Why does the water gurgle?

When water is heated, its boiling point has not yet been reached, small bubbles begin to form in it. They form at the bottom of the container, as they heat up under the bottom, and there the temperature is higher. Bubbles contain air. When the temperature rises, the dissolved air is released from the water. Water evaporates into bubbles. The bubbles are lighter than the surrounding water and therefore begin to rise to the upper layers. Here the temperature is even lower than at the bottom. Therefore, the steam condenses, the bubbles become smaller and heavier, and descend again. This happens until all the water warms up to the boiling point. At this time, a noise preceding the boil is heard.

When the boiling point is reached, the bubbles no longer go down, but float to the surface and burst. Steam escapes from them. At this time, it is no longer a noise that is heard, but the gurgling of liquid, which indicates that it has boiled.

During boiling, as well as during evaporation, a transition of liquid into vapor occurs. Evaporation occurs only on the surface of the liquid, boiling is accompanied by the formation of bubbles containing steam throughout the volume. Also, unlike evaporation, which occurs at any temperature, boiling is possible only at a certain temperature.