When water is heated, its molecules begin to move faster and gain kinetic energy. As the temperature rises, some molecules at the bottom of the pot gain enough energy to overcome the attractive forces holding them in the liquid state. These high-energy molecules turn into water vapor-a gas-and form bubbles. These bubbles are essentially pockets of water vapor trapped within the liquid.

At first, when water is only warm and not yet boiling, small bubbles you see are mostly air that was dissolved in the water. As the water gets hotter, this air escapes and the real boiling process begins. Once the temperature reaches the boiling point (100°C at sea level), the water vapor bubbles grow larger and rise to the surface. At this point, the vapor pressure inside the bubbles equals the atmospheric pressure outside, allowing them to burst at the surface and release steam.

The rising and bursting of bubbles is what we observe as "boiling." The movement helps distribute heat more evenly and is an indicator that the water has reached a temperature suitable for cooking, sterilizing, or brewing beverages.

The size and behavior of the bubbles can change depending on the heat source, altitude, and impurities in the water. For example, salt or sugar can alter bubble formation slightly by changing the boiling point.

So, the bubbles formed during boiling are a result of water molecules gaining enough energy to escape into the gas phase. It’s a visual and physical indicator that the water has reached its boiling point and is undergoing a phase change from liquid to gas.