You are heating water in a pot on the stove. Shortly before the water boils properly, bubbles of steam begin rising from the bottom of the pot but vanish before they reach the water’s surface. What happened to those bubbles?

they popped

The phenomenon you're observing in this scenario is called "Leidenfrost effect." It occurs when a liquid, in this case water, comes into contact with a surface significantly hotter than its boiling point.

Here's what happens:

1. Initially, the bottom of the pot heats up, raising the temperature of the water above its boiling point.
2. As the water temperature exceeds the boiling point, some water molecules at the bottom start vaporizing, forming bubbles of steam.
3. However, due to the difference in temperature between the bottom of the pot and the surface of the water, the steam bubbles created are rapidly surrounded by a layer of vapor.
4. This vapor layer acts as an insulator, preventing direct contact between the hot surface and the water, and significantly hindering heat transfer.
5. The trapped steam bubbles then levitate on this cushion of vapor and move freely in random patterns around the pot.
6. It's important to note that these steam bubbles are not stable or long-lasting due to the presence of the vapor layer. They quickly collapse or vanish before reaching the water's surface.
7. Once the water in the pot reaches a temperature where the Leidenfrost effect is no longer sustained, the vapor layer collapses, and the bubbles can rise to the surface as visible steam.

In summary, the steam bubbles that vanish before reaching the water's surface are being pushed away from the bottom of the pot by a layer of vapor, creating a temporary floating effect known as the Leidenfrost effect.