When a mixture of hydrocarbons is heated, which component boils first?

• A. The lightest
• B. The heaviest
• C. The middle fraction
• D. The most reactive component

Answer: (A)

When you heat a mixture of hydrocarbons, the component with the lowest boiling point vaporizes first. This is because boiling point tends to rise with molecular size in hydrocarbons: shorter chains have weaker intermolecular (van der Waals) forces, so they require less energy to break free into vapor. The lightest hydrocarbon thus reaches its boiling point first and turns to vapor, while heavier ones stay liquid until you raise the temperature further. The middle fraction would vaporize at a higher temperature than the lightest but lower than the heaviest, and the most reactive aspect isn’t the deciding factor for boiling in this context.