finely divided naturally calcium sulphate . The presence of calcium causes permanent hardness to water .
In the laboratory, calcium sulphate is prepared by the action of dil H2SO4 on lime (CaO) or CaCO3 . On concentration of the solution , gypsum CaSO4.2H2O separates out .
CaO + H2SO4 + H2O ---- CaSO4.2H2O
CaCO3 + H2SO4 + H2O ---- CaSO4.2H2O + CO2
Calcium sulphate or gypsum is sparingly soluble in water . Its solubility decreases on heating . It dissolves in solutions like that of ammonium sulphate due to double salt formation . When heated to 120oC to 130oC , a half hydrate or a hemihydrate ( CaSO4. H2O ) is formed . The hemihydrate is often referred to as plaster of paris . Both gypsum and plaster of paris lose water of hydration at 200oC and are converted into anhydrous calcium sulphate , known as "Dead burnt" CaSO4 . This is so called because it does not set with water .
Plaster of paris , when mixed with water ( half of its amount by weight ) becomes a paste which sets to a hard mass on standing . The process is known as "setting" . As the mass becomes hard , its volume increases and finally changes to CaSO4.2H2O .
There are two stages in the setting of plaster of paris . In the first stage , known as "setting stage" , the hemihydrate changes into orthorhombicdihydrite . In the second phase orthorhombic dihydrate is converted into mono-clinic dihydrate , gypsum . This stage is known as "hardening stage" .
Plaster of paris is used in the manufacture of crucibls , models etc . Surgeons use it for setting the fractured bones in position . Dentists also use it in hospitals . Toys are made with it . In the preparation of chalks used in class room teaching , plaster is used .
Gypsum is used in the preparation of cement and plaster of paris .
Mortar is an important compounds of calcium
An intimate mixture of 1 part of slaked lime , 3 parts of sand and water is known as lime "mortar" . Sand makes mortar porous and also harder . While hardening , sand prevents the cracks on contraction . Even though mortar was used in the construction of houses for several decades , it is not clearly known how it becomes so hard . Evaporation of water may be one of the reasons . Slaked lime , when mixed with sand , hardens slowly , due to formation of calcium silicate .
Ca(OH)2 + SiO2 gives CaSiO3 + H2O
Similarly , it may combine with CO2 to form calcium carbonate .
Ca(OH)2 + CO2 gives CaCO3 + H2O
Mortar becomes hard with time due to several chemical reaction taking place at the centre of the mass .
Mortar , mixed with cement is called "cement mortar" . This is stronger than mortar .
Lime stone and clay when heated together give "hydraulic mortar" .This can set like cement on addition of water . This is used for bleaching purposes and as an antiseptic .
Uses of Calcium
It is used as a reducing agent in the extraction of metals like uranium , zirconium and thorium .
It is also used as an alloying agent in the production of beryllium , aluminium , copper , lead and magnesium alloys .
It is also used as an deoxidizers , desulfurizer for various ferrous and non-ferrous alloys .
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