Boiling a pot of water always seems to take forever, and there's a real reason. Water is extremely thirsty for heat — it takes a lot of energy to warm it up even a little. If you put the same flame under a chunk of metal that weighs the same as your water, the metal would get scorching hot in the time the water barely warms. That's because warming one gram of water by a single degree needs about ten times more energy than warming a gram of iron the same amount. So the same flame races the metal up and crawls the water along. The flip side is that water cools slowly too, which is why the sea keeps seaside towns mild. Put equal flames under water and metal in the simulator and watch the two temperatures split apart.
Most people think a slow kettle is the stove's fault. In fact the culprit is water itself: warming a gram of water by one degree needs about ten times the energy iron does, because much of the heat goes into flexing and breaking hydrogen bonds rather than speeding the molecules up.
What's actually happening
It's tempting to blame a slow kettle on the stove, but the real culprit is the water itself. Every substance has a 'specific heat', a measure of how much energy you must pour in to warm it by one degree, and water's is remarkably large. It takes about 4.2 joules to lift a single gram of water by one degree. The same gram of iron needs only about 0.45 joules, and copper even less. So when you light identical flames under equal weights of water and metal, you are feeding them the same energy per second, but the water demands roughly ten times as much of it for each degree of rise. The metal sprints; the water plods.
The reason water is so greedy lies in how its molecules are stuck together. Water molecules cling to each other with hydrogen bonds — weak but numerous links between them. When you add heat, not all of it goes into making the molecules move faster (which is what a thermometer reads as temperature). A big share goes into stretching and partly breaking those bonds, an invisible bill that has to be paid before the temperature can climb. Metals have no such network to feed, so almost all the energy you give them turns straight into faster, hotter motion. The mathematics is tidy: with the same heat in and the same mass, the temperature rise goes as one divided by the specific heat, so water rises about ten times less than iron.
That sluggishness sounds like a nuisance, but it quietly runs the planet. Because water both warms slowly and cools slowly, the oceans act as an enormous heat store, soaking up warmth in summer and releasing it in winter — which is why a coastal town swings between mild seasons while an inland city of the same latitude roasts and freezes. The same property makes water the coolant of choice in car engines and power stations: it can carry away a great deal of heat without its own temperature shooting up. And it's why a hot-water bottle stays warm in your bed for hours, while a metal one of the same size would go cold almost at once. Water's reluctance to change temperature is one of its most useful habits.
Water's huge thirst for heat makes it slow to warm and slow to cool, which is why the sea keeps coasts mild and why engines use it as coolant.
- 1Heat equal amounts of water and cooking oil in two identical pans on identical rings, and time how long each takes to get hand-hot.
- 2The oil warms noticeably faster, because its specific heat is roughly half that of water — the same flame lifts its temperature about twice as quickly.
- 3Take both off the heat and time them cooling, too: the water clings to its warmth far longer, the same thirst for heat now working in reverse.
Common questions
Water molecules cling together with hydrogen bonds. Adding heat partly stretches and breaks those bonds rather than only making the molecules move faster, an invisible bill that must be paid before the temperature can climb.
Yes, the same thirst for heat works in reverse. That is why a hot-water bottle stays warm in bed for hours while the same mass of metal would go cold almost at once.
Because water warms and cools so slowly, the ocean stores summer heat and releases it through winter. A seaside town stays gentler than an inland city at the same latitude that bakes and freezes.