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Boiling water bubbles

  1. Jul 23, 2013 #1
    Just curious,
    When my mom was boiling water i saw that the rate of bubbles coming out of water was increasing as time passed.But i want what is the rate of bubbles coming out is proportional to?
    And one thing i noticed is that when the gas is shut off the bubbles stop instantaneously.Why is it?
     
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  3. Jul 23, 2013 #2

    Drakkith

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    I believe it is roughly proportional to the temperature of the bottom of the pot. I'm sure the water temperature plays a part too.

    The bubbles stop nearly instantly once you turn off the flame because when the water boils and turns to a gas at the bottom, it removes heat from the metal. Once you turn the flame off there is nothing to replace this lost heat, and thus the water stops boiling.
     
  4. Jul 24, 2013 #3
    I think the rate will stabilize if you wait long enough. It's all about energy balance. The rate of bubbles corresponds to the amount of water turned into vapor per second. Turning water into vapor requires energy. The flow of energy coming in is proportional to the temperature difference between hot gases and the bottom of the pot. This energy flow goes into raising the temperature of the water and vaporization (some small part is also transferred to to the surrounding air though the walls and surface of the water). As the water temperature rises, the balance is shifted towards vaporization until the entire bulk of water is a 100°, when a sort of equilibrium is reached. When you switch the gas off, incoming energy flow ceases pretty much instantly.
     
  5. Jul 24, 2013 #4

    Andy Resnick

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    Do the experiment: measure/estimate the 'bubble creation rate' with the gas burner setting as a variable. As for your second question, think about where the energy is coming from to heat the water, keeping in mind that as the water boils away the temperature is constant.
     
  6. Jul 24, 2013 #5

    cjl

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    I don't think that would be the case. Whether it is at a gentle boil or a vigorous boil, the temperature at the bottom of the pot should be extremely close to the boiling point of the water (100C, assuming standard conditions). I would guess instead that the rate of bubble formation is related to the heat flux into the pot.
     
  7. Jul 24, 2013 #6

    Drakkith

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    Considering that the heat flux into the pot is steady, I assume it's either that the pot gradually heats up, or the bubbles form faster and faster because the temperature of the water is increasing. Or both.
     
  8. Jul 24, 2013 #7
    The rate of bubble formation will depend on the temperature of the pot. More specificity the difference between the temperature of the pot and water's saturation temperature. (The difference between the temperatures is strongly related to the heat flux). The rate of bubble formation also depends on the number of nucleation sites on the surface of the pot. Thus the smoothness and cleanliness of the pot can affect the rate of boiling. Finally the material the pot is made out can also affects the rate of bubble formation.

    Boiling only occurs when the temperature of the pot is above the saturation temperature of water. When you turn the heat from the oven off, boiling can only continue until the temperature of the pot reaches the saturation temperature (its actually a little above the saturation temp) However, it also turns out that boiling is a very effective means of cooling an object and most metals have low heat capacities. Thus boiling will quickly cool the pot to the saturation temperature, and then stop.

    Technically it is the temperature of the pot that determines rate of bubble formation. However in this set up and in many problems we control the heat flux. Which in turns determines the temperature. This is a subtle point, but has a significant impact of the behaviour of the system at and above the critical heat flux.

    As a final comment. "Nucleate boiling" is a classic problem in two-phase flow. The behaviour of the boiling changes dramatically as the temperature (or heat flux) is increased, and it is an active area of open research with many application beyond cooking.
     
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