Final answer:
At the molecular level, boiling occurs when molecules in a liquid gain enough thermal energy to overcome intermolecular forces and form vapor bubbles, transitioning to the gas phase. This takes place at the substance's specific boiling point, where liquid and gas can coexist.
Step-by-step explanation:
When a substance boils, several changes happen at the molecular level. Initially, as energy is introduced into the system, for instance through heating, the molecules of the substance begin to move with increased velocity. This process continues until the molecules have sufficient thermal energy to overcome the intermolecular attractive forces that have been holding them together in the liquid state.
As the substance reaches its boiling point, which is a fixed temperature specific to each substance and dependent on pressure, the molecules have enough energy to form bubbles of vapor within the liquid. This creation of vapor bubbles signifies the transition from the liquid phase to the gas phase. It is important to note that at the boiling point, the liquid and gas phases can coexist. The substance remains at its boiling point temperature until the phase change is complete, during which it absorbs energy without increasing in temperature, a concept known as latent heat.