Question

An ionic compound is dissolved simultaneously in heavy water and simple water. Its solubility is
A. Large in heavy water
B. Smaller in heavy water
C. Solubility is same in both
D. smaller in simple water

Answer 1

Hint: Ionic compounds dissolve in water and the main driving force for this solubility is the value of the dielectric constant. The higher the value of the dielectric constant of any solvent, the higher will be the solubility of ionic compounds. Because this increasing dielectric constant value reduces the electric field between the opposite charges of ions.

Formula used: The coulomb's force of attraction is related to the dielectric constant by the following equation:
$F=\frac{1}{4\pi {{\in }_{O}}}\frac{\left| {{q}_{1}} \right|\left| {{q}_{2}} \right|}{{{r}^{2}}}$
Here F= Force of attraction between two oppositely charged ions
${{\in }_{o}}$= Dielectric constant of the solvent
$r=$ Distance between two oppositely charged ions
${{q}_{1,}}{{q}_{2}}$are the charge of the respective ions present in that medium.

Complete Step by Step Answer:
From the formula, it is found that the coulomb's force of attraction between two oppositely charged ions is inversely proportional to the dielectric constant of the solvent. Therefore solubility of ionic compounds depends on the dielectric constant of the solvent. The dielectric constant value of water (${{H}_{2}}O$) is greater than heavy water or deuterium water (${{D}_{2}}O$).

Solvent	Simple $H_2O$	$D_2O$
Dielectric constant $/{{C}^{2}}/N.{{m}^{2}}$	$78.39$	$78.06$

As the force of attraction between the ions decreases, ions freely move in that particular solvent, hence making the compound more soluble. The dielectric constant has a special capacity to weaken the force of attraction between charges of ions so that solubility increases. Here we see from the table the value of the dielectric constant of simple water is more which means the solubility of an ionic compound in simple water is greater and solubility in heavy water is smaller.
Hence, option (B) is correct.

Note: Heavy water is the oxide of heavy hydrogen known as deuterium oxide (${{D}_{2}}O$). It is used in neutron moderators, as a tracer compound. Heavy water can also be used for exchanging labile hydrogen with deuterium completely or partially. Heavy water reacts slower than simple water as it forms strong $O-D$ bonds than $O-H$ bonds.