Why does Water Boil and Can Water Get Hotter Than the Boiling Point?

Sorry, but once the water has begun to boil, it’s as hot as it will ever get, even if you use a flamethrower. No matter how furiously you might get the water to boil, it won’t get any hotter than the boiling point of water: 212 degrees Fahrenheit (100 degrees Celsius), plus or minus.

Let’s take a close look at what’s happening inside the water as you heat it to boiling. When you first start heating the water, its temperature rises. That is, the water molecules take on the heat energy and show it by moving faster and faster.

Eventually, some of the molecules will have so much energy that they can actually break away from their buddies, to whom they had been attached by rather strong attractions. The energetic molecules may even elbow their buddies aside to make spaces in the liquid, bubbles, which then rise and erupt at the surface as blurps of water vapor a gas. We can’t see this gaseous water until it gets away from the surface, cools a bit, and condenses into a cloud of tiny liquid droplets that we call steam.

We refer to this whole complex process as “boiling.” The bottom line is that the water is absorbing the heat that you’re putting in and using it to change from a liquid to a gas.

Turning liquid water into gaseous water uses up heat energy, because it takes energy to break the molecules away from each other. If the molecules didn’t stick together, water wouldn’t be a liquid; it would always be a gas: loose molecules, flying around independently. Every liquid has its own degree of molecule-to-molecule stickiness, and therefore its own breakaway energy, and therefore its own boiling temperature. It requires a temperature of 212 degrees Fahrenheit (100 degrees Celsius) to break water molecules away from each other.

Now let’s turn up the heat. The more heat energy per second we pump in from the fire, the more water molecules per second will be acquiring enough energy to break away and shoot off as gas. The water will boil more vigorously and it will boil away faster. But all that extra heat doesn’t make the temperature of the water go up, because any extra energy that a molecule may acquire, beyond what it needs to break away, simply goes flying off along with the molecule.

As soon as any molecule gets more than the necessary breakaway energy, it will depart at a higher-than-usual speed. The extra energy, and the higher temperature that goes along with it, winds up in the steam, not in the liquid that remains behind in the pot. Its temperature will remain the same, at the “boiling point” of water, until the water has all boiled away.

Moral: You can’t cook your spaghetti any faster by turning up the heat. Save your energy.

You can check this for yourself by holding a candy or meat thermometer (with tongs) in the steam just above a pot of boiling water and in the water itself, both when it’s boiling gently and when it’s boiling vigorously. The water temperature will stay the same, but the steam will be a little hotter when the boiling is more vigorous.

No matter how hot a fire you put under a pot of boiling water, the water won’t get any hotter.