How come when you put sugar into boiling water, it fizzes? asks Edward Drosse, of Smithtown, NY.
Ever add sugar to a cup of microwaved tea, only to have the tea (startlingly) boil over? Boiling depends on bubbles, and sugar can make hot water more bubbly.
Boiling is evaporation, but fast and furious. We can actually see the water leaving, as a cloud of steam. It can take days for a room-temperature glass of water to evaporate, but a small pan of water can boil away in a matter of minutes. That’s because when water reaches its boiling temperature (212 F. at sea level), it evaporates not just from the surface, but from deep within.
Adding sugar (or other ingredients, like salt) can make extremely hot water boil, or cause already-boiling water to boil faster. How come? Boiling begins with bubbles. The first bubbles to appear on the walls of a heating pan of water are actually air that was dissolved in the water, re-emerging as a gas. But as the bottom of the pan gets hot, liquid water itself begins to turn to gas.
Water begins forming vapor bubbles at hot spots here and there on the pan bottom. Steam bubbles form most easily on a rough, uneven surface. Tiny crevices are ideal “nucleation sites,” places where bubbles can get a foothold and grow.
When steam bubbles inflate, break free, and begin to rise, their journey is short-lived. As a bubble floats up into cooler water in the middle of the pan, it collapses like a deflated balloon. Why? At lower temperatures, a bubble’s pressure drops, allowing the kitchen air — the local part of the Earth’s atmosphere — to crush it.
But when the water’s temperature reaches the boiling point throughout, the pressure in the vapor bubbles increases to that of the air. Bubbles from down under can then rise to the surface. Where, with tiny pops, they release their vapor into the air.
When you drop sugar (or salt, or powdered sweetener) into boiling water, the crystals provide a raft of new nucleation spots. Presto–a crowd of new vapor bubbles forms, creating a short-lived fizzy effect. But add sugar to microwaved water, and the effect can be much more dramatic.
When you heat a cup of water or tea in the microwave, its temperature can rise several degrees above the boiling point — while the liquid remains still and bubble-free. Why? When water isn’t heated from the bottom up, there are fewer hot spots. Meanwhile, in a smooth glass or ceramic cup, there are few nucleation spots. The result: Water can “superheat,” without boiling.
Remove your superheated cup of tea and add sugar — or drop a teabag into superheated water — and the results can be explosive. Runaway nucleation can cause the tea or water to suddenly boil furiously. Scalding liquid may pour over the sides of the cup, or even spray into the air. The lesson: You can’t tell how hot the water is just by looking. When in doubt, let microwaved liquids sit a while before moving.