Bubbles in water

I didn’t pay attention to this before: lol

Purity is only relative; there should be bubbles on the walls of the heating medium

Pure water contains few impurities, resulting in fewer bubbles.

What role do impurities play in water?

Small bubbles need to grow larger before they can be seen; therefore, water containing impurities appears to have more bubbles

First, let’s look at the source of the bubbles. There are two in total; the first is gases dissolved in water. The second is the water vapor produced by the vaporization of water. First, on the heated wall surface (such as the bottom of a pot), there are many vaporization nuclei (what are vaporization nuclei? . . . I haven’t looked up any literature yet, and I don’t have a specific definition for it either; it’s probably something like very small pits on the surface, etc. On these vaporization cores, water first vaporizes to form tiny bubbles (as for why vaporization occurs first in these areas and how it happens there, there are likely many theories and models at present, with no definitive answer yet). At first, these small bubbles remain attached to the vaporization cores, and the water surrounding the bubble boundaries continues to vaporize and enter the bubble, causing it to grow larger. When it expands, there are roughly two outcomes: one is rupture (the reason was covered in the two-phase flow course; I’ve forgotten it T T), and the other is no rupture. Unbroken bubbles, due to their relatively large size, experience buoyant force that is greater than the attractive force exerted by the bubble’s surface on it (something like surface tension). As a result, the bubble is carried away by the buoyant force (along with other forces, but mainly buoyancy). There are probably also two possible outcomes during the ascent of the bubbles. First, it failed halfway through. Probably because the water temperature above is low, the steam inside the bubbles has liquefied. The second step is for it to rise to the surface smoothly, forming the bubbles that we see bubbling up. Thus, the source of these bubbles during boiling is water vapor formed by the vaporization of water near the wall surface.