B. Solubility, Temperature, and Crystallization Solubilities as a function of temperature 71 9.81 ‘ . 31 n 100 Temperature, Molarities M NaNO2, 45 M KCl; 45 M NaCl;
The Correct Answer and Explanation is:
Correct Answer:
As temperature increases, the solubility of most salts increases, but not uniformly for all compounds. Here’s how solubility behaves for the three given salts:
- NaNO₂ (Sodium nitrite): Solubility increases significantly with temperature.
- KCl (Potassium chloride): Solubility increases moderately with temperature.
- NaCl (Sodium chloride): Solubility increases slightly with temperature.
Explanation
Solubility is defined as the maximum amount of a solute that can dissolve in a solvent (usually water) at a given temperature to form a saturated solution. Temperature plays a key role in solubility, especially for solid solutes in liquid solvents. As temperature increases, most solid salts become more soluble due to the endothermic nature of their dissolution processes—more heat allows more solute particles to escape into the solution.
Let’s examine the three salts in question:
- NaNO₂ (Sodium nitrite): This salt has a strong positive temperature-solubility correlation. It dissolves endothermically, meaning heat absorption helps break its ionic lattice and integrate ions into the solvent. Thus, increasing temperature significantly enhances NaNO₂’s solubility.
- KCl (Potassium chloride): Its solubility also increases with temperature, but the effect is less dramatic than with NaNO₂. Potassium chloride’s dissolution is slightly endothermic, so increased thermal energy helps, but not as significantly.
- NaCl (Sodium chloride): NaCl is an exception among many salts. Its solubility is nearly temperature-independent, showing only a slight increase with rising temperature. This is because the dissolution of NaCl is nearly thermoneutral—it doesn’t absorb or release much heat—so temperature has minimal effect.
Crystallization:
When a hot saturated solution of one of these salts cools down, the excess solute precipitates out as crystals. This is used in recrystallization techniques for purification. Salts like NaNO₂ and KCl are excellent for such processes because they show a significant solubility difference across temperatures. NaCl, however, is less suitable because its solubility barely changes with temperature.
Understanding these properties is essential in industrial applications, pharmaceuticals, and laboratory recrystallization techniques.
