Question

Temporary hardness of water can be removed by using calcium hydroxide. How$?$

Answer 1

Hint: Temporary hardness of water is caused by the presence of dissolved bicarbonate minerals like calcium bicarbonate and magnesium bicarbonate. Try to think of any reaction that could occur in between the metal bicarbonates and calcium hydroxide, and how could the product formed be beneficial in removal of hardness of water.

Complete step-by-step answer:
Hard water is formed when water percolates through deposits of limestone, chalk or gypsum which are largely made up of calcium and magnesium bicarbonates and sulphates. Although it has high mineral content, it does not form lather with soap, hence it is not suitable for laundry purposes.

Hardness of water can be of two types:
$ \bullet $Temporary Hardness of Water: Temporary hardness of water is caused by the presence of dissolved bicarbonate minerals like calcium and magnesium bicarbonates.
$ \bullet $Permanent Hardness of Water: Permanent hardness of water is caused by the presence of magnesium or calcium sulphates.

Temporary hardness of water can be removed by using calcium hydroxide also known as Clark’s reagent, as calcium hydroxide converts bicarbonates to insoluble carbonates which can be removed by filtration thus softening the hard water.
$Ca{\left( {HC{O_3}} \right)_2} + Ca{\left( {OH} \right)_2} \to 2CaC{O_3} \downarrow + 2{H_2}O$

Additional Information: Temporary hardness of water can also be removed by simply boiling the hard water, as by boiling bicarbonates are converted to insoluble carbonates which can be simply removed by filtration.

$Mg{\left( {HC{O_3}} \right)_2}\xrightarrow{\Delta }MgC{O_3} \downarrow + {H_2}O + C{O_2}$
But for permanent hardness of water boiling does not help as sulphate salts do not decompose on heating.

Note: First of all you must be very clear with the definition of hard water, as many students make mistakes in that. Also you must be careful with the constituents present in temporary and permanent hard water as it is the key factor guiding the reaction with calcium hydroxide.