Question

Which of the following ions will cause hardness in water samples?
[A] $C{{a}^{2+}}$
[B] $N{{a}^{+}}$
[C] $C{{l}^{-}}$
[D] ${{K}^{+}}$

Answer 1

HINT: Hardness of water is caused by carbonates and bicarbonates of certain elements. Here, the correct answer is the one that we commonly find as carbonate and bicarbonate in limestone and cleansing limestone.

COMPLETE STEP BY STEP SOLUTION: When water has high mineral content, it is called hard water. Hard water is formed when water percolates (percolation is the movement of fluids through porous material) through the deposits of porous particles made up of excess minerals like calcium and magnesium carbonates, sulphates and bicarbonates in it.
We know that hardness of water is caused due to the mineral impurities like calcium and magnesium in water.
Presence of bicarbonates, chlorides and sulphates of calcium and magnesium makes the water hard. We can measure this hardness of water through various chemical processes.
Hardness of water can be permanent or temporary.
Temporary hardness is basically caused by the presence of carbonates and bicarbonates. We can remove temporary hardness of water simply by boiling.
Permanent hardness of water is caused by sulphate and chlorides of calcium and magnesium. We cannot remove permanent hardness of water through boiling as they do not precipitate out on increasing temperature. We generally use a water softener or ion exchange column to remove permanent hardness.
Now, let us go through the options to answer the question.
We can understand from the above discussion that hardness of water is caused by calcium and magnesium ions.

Therefore, the correct answer is option [A] $C{{a}^{2+}}$

NOTE: We can measure the hardness of water using EDTA through complexometric titration. At a pH of 10, the dissolved salts of calcium and magnesium form complexes with EDTA and the solution is colourless. We use an indication named Erio T which is wine red coloured in hard water and the end point of the titration, the solution turns blue.
     \[\begin{align}
  & C{{a}^{2+}}+{{H}_{2}}EDT{{A}^{2-}}\to CaEDT{{A}^{2-}}+2{{H}^{+}} \\
 & M{{g}^{2+}}+{{H}_{2}}EDT{{A}^{2-}}\to MgEDT{{A}^{2-}}+2{{H}^{+}} \\
\end{align}\]