Boiling moves heat extremely effectively

Phase change, and why it carries so much energy per degree.

Boiling moves heat extremely effectively because turning a liquid into vapour absorbs a very large amount of energy without the temperature changing at all during the process, letting a boiling surface soak up far more heat, far faster, than the same surface would manage simply by warming a liquid that stays liquid.

Energy spent pulling molecules apart

Heating a liquid that stays liquid raises its temperature steadily, and how much heat that takes depends on the liquid's ordinary heat capacity. Boiling it into vapour instead means pulling its molecules apart against the forces holding them together as a liquid, and that separation costs a large amount of extra energy, delivered without the temperature rising by so much as a degree, since all of it goes into breaking the molecules free and none into speeding them up.

For water the gap is striking. Heating a kilogram from a room temperature of 20 °C to 100 °C takes about 335 kilojoules. Turning that same kilogram, already at 100 °C, into steam takes about 2,260 kilojoules more, nearly seven times as much, with nothing on a thermometer to show for it. That is why a kettle that has reached a rolling boil still takes far longer again to boil dry, and why a pan at a rolling boil on the hob stays at the same temperature however hard the flame is turned up, with the extra heat going into making steam faster.

It is also why boiling is such an effective way to pull heat away from a hot surface. Every bit of liquid that turns to vapour right at the surface carries away a large parcel of energy, then lifts away and is replaced by fresh, cooler liquid. A liquid that stays liquid can only carry heat off as warmer liquid, limited by how hot it is allowed to get, while a boiling one holds the surface at a stable temperature even as very large amounts of heat pour in.

Ice in a drink runs the same trick

A cold drink with ice floating in it stays cold far longer than the same drink at the same starting temperature without ice. As the ice melts it absorbs heat from the drink, and that energy goes into breaking the ice's solid structure apart, so the drink stays cold for as long as unmelted ice remains and only starts warming properly once the last of it has gone. Melting a kilogram of ice takes about the same energy as heating a kilogram of water through 80 degrees, all of it absorbed at 0 °C. Once the last cube has gone, only the drink's ordinary heat capacity stands between it and the warm room, and with no melting left to soak up the incoming heat, the drink's temperature starts climbing steadily toward the room's.

Boiling uses the same kind of hidden absorption one step further along, a liquid pulling in heat to become vapour where the ice was a solid pulling in heat to become liquid, and in both cases the phase change does far more of the work than the thermometer suggests. A thermometer measures how fast molecules are moving, and during a phase change the energy is going somewhere a thermometer cannot see.

When the vapour gives it back

The latent energy comes back in full when the process runs in reverse. Vapour condensing into liquid releases exactly as much heat as it absorbed while boiling, without its own temperature needing to drop first. This is why a scald from steam does more damage than boiling water at the same temperature: the steam condenses directly on the skin and hands over all that latent heat as it does. It is also why heat pipes and refrigeration loops, which deliberately cycle a fluid between boiling in one place and condensing in another, can move very large amounts of energy through small, light components, and why boiling and evaporative cooling appear wherever compactness and cooling power both matter.

A pan that cannot scorch while it is wet

On a domestic scale, the same physics makes boiling water a forgiving way to cook, holding food at a steady temperature that will not climb no matter how vigorously the water bubbles, unlike the far less consistent surface of a hot, dry pan. A pan is also hard to scorch while liquid is still boiling in it, since the boiling pins its wetted surface to that bounded temperature. Once the last of the liquid has gone, the pan's temperature runs free toward whatever the flame can drive it to, often fast enough to scorch or warp within a minute of running dry.