Final answer:
When a liquid boils, the molecules gain enough thermal energy to overcome intermolecular forces and transition into the gas phase, spreading out as vapor. This happens when the liquid's vapor pressure is equal to the external pressure. The process involves energy consumption called the enthalpy of vaporization.
Step-by-step explanation:
When a liquid boils, there is sufficient thermal energy in the system such that the liquid molecules at the surface of the solution can go into the gas phase. These gaseous molecules spread out and move freely, becoming part of the surrounding air as vapor. Boiling occurs when the liquid's vapor pressure equals the atmospheric pressure, allowing bubbles of vapor to form throughout the liquid, not just on the surface. The molecules within the boiling liquid gain energy from the thermal input, and when this energy is enough to overcome the intermolecular forces holding the molecules together, they escape into the air as gas.
The transition from liquid to gas involves an energy consumption known as the enthalpy of vaporization. In a closed system, the gaseous molecules could potentially condense back into the liquid if they lose energy. However, in an open system, such as boiling water on a stove, the gaseous molecules simply escape into the atmosphere.