The boiling point of H₂O (water) is 100 °C (212 °F) at standard atmospheric pressure. That’s the textbook answer, and it holds true at sea level where air pressure sits at one atmosphere. But the actual temperature at which water boils shifts depending on where you are, what’s dissolved in it, and the pressure surrounding it.
Why Water Boils at 100 °C
Boiling happens when the pressure of water vapor escaping from the liquid matches the pressure of the air pushing down on its surface. At sea level, that balance point lands at exactly 100 °C. Heat water beyond that and it can’t stay liquid; molecules escape into the air as steam faster than the atmosphere can hold them back.
This is different from evaporation, which happens slowly at any temperature from a puddle or a glass left on the counter. Boiling is the rapid, vigorous version: bubbles form throughout the liquid, not just at the surface, because the vapor pressure inside those bubbles is strong enough to push against the surrounding air.
How Altitude Changes the Boiling Point
Air pressure drops as you gain elevation, and lower pressure means water molecules don’t need as much energy to escape into vapor. The result: water boils at a lower temperature the higher you go. For roughly every 500 feet above sea level, the boiling point drops by about 0.9 °F (0.5 °C).
Here’s how that plays out at common elevations:
- Sea level (0 ft): 212 °F / 100 °C
- 5,000 ft (Denver, CO): 203 °F / 95 °C
- 10,000 ft (Leadville, CO): 193.6 °F / 89.8 °C
This is why cooking instructions change at high altitude. Pasta, rice, and beans take longer because the water surrounding them is cooler than it would be at sea level, even at a full rolling boil. Pressure cookers solve this by trapping steam, raising the internal pressure, and pushing the boiling point back up above 100 °C.
How Dissolved Substances Raise It
Adding salt or sugar to water raises the boiling point slightly. Dissolved particles make it harder for water molecules at the surface to escape into vapor, so the liquid needs more heat before boiling begins. A pot of heavily salted pasta water boils at roughly 100.5 to 101 °C depending on how much salt you’ve added. Ocean water, with its average salt concentration of about 3.5%, behaves similarly. The effect is real but small enough that you won’t notice a meaningful difference in cooking time.
Heavy Water Boils Slightly Higher
Most water molecules contain ordinary hydrogen, but a small fraction of naturally occurring water uses deuterium, a heavier form of hydrogen. Pure deuterium oxide (D₂O), often called heavy water, boils at 101.42 °C under normal pressure. That 1.4-degree difference comes from the extra mass in each molecule: heavier molecules need more energy to break free from the liquid. You’ll never encounter enough heavy water in your tap to matter, but it’s a useful illustration of how molecular weight shifts the boiling point.
Superheating: When Water Gets Hotter Than 100 °C Without Boiling
Under certain conditions, water can exceed its boiling point and remain liquid. This is called superheating, and it happens most often in a microwave. Stovetop heating creates convection currents and contact with rough pot surfaces, both of which give bubbles natural starting points. A microwave heats water more evenly and a smooth ceramic mug offers fewer of those nucleation sites, so the water can sit quietly at 101 or 102 °C with no visible bubbling.
The danger is that any disturbance, like dropping in a spoon or a tea bag, can trigger a sudden, explosive burst of steam. To avoid this, place a wooden chopstick or stir stick in the cup before microwaving. The rough surface gives bubbles a place to form continuously, preventing the liquid from quietly overheating.
The Upper Limit: Water’s Critical Point
If you keep increasing pressure and temperature, water eventually reaches a point where the distinction between liquid and gas disappears entirely. This happens at 374 °C and about 218 atmospheres of pressure. Above this threshold, water exists as a supercritical fluid, something that behaves like both a liquid and a gas simultaneously. You won’t encounter this in everyday life, but it’s the physical ceiling for water’s boiling point: no matter how much pressure you apply beyond this point, liquid water as we know it simply doesn’t exist.