|MLA Citation:||Bloomfield, Louis A. "Question 1183"|
How Everything Works 19 Apr 2018. 19 Apr 2018 <http://howeverythingworks.org/print1.php?QNum=1183>.
Your suggestions for why the bubbles appear raise two interesting points. First, in a thermal system such as hot water, you can't identify some molecules as being boiling hot and others as being cooler—temperature is a property of the entire system and not of individual molecules. However, at a given instant, there are molecules with more energy than their neighbors and it is these energetic molecules that may break free of their neighbors to form a bubble nucleus.
Second, water often contains dissolved gases and these gases come out of solution when the water is heated. While many of the gas molecules leave through the water's surface, some of them may leave as bubbles from within the water. This gas bubble formation requires nucleation as well, which is why these bubbles often appear on the inner surfaces of a metal pot on the stove—flaws in the pot's surface assist bubble nucleation. But these gas bubbles aren't what you observed; there just isn't that much dissolve gas. You can prove that the bubbles you observe are steam: repeat the experiment several times with the same water. Each time you heat the water and add sugar, it bubbles wildly—something that wouldn't be possible if you were simply releasing dissolved gases from the water.