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?
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).