A saturated liquid is a thermodynamic state where a substance exists entirely as a liquid but is poised at the exact threshold of vaporization. This means the liquid is at its boiling temperature, known as the saturation temperature, for the existing pressure. It has absorbed the maximum possible amount of thermal energy as a liquid without transitioning into the gaseous phase. This state defines a boundary in the study of phase changes.
Defining the Saturated Liquid State
A liquid is considered saturated when it contains the highest amount of internal energy it can hold before the liquid-to-vapor phase change begins. At this point, the liquid is in thermal equilibrium with its vapor. The liquid is on the verge of boiling, often defined as a “quality” of zero on thermodynamic property charts, indicating no vapor content.
Any addition of heat to this saturated liquid will cause some of the liquid to flash into vapor, initiating the boiling process. Conversely, if energy is removed, the liquid will drop below the saturation temperature and become a subcooled liquid. The energy required to convert a saturated liquid completely into a saturated vapor, without changing its temperature, is termed the latent heat of vaporization.
The Interplay of Saturation Temperature and Pressure
The state of a saturated liquid is not defined by a single fixed temperature but rather by a dependent relationship between temperature and pressure. For a pure substance, the saturation temperature is the boiling point corresponding to a specific pressure, known as the saturation pressure. This dependency means that if one of these properties is fixed, the other is automatically determined.
This direct link contrasts with non-saturated states where temperature and pressure can be varied independently. A common example is water, which boils at 100°C at standard atmospheric pressure (101.3 kPa). If the pressure is lowered, such as at high altitudes, the saturation temperature decreases, causing water to boil at a lower temperature, perhaps 90°C or less. Conversely, increasing the pressure in a sealed container forces the boiling point to rise above 100°C.
Saturated Liquid vs. Subcooled Liquid
The saturated liquid state is distinguished from a subcooled liquid, also called a compressed liquid, by its energy content and proximity to boiling. A subcooled liquid exists at a temperature lower than the saturation temperature for its given pressure. For example, water at 20°C at standard atmospheric pressure is subcooled because its saturation temperature is 100°C.
Subcooled liquids are thermodynamically stable and not on the brink of phase change, requiring a significant amount of heat to reach the saturation point. Due to their lower energy and temperature, subcooled liquids typically have a slightly higher density than saturated liquids. A subcooled liquid can absorb a considerable amount of heat without boiling, whereas a saturated liquid converts immediately to vapor upon heat addition.
Practical Importance of the Saturation Point
Understanding the saturated liquid state is important for the design and operation of many industrial and commercial systems. Engineers rely on the properties of saturated liquids when working with thermodynamic cycles, particularly in power generation and refrigeration. For instance, in a steam power plant, water must be heated to the saturated liquid state before it can be vaporized to drive turbines.
The concept is also central to refrigeration and air conditioning systems, which use refrigerants that cycle through saturated states to absorb and release heat. The principle is applied in everyday devices like pressure cookers, which increase the pressure to raise the saturation temperature of water, allowing food to cook faster at temperatures above 100°C.