Question

How the temporary and permanent hardness is removed?

Answer 1

Hint:The hardness of water is determined by determining the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than \[ + 1\] . Usually, the cations have the charge of \[ + 2\] . The most common cations that are found in hard water include \[C{a^{2 + }}\;\] and \[M{g^{2 + }}\] .

Complete step by step answer:
Temporary hardness is a type of water hardness which is caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved, these type of minerals yield calcium and magnesium cations (\[C{a^{2 + }}\;\],\[M{g^{2 + }}\]) and carbonate and bicarbonate anions ($CO_3^{2 - }$ and $HCO_3^ - $). The occurrence of metal cations in the water makes the water hard. However, unlike the permanent hardness caused by sulfate and chloride compounds, this "temporary" hardness can be reduced either by boiling the water, or by the addition of lime (calcium hydroxide) through the process of lime softening. The boiling of hard water promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of the solution, leaving behind pure water in the container that is softer upon cooling.
Permanent hardness (the mineral content dissolved in the water) is generally difficult to remove by the process of boiling. If this occurs, it is generally caused by the presence of calcium sulfate or calcium chloride and/or magnesium sulfate or magnesium chloride in the water, which do not precipitate out as the temperature increases. Ions that cause the permanent hardness of water can be removed by using a water softener, or an ion exchange column.

Therefore, option (C) is correct.

Note: A common method for softening of the hard water involves the use of ion exchange resins, which replace ions like \[C{a^{2 + }}\;\] by twice the number of the monovalent cations such as the sodium and the potassium ions present in the hard water during that time.