The change from liquid to gas is called vaporization, a physical process where molecules gain enough energy to break free from each other and spread apart. It happens in two ways: evaporation, which occurs gradually at the surface of a liquid at any temperature, and boiling, which happens throughout the entire liquid once it reaches a specific temperature. Water turning to steam in a pot is the most familiar example, but every liquid can make this transition.
What Happens to Molecules During the Change
In a liquid, molecules are loosely bonded to each other by attractive forces. They slide past one another but stay relatively close together. Temperature is really a measure of how fast these molecules are moving. As you add heat energy, the molecules move faster and faster, and some gain enough speed to overcome the pull of their neighbors.
Once a molecule’s motion energy exceeds the attractive forces holding it in the liquid, it escapes into the air as a gas particle. In a gas, molecules are spread far apart and move freely, which is why gases expand to fill any container. The key difference between a liquid and a gas comes down to this balance: in a liquid, the attractive forces win and keep molecules clustered. In a gas, motion energy wins and molecules fly independently.
Evaporation vs. Boiling
Evaporation and boiling are both liquid-to-gas transitions, but they work differently. Evaporation happens at the surface of a liquid, at temperatures below the boiling point. Even in a glass of room-temperature water, some molecules near the surface are moving fast enough to escape into the air. That’s why a puddle dries on a warm sidewalk without ever reaching 100°C.
Boiling, on the other hand, happens throughout the entire body of the liquid. It starts when the liquid reaches a temperature where the internal pressure of forming vapor matches the air pressure pushing down on it. At that point, bubbles of gas form inside the liquid, rise to the surface, and burst. For water at standard atmospheric pressure (sea level), this happens at 100°C (212°F).
A simple way to remember: evaporation is a surface event that happens slowly at any temperature, while boiling is a bulk event that happens rapidly at one specific temperature.
Why Boiling Point Changes With Altitude
The boiling point of a liquid isn’t fixed. It depends on the air pressure above it. At sea level, atmospheric pressure pushes down on the water’s surface, and the water must reach 100°C before its internal vapor pressure can match that force. At higher elevations, there’s less air overhead, so the pressure is lower. Water doesn’t need to get as hot to start boiling.
This is why water boils at roughly 95°C in Denver (about 1,600 meters elevation) and closer to 70°C on top of Mount Everest. Cooking takes longer at altitude because the water is boiling at a lower temperature, meaning less heat is being transferred to your food. The reverse is also true: in a pressure cooker, the sealed environment raises the pressure above normal atmospheric levels, pushing the boiling point higher and cooking food faster.
The Energy Cost of Vaporization
Turning a liquid into a gas requires a surprisingly large amount of energy, called the heat of vaporization. For water, this is about 2,260 joules per gram at 100°C. To put that in perspective, heating one gram of water from 0°C to 100°C takes about 418 joules. Converting that same gram of already-boiling water into steam takes more than five times as much energy. All of that extra energy goes into pulling molecules apart from each other rather than raising the temperature.
This is why a pot of water seems to sit at a rolling boil for a long time before it all evaporates. The temperature stays at 100°C the entire time because every bit of incoming heat is being used to break molecular bonds, not to make the water hotter. Scientists call this “latent heat” because the energy is hidden inside the phase change rather than showing up as a temperature increase.
How Your Body Uses This Process
Evaporative cooling is the reason you sweat. When liquid sweat sits on your skin and transitions to gas, it pulls heat energy away from your body. Each gram of sweat that evaporates removes roughly 2,426 joules of heat from your skin. This is the primary way your body regulates temperature during exercise or hot weather.
The catch is that sweat only cools you when it actually evaporates. In humid conditions, the air is already saturated with water vapor, so sweat evaporates much more slowly. Some of it simply drips off your body without ever making the liquid-to-gas transition, providing no cooling benefit at all. This is why 30°C with high humidity feels far more oppressive than 30°C in dry air. Your body’s main cooling mechanism is effectively throttled.
The Critical Point: Where Liquid and Gas Merge
There’s a temperature and pressure at which the distinction between liquid and gas disappears entirely. For water, this critical point occurs at about 374°C and 220 times normal atmospheric pressure. Above these conditions, water exists as a “supercritical fluid” that has properties of both a liquid and a gas simultaneously. It can dissolve materials like a liquid but expand and flow like a gas. While this is far outside everyday experience, it illustrates that the liquid-to-gas boundary isn’t as rigid as it seems. It’s a spectrum governed by temperature and pressure, and at extreme enough conditions, the two phases blend into one.