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How come we see bubbles on the bottom of a pan of water, long before the water actually boils? asks a reader.

If you've ever filled a pot with water, then set it on the stove to heat, you've probably noticed them: tiny bubbles, quickly collecting on the bottom of the pan. Oh good, you may think -- the water's begun to boil.

But don't dump that pasta in just yet. When the first bubbles form, the water may still be lukewarm. In fact, these teeny bubbles actually have nothing to do with the bubbles of boiling. Those bubbles are full of hot, vaporized water. The first bottom-dwelling bubbles, however, are full of plain old air.

Here's how it works: Both air and water vapor bubbles form at hot spots on the pan bottom. Bubbles form more easily in a pan whose surface is a little scratchy, rather than glass-smooth. That's because a rough surface provides more "nucleation sites" where bubbles can get started and grow.

Air bubbles form first, long before heating water reaches its boiling point. Tap water contains dissolved air. As the temperature at the bottom of the pan rises, hidden air is forced out of solution, forming visible bubbles.

According to Jearl Walker, a physicist at Cleveland State University, the air bubbles expand like tiny balloons, eventually pulling away from pan crevices. And because air is lighter than water, the bubbles rise to the surface, like the air exhaled by a scuba diver.

At the abandoned nucleation sites, new air bubbles form in the escaped bubbles' places. As the water continues to heat, its dissolved air supply dwindles to nearly nothing.

So what about real boiling? Water at sea level on Earth boils at 212 F. Boiling begins near the source of heat. When the pan bottom becomes hot enough, H2O molecules begin to break their bonds to their fellow molecules, turning from sloshy liquid to wispy gas. The result: hot pockets of water vapor, the long-awaited, boiling-up bubbles.

The first true vapor bubbles rise quickly through the heating water, only to collapse when they encounter cooler water above. But the switch from vapor back to liquid is only temporary. As the pan of water heats to the very top, vapor bubbles are able to rise higher and higher. Less dense than the liquid water around them, the bubbles climb like helium-filled balloons, collecting more water molecules as they float up.

Finally, at the surface, bubbles make their great escape, releasing vapor into the air with a tiny, audible pop. As the pot releases steam, some of the vapor will condense back to liquid, visible as a cozy fog on your wintry kitchen windows.

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