Boiling Water

Technically, what the candy thermometer measures is the syrup’s boiling point, which is another unusual characteristic of water. Normally, the boiling points of lightweight molecules are low. Due to low intermolecular forces, it doesn't take much kinetic energy for a molecule to escape into the air. For instance, the boiling point of H₂S (a heavier molecule than H₂O) is -60°C. Water’s boiling point is unusually high because of the hydrogen bond. It takes 4.18 Joules of energy to raise the temperature of one gram of water by one degree. This number is called the specific heat capacity of a substance. In comparison, the specific heat of copper is 0.385 J/g·K, air: 1 J/g·K, meat: 1.7 J/g·K, olive oil: 2 J/g·K. Therein lies the answer to one of life’s mysteries: why does water take so long to boil? The highest power consumption of a countertop appliance in the United States is 1500 W. To boil 1 kilogram of water beginning at 20°C ambient temperature, and assuming a generous 80% heating efficiency, it will take 4.18 x 1000 x (100–20) / (1500 x 0.8) = 278 seconds or almost 5 minutes with a countertop water kettle.

When there are foreign substances in water, they elevate the water's boiling point. The extra molecules get in the way of the escaping water molecules, so the water molecules need to have higher energy to break free. The macro manifestation of higher water molecule energy is higher temperature. However, during normal cooking, the salt concentration is not high enough to meaningfully change the boiling point of the liquid.

So how does water boil? On the stovetop, a pot of water is heated from the bottom. As the water at the bottom heats up, it rises and is replaced by the colder water that sinks to the bottom. These movements are called convection currents. The French have a word frémir, meaning to quiver or to tremble, that refers to this pre-bubble stage when the surface of the liquid is visibly moving.

The first bubbles show up around the bottom and the sides of the pot. These are not steam. They are air dissolved in water. Unlike sugar, air’s solubility decreases as temperature increases (that’s why you should keep your opened champagne in the fridge). As the temperature rises further, some hot water near the bottom turns into steam and starts to float up. But most of them can’t reach the surface because the cold water they encounter along the way condenses them back to liquid water. When you see a lot of bubbles rise to the top and pop, the water is boiling at 100°C. Measuring boiling water is a great way to calibrate your thermometer.