You are heating water in a pot on the stove. Shortly before the water boils properly, bubbles of steam begin rising from the bottom of the pot but vanish before they reach the water’s surface. What happened to those bubbles?
They condense back into liquid water. When steam condenses to water that releases latent heat. So, these bubbles when they condense into water are transerring heat from the bottom to the water near the surface.
The bubbles that rise from the bottom of the pot and vanish before reaching the surface are called "nucleate boiling" or "boiling nuclei". This phenomenon occurs when the water is heated to a temperature just below its boiling point.
Here's how it works: When water is heated, the thermal energy causes the water molecules to move faster. As the temperature increases, some water molecules gain enough energy to break their intermolecular bonds and become vapor molecules, forming bubbles. However, these bubbles are immediately condensed back into liquid as they rise because the surrounding water is still cooler and can't sustain the vapor phase.
The reason these bubbles form at the bottom of the pot is due to imperfections on the surface or impurities in the water, also known as "nucleation sites." These nucleation sites provide a place for the vapor bubbles to form, but they collapse before reaching the water's surface because the heat from the stove is not evenly distributed throughout the pot.
As the heat continues to transfer from the bottom to the rest of the water, the temperature gradually increases, leading to a point where the entire volume of water reaches a temperature high enough to sustain continuous boiling or the formation of stable bubbles that reach the surface. This is when the water finally boils properly and you see a rolling boil with continuous bubbles rising to the surface.