Purity of Benzene by Crystallisation?

📌 CONCEPT: Crystallisation is a method used to purify a substance by dissolving it in a solvent and then allowing it to crystallise out of the solution, resulting in a purified solid.

📐 RULE / FORMULA: According to the principle of crystallisation, the purity of the crystallised substance depends on the rate of cooling and the presence of a 'seed crystal'. If the rate of cooling is too slow or too fast, or if the seed crystal is not pure, the quality of crystallisation will be poor.

💡 WORKED EXAMPLE: To improve the purity of benzene by crystallisation, first, a pure seed crystal of benzene is obtained by slow cooling of a pure benzene solution. Then, the impure benzene solution is cooled slowly in the presence of the pure seed crystal, allowing the pure benzene to crystallise out of the solution. Finally, the crystallised benzene is filtered and recrystallised to achieve higher purity.

⚠️ COMMON MISTAKE: Students often get confused about the importance of the rate of cooling and the purity of the seed crystal in achieving high-quality crystallisation. They may assume that simply dissolving the impure substance in a solvent and then allowing it to crystallise will result in a purified solid, without considering the factors that affect the crystallisation process.