Prevent hydrolysis and ensure clear crystal formation.
Explanation
Crystallization is a method used to purify solids. When preparing crystals of salts derived from weak bases and strong acids, it is essential to maintain the chemical stability of the salt in an aqueous solution to ensure high purity and clarity.Detailed Analysis:
- Hydrolysis of Copper Sulphate: Copper sulphate ($CuSO_4$) is a salt formed from a strong acid (sulphuric acid) and a weak base (copper hydroxide). In water, the copper ions ($Cu^{2+}$) tend to react with water molecules (hydrolysis).
- Formation of Turbidity: The hydrolysis reaction produces trace amounts of copper hydroxide ($Cu(OH)_2$), which is insoluble and creates cloudiness or turbidity in the solution.
Reaction: $Cu^{2+} + 2H_2O \rightleftharpoons Cu(OH)_2 + 2H^+$ - Role of Dilute Sulphuric Acid: Adding dilute sulphuric acid introduces excess hydrogen ions ($H^+$) into the solution. According to the common ion effect and Le Chatelier's principle, the increased concentration of $H^+$ shifts the equilibrium backward, suppressing the hydrolysis. This prevents the formation of the cloudy precipitate and ensures the crystallization of pure, clear blue copper sulphate.
- Analysis of Incorrect Options:
- Increase the boiling point of the solvent.: While adding solutes elevates the boiling point, this is not the functional reason for adding acid in this specific process.
- Induce a chemical change in the copper salts.: Crystallization is a physical change aimed at purification; the goal is to preserve the chemical identity of the salt, not change it.
- Prevent the rapid evaporation of water.: The presence of a small amount of acid does not significantly impact the rate of evaporation.
Key Takeaway: Dilute acid is added during the crystallization of salts of weak bases to suppress hydrolysis, thereby preventing the precipitation of hydroxides and ensuring the formation of clear crystals.