The question of the temperature at which water turns into a gas is common, often leading to the misconception that $100^{\circ}\text{C}$ ($212^{\circ}\text{F}$) is required. In reality, water evaporates at virtually any temperature above its freezing point of $0^{\circ}\text{C}$ ($32^{\circ}\text{F}$). This process allows puddles to dry on a cool day and wet clothes to air-dry indoors. The speed of this phase change changes with temperature, not the possibility of it occurring.
The Molecular Mechanics of Evaporation
Evaporation is possible at low temperatures because individual water molecules are not all moving at the same speed. Temperature reflects the average energy of all molecules, but their specific energies are distributed across a wide spectrum. This means that even in cool water, a small fraction of molecules possess higher kinetic energy than the average.
These high-energy molecules are mostly located at the liquid’s surface. To escape and become a gas, a molecule must overcome the cohesive forces of attraction, primarily hydrogen bonds, that hold it to its neighbors. Only those with enough kinetic energy can successfully transition into water vapor.
As the liquid’s temperature increases, the energy distribution shifts, meaning a larger percentage of molecules reach the energy threshold required to escape. This explains why water evaporates much faster on a hot day than a cold one.
Evaporation Versus Boiling
The distinction between evaporation and boiling comes down to where the phase change occurs and its relationship to atmospheric pressure. Evaporation is a quiet, slow process that occurs only at the exposed surface of the liquid. It happens when the vapor pressure is less than the atmospheric pressure pushing down on the surface.
Boiling, by contrast, is a rapid, turbulent phase transition that takes place throughout the entire liquid body. This occurs at the boiling point, when the liquid’s vapor pressure equals the surrounding atmospheric pressure. Once this balance is achieved, water molecules form vapor bubbles within the bulk of the liquid and rise to the surface.
For water at standard sea-level pressure, the boiling point is $100^{\circ}\text{C}$ ($212^{\circ}\text{F}$). If atmospheric pressure is lowered, such as at high altitude, the boiling point decreases because less external pressure must be overcome.
Factors Influencing the Rate of Evaporation
Since water evaporates at almost any temperature, the speed of the process is governed by several external variables:
- Surface Area: Because evaporation is a surface phenomenon, a larger exposed area allows more high-energy molecules to escape simultaneously, accelerating the process. This is why spreading wet clothes out helps them dry faster.
- Air Movement: Wind continuously removes the layer of air immediately above the liquid. This layer becomes saturated with water vapor, and wind replaces this humid air with drier air, promoting faster evaporation.
- Humidity: The amount of water vapor already present in the surrounding air. Highly humid air holds fewer additional water molecules, which reduces the rate of evaporation.
- Atmospheric Pressure: Lower atmospheric pressure can slightly increase the evaporation rate, as there is less downward force resisting the escape of water molecules from the liquid surface.
The Cooling Effect of Evaporation
The act of evaporation results in a cooling effect on the remaining liquid and its surroundings. When the highest-energy water molecules break free and escape as vapor, they take their thermal energy with them. This energy is referred to as the latent heat of vaporization.
The removal of these high-energy molecules lowers the average kinetic energy of the molecules left behind in the liquid, causing the temperature of the remaining liquid to drop. This explains why sweating helps regulate body temperature and why a person feels cool after getting out of a pool.
When sweat evaporates from the skin, it draws the heat required for the phase change directly from the body’s surface. A similar effect occurs when rubbing alcohol is applied to the skin; its rapid evaporation pulls heat away quickly, creating an immediate sensation of cold.