Assessment and Qualifications Alliance (AQA) GCSE Biology Paper 2 Practice Exam

Question: 1 / 400

How does the boiling point of hydrocarbons change with increasing molecular size?

It decreases

It remains constant

It increases

The boiling point of hydrocarbons increases with increasing molecular size due to the greater strength of intermolecular forces, specifically London dispersion forces, as the size of the hydrocarbons increases. Larger hydrocarbons have more electrons and a larger surface area, which enhances the ability to induce temporary dipoles in neighboring molecules. This leads to stronger attractive forces between the molecules, requiring more energy in the form of heat for the molecules to overcome these forces and transition from the liquid phase to the gas phase.

As a result, larger hydrocarbons have higher boiling points compared to their smaller counterparts. For example, as you move from methane (the smallest alkane) to octane (a larger alkane), the boiling points steadily increase, demonstrating the direct relationship between molecular size and boiling point in hydrocarbons.

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It fluctuates unpredictably

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