The boiling point of water is indeed 100 degrees Celsius (at sea level and standard atmospheric pressure). However, the process of boiling is more complex than simply reaching the boiling point temperature.
When a pot or kettle of water is heated, the heat energy is transferred to the water molecules, causing them to gain energy and move more vigorously. As the temperature increases, the water molecules start to move faster and faster.
At the boiling point, the water molecules have gained enough energy to overcome the attractive forces holding them together in the liquid phase. This allows them to escape from the liquid surface and enter the gas phase, forming water vapor or steam.
However, for the entire pot or kettle of water to evaporate, it requires continuous heat supply to maintain the boiling process. The heat energy needs to be transferred throughout the water mass, and this transfer takes time. The heat from the heat source must continuously replenish the energy lost through the evaporation process.
Furthermore, the pot or kettle acts as a barrier, enclosing the water and preventing the rapid escape of water vapor. The steam that forms at the boiling point is contained within the pot, and it creates a layer of vapor above the liquid surface. This vapor layer insulates the remaining water from direct contact with the heat source, reducing the rate at which the water gains heat and reaches its boiling point.
In summary, the boiling point is the temperature at which water molecules have enough energy to escape from the liquid phase and enter the gas phase. However, the entire pot or kettle of water doesn't instantly evaporate because the heat transfer process takes time, and the container itself helps retain the water and slows down the rate of evaporation.