During fractional distillation, which property is primarily used to separate fractions?

• A. Boiling point
• B. Density
• C. Color
• D. Reactivity

Answer: (A)

The key idea here is that fractional distillation relies on differences in boiling points to separate liquids. When the mixture is heated, the component with the lower boiling point turns into vapour first. The fractionating column provides many opportunities for this vapour to condense and re-evaporate, so the lower-boiling components rise higher and are collected separately from the higher-boiling ones. Because the separation is based on how easily each liquid turns into vapour, boiling point is the property that matters most. Density, color, and reactivity don’t control the distillation process: density isn’t what drives vaporization, color is just an appearance and won’t affect phase change, and reactivity would involve chemical change rather than a physical separation.