The boiling point of a substance is generally affected by its molar mass, and as the molar mass increases, the boiling point tends to increase as well. This relationship can largely be explained through the intermolecular forces present in a substance.
When we consider molecular compounds, larger molecules usually have stronger van der Waals forces (or London dispersion forces) due to their increased size and mass. These forces arise from the temporary dipoles created when electrons move around the nuclei of atoms. The more massive the molecule, the greater the number of electrons and the stronger the temporary dipoles that can form, leading to stronger intermolecular attractions.
As a result, substances with higher molar masses often require more energy to break these intermolecular forces when transitioning from a liquid to a gas, which means they will have higher boiling points. For example, consider the series of hydrocarbons: as you go from methane (CH₄) to decane (C₁₀H₂₂), the molar mass and, consequently, the boiling point increases due to stronger intermolecular forces.
However, it’s essential to remember that while a trend exists, other factors such as the type of intermolecular forces (like hydrogen bonding, dipole-dipole interactions, and ionic bonds) and molecular structure also play crucial roles in determining boiling points. For instance, two substances might have similar molar masses but very different boiling points due to the presence of hydrogen bonds in one and weaker van der Waals forces in the other.