Question

3 multiple choice 1 point what is the main reason for adding water dropwise to the boiling methanol solution? to neutralize the impurities. to increase the polarity of the solvent mixture gradually, promoting controlled crystallization. to lower the boiling point of methanol. to prevent methanol evaporation.

Explanation:

• Option "To neutralize the impurities": Water adding here isn't for neutralizing impurities (neutralization usually involves acid - base reactions, not typical in this solvent - crystallization context).
• Option "To increase the polarity of the solvent mixture gradually, promoting controlled crystallization": Methanol is less polar than water. Adding water dropwise to boiling methanol increases the solvent's polarity slowly. This gradual change helps in controlled crystallization (preventing rapid, uncontrolled crystal growth with impurities).
• Option "To lower the boiling point of methanol": Adding water (a higher - boiling - point solvent) to methanol would increase the boiling point of the mixture, not lower methanol's boiling point.
• Option "To prevent methanol evaporation": Adding water doesn't prevent methanol evaporation; in fact, the mixture is boiling, so evaporation is ongoing, and water addition isn't for this purpose.

Answer:

B. To increase the polarity of the solvent mixture gradually, promoting controlled crystallization.