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Whenever we boil water there, there comes bubble from the bottom surface of the kettle when the water reaches 100 degree Celsius. Why is that? We also see some bubble sticking to the surface even before the water reaches the boiling temperature. Why is that also. But we do not see the same phenomenon in time of heating oil. Why? Is is because oil is more dense? Or something else?

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    $\begingroup$ It happens in oil too if the oil actually boils. $\endgroup$ – A. C. A. C. Jul 24 '17 at 18:17
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The bubbles you see come from water vapor collecting at nucleation sites and rising to the surface . When water boils, its vapor pressure equals atmospheric pressure, which is why water boils at lower temperature at higher elevations where atmospheric pressure is lower. The bubbles originate from the bottom because that is where the heat source is and, therefore, where the temperature is greatest. It is also where the most nucleation sites in the form of microscopic particles are likely to exist.

You will see the same thing happen to oil. However, you have to get oils a lot hotter for them to boil. For instance, canola oil boils at 355°C but smokes at only 154 and ignites at 431 (source, pg 24).

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  • $\begingroup$ Great explanation. Thank you so much. But we see some more bubble sticking to the surface of the kettle. What're those for? $\endgroup$ – Saiful Islam Jul 26 '17 at 11:05
  • $\begingroup$ I'm not sure what you mean by "surface of the kettle." My first inclination was that you were referring to the sides. Are you referring to the surface of the water itself? $\endgroup$ – BillDOe Jul 26 '17 at 23:05
  • $\begingroup$ Yes, I am referring to the side of the kettle. We can see bubble there even before the water starts boiling. Why is that? $\endgroup$ – Saiful Islam Jul 28 '17 at 0:15
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    $\begingroup$ Just like the bottom, the sides are going to have nucleation sides, as well, and that's what the water vapor will form bubbles around. I'm curious what metal your kettle is made from. I'm guessing copper. $\endgroup$ – BillDOe Jul 28 '17 at 19:29

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