Q: Why — when water gets to its boiling point, does its temperature not change anymore??!  Is that just a characteristic, or is there a real explanation? (Eliza, Quito, Ecuador)

A: There is a real explanation; basically it’s because water cools as it boils. So, heating a pot of water to keep it boiling increases its temperature but the act of boiling decreases the water temperature. The temperature increase and decrease essentially balance, thus keeping the temperature more or less constant. That’s the big picture but there’s more to it.

A vigorously boiling pot of water. Note the wealth of bubbles. Courtesy of Wikipedia

When we step out of a shower to dry off, we get chilly as the water evaporates from our skin. Water cools as it evaporates. Water molecules zip around constantly, some faster than others. The faster (hotter) ones near the surface can escape the molecular bonds of their neighbors and move as water vapor into the air above. They evaporate. That leaves the slower (cooler) molecules still in the liquid water. The slower the molecule, the lower the temperature since temperature decreases as the kinetic (motion) energy of the molecules decrease. So, as the fast molecules escape, the temperature of remaining water drops. That’s why evaporation cools. See Figure 2, top drawing.

Figure 2. The top drawing shows the fast water molecules (above the water surface) that have evaporated and, in so doing, escaped. The bottom drawing shows water boiling — where evaporation (and bubbles) occur throughout the pot. Courtesy of Rod Nave, copyright, used with permission, HyperPhysics

Water boils when evaporation occurs throughout the pot — not just on the surface — and forms bubbles that are buoyed to the top where they escape. See Figure 2, bottom drawing. "Boiling means bubbles," writes meteorologist Craig Bohren in What light through yonder window breaks.

Here’s how it works. I fill a pot with tap water, and thereby trap a bunch of micro-bubbles along its sides and bottom. But I can’t see them yet; they’re too tiny. I put the pot on the stove and turn on the burner. As the water temperature rises, its air solubility drops. Soon small visible bubbles form along the sides and bottom of the pot as the air diffuses out of the water, and joins the micro-bubbles.

As the temperature continues to rise, the bubbles expand, and break free. They rise towards the top but disappear as they reach the cooler water near the top. The water isn’t boiling yet. "A rising bubble that is visible is mostly water vapor.  When the bubble rises into cooler water, the vapor pressure decreases and, hence, the bubble shrinks," says Bohren.

The temperature continues to rise. Now, much water evaporates into the bubble on its way up; it expands, and makes it to the top. There the hot bubble steam bursts out into the air, and the pot's water temperature drops. Thus, the water temperature stays at near the boiling point of water. If I turn up the burner heat, I just increase the rate of bubble formation, bursting at the surface and cooling the remaining water by evaporation. So, the temperature stays nearly constant, about 212 degrees Fahrenheit (100 C) at sea level — the boiling point.

By the way, 100 C is the nominal boiling point of pure water at sea level, but your chances of actually measuring this value are small unless you take great care, says Bohren.  A "very frustrated" former student of his kept measuring 102 C when helping her young daughter do a boiling experiment for school. 

One can superheat water above its normal boiling point by reducing the source of micro-bubbles, and therefore, suppressing boiling. Because, boiling means bubbles. In 1874, John Aitken may have been the first to experiment in this fashion. With a super-smooth container (to minimize bubble formation), he managed to heat the nearly pure water to 244 F (118 C) before the water boiled. Then, as he relates, it "exploded" — a danger he managed to survive.

Further Reading:

Evaporation and boiling by Rod Nave, HyperPhysics

What light through yonder window breaks? by Craig Bohren. New York: John Wiley & Sons, 1991.

Conceptual physics, ninth edition, by Paul G. Hewitt. New York: Pearson/Addison Wesley, 2002.

Superheating, Wikipedia

(Answered April 11, 2006)