Formation mechanisms of bubbles under two boiling conditions
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As we all know, water boils at 100 degrees Celsius at standard pressure, accompanied by bubbles rising from the bottom up. Meanwhile, another method – reducing the pressure on water at room temperature, that is, creating a vacuum to a certain level – can also cause the water to boil and produce bubbles. I really would like to hear from other sailors what they think – is the mechanism behind bubble formation the same in these two situations? I would be very grateful if you could provide detailed information.
Actually, it’s the same. They are all equilibria saturated under certain temperature and saturation vapor pressure conditions; the disruption of equilibrium occurs when either of these conditions is altered. The first method involves adding energy to the water, which increases its energy level; this energy then naturally moves towards the atmosphere. The areas at the bottom receive more of this energy and thus become part of the atmosphere first. The second method involves reducing the energy of the atmosphere, and in this case vaporization may start from the top. This is just my personal opinion, as I have never seen the latter method in action. I hope everyone will actively participate in the discussion.
This post was last edited and replied to by phlpf2009 on 2010-12-22 at 12:00. Reply 1# Yanyang’s Steel: I think the two situations are different. During heating, at the point where the surface of the heating device comes into contact with water, heat is transferred to the water. The water at this location superheats and vaporizes to form bubbles, which then rise upward. When the temperature of the water in the upper layers is still low, the bubbles do not rise to the surface due to heat exchange. Once the water temperature reaches the boiling point, the bubbles can rise to the surface, resulting in boiling. Bubbles always form on the heated surface. One can observe the process of water boiling. Under reduced pressure, and before boiling, water evaporates at the surface of the liquid. When the pressure is lower than the saturated vapor pressure of water, the water is in a superheated state, and bubbles form on the container walls and on the surfaces of impurities in the water, resulting in boiling. As long as the pressure is kept below the saturated vapor pressure of water, bubbles always form on the container walls and on the surfaces of impurities in the water, resulting in boiling. Therefore, in vacuum reduction operations, controlling the vacuum level can sometimes provide guidance for specific processes such as concentration and crystallization.
I largely agree with the view above; in experiments, the bubbles formed by heating generally originate from the bottom of the beaker, while during suction filtration, the bubbles appear at or within the liquid surface.
I agree with the view from the 3rd floor: if boiling occurs due to an increase in heating temperature, then the overall boiling pattern will change from nucleate boiling to film boiling, that is, a gas film forms at the vessel walls, which ultimately slows down the heat transfer rate. On the other hand, if boiling is caused by reduced pressure, or in other words, by a lower boiling point, there are usually not many bubbles at the vessel walls; most bubbles appear on the surface. Of course, if a heat source is used, a gas film will eventually form at the bottom vessel walls as well
The mechanism is the same; the conditions for boiling and bubble formation are superheated liquid and a vaporization nucleus, and both heating and pressure reduction are ways to superheat the liquid.