Question

Soaps do not work in hard water containing calcium and magnesium ions because

• A. $Ca^{2+}$ and $Mg^{2+}$ ions form insoluble calcium and magnesium salts in the form of scum
• B. $Na^{+}$ and $K^{+}$ present in soap react with $Ca^{2+}$ and $Mg^{2+}$ and hinder cleansing process
• C. a large amount of soap is to be used in presence of $Ca^{2+}$ and $Mg^{2+}$
• D. scum formed by combination of $Na^{+}, Ca^{2+}$ and $Mg^{2+}$ stick to the cloth and are not removed on agitation

Answer 1

Answer: (A)

$Ca^{2+}$ and $Mg^{2+}$ ions form insoluble calcium and magnesium soaps respectively when sodium or potassium soaps are dissolved in hard water.
$\underset{\text{Soap}}{2C_{17}H_{35}COOHNa}+CaCl_{2} \to 2NaCl + \underset{\text{Insoluble calcium stearate (soap) }}{(C_{17}H_{35}COO)_2Ca}$
These insoluble soaps separate as scum in water and are useless as cleansing agent.