Vaporization is the process by which a substance changes its physical state from a liquid to a gas, also known as vapor. This phase transition requires an input of energy. The energy is necessary for the liquid’s molecules to overcome the attractive forces holding them together. The two distinct examples of vaporization are evaporation and boiling, which differ primarily in their speed and the location within the liquid where the change takes place.
Evaporation: Slow Conversion to Gas
Evaporation is a gradual form of vaporization that occurs only at the exposed surface of the liquid, below the substance’s boiling point. The process is driven by the distribution of kinetic energy among the liquid’s molecules. Some individual molecules possess enough kinetic energy to break free from the liquid’s surface tension and escape as a gas.
Because the highest-energy molecules are the ones escaping, the average kinetic energy of the remaining liquid decreases, which results in a cooling effect known as evaporative cooling. A common example is the cooling sensation experienced when sweat dries from the skin, as the liquid water absorbs excess body heat to fuel its conversion into water vapor. The slow disappearance of a puddle after a rain shower also demonstrates this surface phenomenon. The process of clothes drying on a line relies on evaporation, where water leaves the fabric surface without the need for the entire mass of water to reach the boiling temperature.
Boiling: Rapid Conversion to Gas
Boiling is the rapid form of vaporization, characterized by the phase change occurring throughout the entire volume of the liquid, not just at the surface. This process is initiated only when the liquid reaches a specific temperature known as the boiling point. At this temperature, the liquid’s vapor pressure equals the external atmospheric pressure. When the vapor pressure equals the atmospheric pressure, the gaseous phase can form internally, leading to the rapid formation of bubbles that rise and release vapor from beneath the surface.
An example is water heating on a stove, where vigorous bubbling marks the moment the liquid reaches 100 degrees Celsius (at standard sea-level pressure) and begins bulk vaporization. Generating steam for industrial power also uses boiling, where water is rapidly converted to high-pressure vapor in enclosed systems. Unlike evaporation, the temperature of a pure liquid remains constant throughout the boiling process, even with continued heating, as all added energy is consumed in converting the liquid to gas.
Condensation: The Reverse Process
Vaporization is part of a continuous cycle of phase changes; its reverse process is called condensation. Condensation is the transition from a gas or vapor state back into a liquid. Unlike vaporization, condensation releases energy, often referred to as the latent heat of condensation.
This energy release occurs because the gaseous molecules must lose kinetic energy for the attractive forces to pull them back together into a liquid. Examples include the formation of dew drops on grass overnight as water vapor cools and turns back into liquid. The fogging of a bathroom mirror after a hot shower occurs when warm, moisture-rich air contacts the cooler surface, causing the water vapor to condense. In the atmosphere, condensation forms clouds when water vapor cools and collects around microscopic particles.