The boiling point of a substance is primarily determined by the intermolecular forces acting between its molecules.

Intermolecular forces refer to the attractive interactions that occur between molecules. The strength of these forces is influenced by the types of bonds present between the atoms in the molecule. Molecules exhibiting strong intermolecular forces necessitate a greater amount of energy to separate them, leading to a higher boiling point. For instance, hydrogen bonding is a particularly strong type of intermolecular force that arises between molecules containing hydrogen and either oxygen, nitrogen, or fluorine. This results in elevated boiling points for substances capable of forming hydrogen bonds, such as water.

Conversely, molecules with weak intermolecular forces require less energy to break apart, which results in lower boiling points. An example of this is molecules that only exhibit London dispersion forces, recognized as the weakest form of intermolecular forces. Although these forces exist between all molecules, they play a significant role in non-polar molecules, such as hydrocarbons, which typically have low boiling points.

Furthermore, the size and shape of molecules can significantly influence their intermolecular forces and, consequently, their boiling points. Generally, larger molecules tend to possess stronger intermolecular forces and higher boiling points compared to smaller molecules. This trend is attributed to the greater number of electrons present in larger molecules, which can enhance London dispersion forces. Additionally, molecules with more complex geometries may exhibit stronger intermolecular interactions due to the increased surface area available for these interactions.

In summary, the boiling point of a substance is affected by its intermolecular forces, which are influenced by factors such as the types of bonds between atoms, the size and shape of the molecules, and the presence of functional groups capable of forming hydrogen bonds.