Question:

Permanent hardness in water cannot be cured by :

Updated On: Jul 6, 2024

Boiling
Ion exchange method
Calgon's method
Treatment with washing soda

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The Correct Option is A

Solution and Explanation

Only temporary hardness which is due to $HCO^-_3$ (bicarbonate) ions is removed by boiling.

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Concepts Used:

Hydrogen Bonding

Hydrogen bonding implies the formation of hydrogen bonds which are an attractive intermolecular force. An example of hydrogen bonding is the bond between the H atom and the O atom in water.

A special type of intermolecular attractive force arises only in the compounds having Hydrogen atoms bonded to an electronegative atom. This force is known as the Hydrogen bond. For instance, in water molecules, the hydrogen atom is bonded to a highly electronegative Oxygen.

The conditions for hydrogen bonding are:

The molecule must contain a strongly electronegative atom that is bound to the hydrogen atom. The higher the electronegativity, the more polarized is the molecule.
The electronegative atom must be small. The smaller the size, the greater the electrostatic magnetism.

Effects of Hydrogen Bonding on Elements:

Association: The molecules of carboxylic acids exist as dimer because of the hydrogen bonding. The molecular masses of such compounds are found to be double than those calculated from their simple formula.

Dissociation: In aqueous solution, HF dissociates and gives the difluoride ion instead of fluoride ion. This is due to hydrogen bonding in HF. The molecules of HCl, HBr, HI do not form a hydrogen bond. This explains the non-existence of compounds like KHCl2, KHBr2, KHI2.

Types of Hydrogen bonding

Intramolecular Hydrogen bonding: When hydrogen bonding takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding.
Intermolecular hydrogen bonding: The hydrogen bonding which takes place within a molecule itself is called intramolecular hydrogen bonding.
Symmetrical Hydrogen bonding: The symmetric hydrogen bond is a type of a three-centre four-electron bond.