When you place a kettle on the stove, the heating element underneath the kettle transfers heat energy to the bottom of the kettle. This heat is conducted through the metal or other material of the kettle, reaching the liquid inside. The liquid in direct contact with the hot bottom of the kettle absorbs the heat and begins to warm up rapidly.
The process of boiling occurs when a liquid reaches its boiling point, which is the temperature at which its vapor pressure equals the atmospheric pressure. As the liquid inside the kettle absorbs heat, it heats up and reaches its boiling point faster than the rest of the contents because the direct contact with the hot bottom of the kettle provides a higher heat transfer rate compared to the liquid at the top.
As the liquid at the bottom reaches its boiling point, bubbles of water vapor start forming at the bottom of the kettle. These bubbles rise to the surface, and as they reach the cooler liquid at the top, they condense back into liquid water. This process is known as nucleation. The cycle of bubble formation, rising, and condensation continues until the entire volume of liquid inside the kettle reaches its boiling point.
The rest of the contents in the kettle heat up more slowly because they rely on heat conduction from the already boiling liquid at the bottom. The heat transfer from the boiling liquid to the rest of the contents occurs through convection, where the hotter liquid rises and displaces the cooler liquid, creating a circulation pattern. This convection process helps distribute the heat throughout the liquid inside the kettle, allowing it to reach the boiling point gradually.
In summary, the liquid inside a kettle boils at the bottom first because it directly receives heat from the hot kettle's bottom, reaching its boiling point faster than the rest of the contents, which heat up more slowly through conduction and convection processes.