Here's why we take the boiling point as when bubbling just ceases:
* Equilibrium: When bubbling ceases, it means the rate of vaporization is equal to the rate of condensation. This indicates an equilibrium state has been reached.
* Constant Temperature: At this equilibrium, the temperature remains constant even though heat is still being added. This constant temperature is the boiling point.
* Pressure Dependence: The boiling point is not a fixed value, but rather dependent on the surrounding pressure. As pressure increases, the boiling point also increases. Therefore, we specify the boiling point at a particular pressure, typically at standard atmospheric pressure (1 atm).
Why not when bubbling starts?
* Nucleation: Boiling starts when vapor bubbles form within the liquid. This requires energy to overcome the surface tension of the liquid.
* Superheating: The liquid can sometimes be heated above its boiling point before bubbles form. This is called superheating. It's a metastable state, and the presence of nucleation sites (like impurities) or agitation can trigger boiling.
* Not Constant Temperature: During the initial stages of boiling, the temperature of the liquid is not constant as it is still increasing.
In summary, the boiling point is defined as the temperature at which the liquid is in equilibrium with its vapor. This is best observed when bubbling ceases, indicating the rate of vaporization equals the rate of condensation and the temperature remains constant.
