Question

TWhich one of the following will remove temporary hardness of water?
A. Slaked lime
B. Plaster of Paris
C. Epsom
D. Hydrolith

Answer 1

Hint: It is an inorganic compound, white powder or colourless crystal. It is used for the treatment of fresh water, removal of temporary hardness of water. To answer this question, first we should know about the hardness of water.

Complete answer:
In water, salts of magnesium and calcium are present as chlorides, bicarbonates and sulphates and this causes water to become hard. There are two types of hardness: temporary hardness and permanent hardness. Hardness caused by the presence of calcium bicarbonates is called temporary hardness. On the other hand, the hardness that is caused by the nitrates, chlorides of magnesium and calcium is termed as permanent hardness.

Hardness from water can be removed by different methods: physical methods and chemical methods. From the above given options, slaked lime can remove the temporary hardness and process of removal of hardness is called Clark’s process.
In Clark's process, the addition of slaked lime into temporary hard water will take place. Calcium carbonate that is insoluble in water precipitates out and there will be no hardness. Similarly, magnesium carbonate will also precipitate out. Slaked lime always be added in calculated amounts because excess of slaked lime can cause hardness of water.
\[Ca{\left( {OH} \right)_2}\left( s \right) + Mg{\left( {HC{O_3}} \right)_2}\left( {aq} \right) \to CaC{O_3}\left( s \right) + MgC{O_3}\left( s \right) + 2{H_2}O\left( l \right)\] \[Ca{\left( {OH} \right)_2}\left( s \right) + Ca{\left( {HC{O_3}} \right)_2}\left( {aq} \right) \to 2CaC{O_3}\left( s \right) + 2{H_2}O\left( l \right)\]

Thus, the correct answer is option A. i.e., Slaked lime.

Note:
There are some important points to be remembered, the chemical formula of slaked lime is $Ca{\left( {OH} \right)_2}$ and it is slightly soluble in the water. Calcium carbonate $CaC{O_3}$ is insoluble in water. Calcium and magnesium bicarbonates are soluble in water.