Question

Identify the correct order of solubility of $N{{a}_{2}}S, CuS,$ and $ZnS$ in aqueous medium:
 (A) $CuS$ > $ZnS$ > $N{{a}_{2}}S$
 (B) $ZnS$ > $N{{a}_{2}}S$ > $CuS$
 (C) $N{{a}_{2}}S$ > $CuS$ > $Zns$
 (D) $N{{a}_{2}}S$ > $ZnS$ > $CuS$

Answer 1

Hint: ‘Like dissolves like’ is a thumb rule for solubility. Water being a polar solvent, will dissolve the compound more which has a higher polarity. The solubility of solute in a solvent is determined by the balance of intermolecular forces between the solvent and solute, and the entropy change that occurs.

Complete step by step answer:
- Intermolecular forces and interionic forces in both the solute and solvent are some of the factors that affect solubility. The solute must be able to break the intermolecular forces of the molecules of the solvent to make itself soluble in it.
- For dissolving a solute in water, a hydrophilic substance will be more soluble. So for checking the order of solubility, we need to check which compound has a higher ionic character. The more the ionic character, the more will be the polarity.
- Ionic compounds are more soluble in water than covalent compounds. Increasing covalent character leads to less solubility. Ionic character is directly proportional to the size of the cation.
- The order of the size of the cation in $N{{a}_{2}}S$, CuS, and ZnS is:
     \[N{{a}^{+}}>Z{{n}^{+2}}>C{{u}^{+2}}\]
- The percentage of ionic character is maximum in Na₂S, hence it undergoes hydration to a maximum extent.

Therefore the correct order of solubility is $N{{a}_{2}}S$ $> ZnS > CuS$, i.e option D.

Note: The solubility is favored by the entropy of mixing ($\Delta S$). The solubility depends on the enthalpy of dissolution ($\Delta H$) and the hydrophobic effect. Factors such as temperature and pressure also affect the solubility of a given solute in a solvent. Depending on the change in Gibbs free energy ($\Delta G$), the solubility of a given compound may increase or decrease with temperature. For most solids and liquids, the solubility increases with an increase in temperature. In water, at high temperature, the solubility of ionic solutes tends to decrease. The pressure dependence of solubility is usually neglected as it occasionally has practical significance.