When thermal energy (heat) is added to a liquid, the substance’s internal energy increases. The liquid’s molecules absorb this energy, causing them to move and vibrate more vigorously. Continuously adding energy ultimately results in a change in the physical state, or phase, of the substance. This transition from a liquid to a gas is driven by the absorbed thermal energy and represents a fundamental rearrangement of molecular structure.
The Initial Response: Increased Molecular Kinetic Energy
The first effect of adding thermal energy is a measurable rise in the liquid’s temperature. This occurs because the energy input is converted into sensible heat, which directly increases the average kinetic energy of the individual molecules. As the molecules absorb this energy, they move faster and further apart, overcoming some of the intermolecular forces that hold them in the liquid state. The temperature will climb steadily until the liquid reaches its boiling point.
This increased molecular motion also causes the liquid to expand slightly. Furthermore, the liquid’s vapor pressure, which is the pressure exerted by molecules escaping into the gas phase, increases significantly during this heating stage. This process weakens the cohesive forces between the molecules, preparing the substance for the eventual phase change.
Reaching the Critical Threshold: The Boiling Point
A liquid reaches its boiling point when its internal vapor pressure equals the external atmospheric pressure surrounding it. This equality is the condition required for the liquid-to-gas phase transition to begin within the bulk of the liquid. At this threshold, the molecules possess enough energy to push against the weight of the air, allowing bubbles of vapor to form and rise.
The boiling point is dependent on the ambient pressure, meaning it is not a fixed property. For example, water boils at 100°C (212°F) at standard sea-level pressure. However, at higher altitudes, where atmospheric pressure is lower, the boiling point decreases. A liquid can also be made to boil at a lower temperature by placing it under a partial vacuum.
The Phase Change: Latent Heat and Vaporization
Once the liquid reaches its boiling point, added thermal energy no longer causes a temperature increase. Instead, the energy is entirely dedicated to changing the substance’s phase. This absorbed energy is known as the Latent Heat of Vaporization, which is required to completely break the remaining intermolecular bonds holding the liquid together. The temperature remains constant throughout the entire process of vaporization, even as heat is continuously supplied.
This energy input transforms the liquid into a gas, resulting in a dramatic increase in volume as the molecules move far apart into the vapor state. The act of boiling is the vigorous formation of vapor bubbles throughout the liquid. For water, the heat of vaporization is particularly high, requiring a substantial amount of energy to convert liquid water into steam at the same temperature.
The Final State: Superheated Vapor
After all the liquid has been converted into saturated vapor, any further addition of thermal energy causes the substance to enter the superheated state. Superheated vapor is defined as a gas that exists at a temperature higher than its boiling point for the pressure it is under. This stage marks a return to sensible heat absorption, meaning the added energy again translates into a rise in the substance’s temperature.
The gas molecules in the superheated vapor absorb the energy, increasing their average kinetic energy and causing them to move much faster. Superheated vapor is a dry gas that cannot coexist with liquid. It is a highly energetic and efficient working fluid often used in power generation turbines. The temperature of this vapor will continue to rise as long as heat is supplied.