Final answer:
Evaporating ½ of the ethanol in a solution before collecting a second crop is done to concentrate the solute, increasing the chance of crystallization during the purification process. The volatility of ethanol facilitates its removal and helps in obtaining more crystals without affecting the solute. In the field of biofuels, ethanol is known for its higher volatility and environmental effects when used in gasoline mixtures.
Step-by-step explanation:
The reason you might evaporate ½ ethanol before collecting a second crop from a solution during a purification process (such as recrystallization) is to concentrate the solute, increasing its supersaturation in the solvent, and thus encouraging crystallization. Ethanol is often used as a solvent because of its ability to dissolve a variety of organic compounds.
During evaporation, the volume of ethanol decreases which increases the concentration of the solute. If the solution has been cooled and a first crop of crystals has already been harvested, evaporating part of the ethanol can lead to the formation of more solute crystals in the second crop.
Also, because of the volatile nature of ethanol, especially in the presence of water, it can be easily removed by gentle heating without decomposing or affecting the solute. This characteristic is particularly beneficial in separation processes where temperature-sensitive compounds are involved.
In the context of biofuels, ethanol is a significant biofuel and is added to gasoline to improve combustion efficiency and reduce emissions. However, ethanol's higher volatility leads to increased evaporative emissions, which can contribute to environmental issues such as ground-level ozone and smog.