Crystallization may be induced by scratching the vessel beneath the surface of t

Question

Crystallization may be induced by scratching the vessel beneath the surface of the solution with a glass rod. How does scratching the inside of the vessel with a glass rod induce crystallization? What properties are desirable in a recrystallization solvent? Give two reasons why vacuum filtration is sometimes preferable to ordinary gravity filtration. Why is water a good solvent for the recrystallization of acetanilide? (Refer to a standard chemistry handbook for the solubility of acetanilide in hot and cold water.) Why must the funnel be heated before the hot acetanilide solution is filtered? Assuming that charcoal and sugar are the main impurities in a sample of crude acetanilide, explain how recrystallization from water would remove each. Explain why, during the recrystallization of p-dibromobenane, the turbidity of the hot solution was used an indicator for the ratio of ethanol to water used. Why can we use a water-ethanol mixture but not a water-hexane mixture but not a water-hexane mixture as a recrystallizing solvent?

Explanation / Answer

3. Scratching the beaker with a glass rod liberates tiny microscopic pieces of glass into the solution. These tiny glass pieces act as nucle and induce nucleation. This gives large cystalline precipitates which cn be easily filtered.

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6. The solubility of acetailide in hot water is 5.5g per 100ml and in cold water is 0.53g per 100ml. Therefore, the solubility of acetanilide decreases significantly when water is cooled and it precipitates. Thus it is a good recrystallization solvent for acetanilide.

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