Assertion : Boiling point of alkanes increases with increase in molecular weight.Reason : van der Waals forces increase with increase in molecular weight

The assertion and reason provided are both true, and they are related to each other. Let's analyze them step-by-step.

1. Assertion Analysis

The assertion states that the boiling point of alkanes increases with an increase in molecular weight. This is generally true for alkanes because, as the molecular weight of the molecule increases, there are more atoms present in the molecule, leading to a greater surface area and more significant van der Waals forces.

2. Reason Analysis

The reason states that van der Waals forces increase with an increase in molecular weight. This is also true because van der Waals forces (which include dispersion forces) arise from temporary dipoles that occur when electrons are distributed asymmetrically in a molecule. Larger molecules (with higher molecular weight) have more electrons and a more extensive electron cloud, leading to stronger dispersion forces.

3. Relationship between Assertion and Reason

The reason explains why the assertion is valid. As molecular weight increases, the strength of van der Waals forces increases, which in turn elevates the boiling point of the alkanes. Thus, the increase in boiling point with molecular weight is a direct result of the increase in van der Waals forces.

4. Conclusion

In conclusion, both the assertion and the reason are true, and the reason effectively supports the assertion. Therefore, the statement hence is valid: the boiling point of alkanes does increase with increasing molecular weight due to the corresponding increase in van der Waals forces.