Analysis:
From the microscopic point of view, the boiling process can be divided into two stages:
The water in the beaker is heated before boiling. Because air is dissolved in water, small bubbles appear on the bottom and wall of the cup. These small bubbles are formed by separating the air adsorbed on the inner wall and bottom of the cup. Small bubbles are surrounded by water, which will evaporate into the bubbles. Small bubbles are small in size, and the gas in them will soon reach saturation, so there is not only air but also saturated water vapor in the bubbles. When the water temperature continues to rise, the saturated vapor pressure in the small bubble will gradually increase, so its volume will also increase. Under the action of buoyancy, bubbles will rise off the bottom and wall of the cup. At the same time, a small amount of air left at the bottom will gradually form new bubbles and rise. When the bubble rises to the relatively cold upper water, because the saturated pressure of water vapor in the bubble is lower than the external pressure, the bubble gradually becomes smaller, so that the saturated steam in the bubble gradually condenses into liquid, and finally only air and a small amount of water vapor escape from the liquid surface.
In the boiling process, with the increase of temperature, the saturated pressure of water vapor in bubbles becomes larger and larger. When the temperature of water rises to a certain extent and the saturated pressure of water vapor in the bubble increases to be equal to the external pressure, the whole water layer is at the same temperature, so the phenomenon of water vapor condensation and volume reduction will not occur during the bubble rising. Because of the constant heat absorption, the water around the bubble quickly evaporates into the bubble, and the volume of the bubble increases continuously during the rising process. Finally, when the bubbles rise to the water surface, they burst and release a lot of steam. At this time, the water in the cup churned up and down, forming a boiling phenomenon.