Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent at a specified temperature. Solubility of a substance depends upon the nature of solute and solvent as well as temperature and pressure. A saturated solution is a solution that is in equilibrium with an excess of solid at a given temperature. A solution that contains a high concentration of solute than does a saturated solution is known as supersaturated solution. If a small crystal of solute is added to a supersaturated solution, the solid will at once crystallize out and an equilibrium will be established. This is known as seeding. More the dielectric constant of a solvent, more is the solubility of polar covalent or ionic compounds in it. Some covalent compounds dissolve in water due to hydrolysis while some other covalent solids dissolve in water due to hydrogen bonding. Ionic compounds dissolve in polar solvents while covalent compounds dissolve in non-polar solvents and this is known as like dissolves like.
The solubilities of ionic compounds often but not always increase with temperature, because energy is required to pull the molecules or ions away from the crystal lattice (lattice energy) \(\Delta H_L\).
Energy is also liberated due to solvation of the solute particles. If the solvent is water, the energy liberated due to solvation is known as hydration energy \(\Delta H_h\).
The energy change when one mole of ionic compound dissolves in water is \(\Delta H_{sol}\).
\[\Delta H_{sol} = \Delta H_h - \Delta H_L\]
Since the hydration energies of the ions and the lattice energy are exothermic changes, they have a negative sign. If \(\Delta H_h\) is more than \(\Delta H_L\), then \(\Delta H_{sol}\) will be negative and the ionic crystal dissolves exothermically.
Which statement best explains the meaning of the phrase “like dissolves like”?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A solvent and a solute with similar intermolecular forces will readily form a solution
The passage states that ionic compounds dissolve in polar solvents while covalent compounds dissolve in non-polar solvents, and that this is known as “like dissolves like”.
A solute dissolves readily when the intermolecular interactions in the solute and in the solvent are similar, because the new solute–solvent attractions can then replace the ones that are broken. So polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents: sodium chloride and sugar dissolve in water, while naphthalene and anthracene dissolve in benzene but not in water.
Why the other options are incorrect
Option (1) A solvent will easily dissolve a solute of similar molar mass — Incorrect. The rule is about the kind of attractive forces, not the size of the molecules. Water (molar mass 18 g mol−1) dissolves sugar (342 g mol−1) readily.
Option (3) True solutions are formed only when water dissolves a non-polar solute — Incorrect. Water is polar, so it does not dissolve non-polar solutes such as naphthalene; and water is not the only solvent that forms true solutions.
Option (4) True solutions are formed only when water dissolves a polar solute — Incorrect. Water does dissolve polar solutes, but the word “only” is wrong: non-polar solutes form true solutions in non-polar solvents, for example naphthalene in benzene.