Short Answer 
The boiling point of alkanes increases with the addition of carbon atoms due to heightened van der Waals forces, which strengthen as molecular size grows. Larger alkanes have increased surface area and more electrons, leading to stronger intermolecular interactions that require more energy to overcome, resulting in higher boiling points.
Step 1: Understanding the Assertion
The assertion (A) states that the boiling point of alkanes increases with the addition of more carbon atoms. This is a valid point since larger alkanes tend to have higher boiling points. As the size of the alkane molecule increases, the number of carbon and hydrogen atoms increases, leading to higher molecular weights.
Step 2: Exploring the Reason
The reason (R) is also accurate as it discusses van der Waals forces, the primary intermolecular forces at play. These forces become stronger with larger molecular sizes due to a larger surface area and the presence of more electrons. Consequently, it makes sense that as the alkane’s molar mass increases, the strength of the van der Waals forces correspondsingly grows, influencing the boiling points significantly.
Step 3: Relating Forces to Boiling Points
The combination of the assertion and reason shows that size matters in alkanes. As their size increases:
- The surface area enhances, leading to stronger interactions.
- More electrons enable significant instantaneous dipole-induced dipole interactions.
- These interactions require more energy to break, thus elevating boiling points.