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On a molecular level, why does condensation form on the outside of a cold glass of water whereas bubbles form on the inside of a hot glass?
Answer 1:

Let's think about what kind of molecular processes are happening in each situation. In the case of the cold glass, the air surrounding the glass is warm relative to the glass. Now, air typically has some water molecules (in the form of vapor/gas) that comprise it, but warm air holds more water than cold air. So when we set a cold glass on a table, the relatively warm air that existed there begins to cool as it comes into contact with the glass. As the air cools, it can't hold as much water, so water beads form on the exterior of the glass. Thus, we are looking at a vapor --> liquid phase transition.

However, in the case of the water that's being heated, the water is undergoing a liquid --> vapor phase transition. As heat energy is transferred to the liquid water molecules, they acquire enough energy to break the hydrogen bonds that keep water molecules together in liquid form. If we happen to be heating the water in a pot on a stove, for instance, you'll notice bubbles tend to form more at the bottom and then rise to the top, since the liquid water molecules at the bottom are the first to get heated and converted into little pockets of gas phase water molecules. They rise to the top because gas water is less dense than liquid water.

Answer 2:

Condensation forms when water transforms from a gas to a liquid. Similarly, bubbles form when water transforms from a liquid to a gas. If there's a cold glass of water sitting on a table, gaseous water molecules in the air touch the cold surface of the glass and condense to form a liquid, which makes the glass look "foggy," and we call "condensation." In the case of a hot bubbly glass of water (water that is almost boiling), the liquid on the inside of the glass is turning into water gas (or vapor), and forming bubbles that are less dense than water liquid. These bubbles rise and then escape. Naturally, water vapor on the outside of the glass is already a gas, so touching the outside of the glass won't change anything (it just heats the gas).

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