Saturated Liquid Line: Need to Know Critical Facts

If we consider the phase diagram of a pure substance, the Saturated Liquid Line and Saturated Vapor Line divide the whole phase diagram into three regions.

A pure substance undergoes state changes as per the variation in temperature, pressure, and volume of the substance. The solid line which separates compressed or sub-cooled liquid from the saturated mixture is known as Saturated Liquid Line.

A simple saturated liquid line for water at constant pressure is shown in the Temperature-Volume diagram as below:

Image Credit: T-v diagram for the heating process of water at constant pressure

The saturated liquid line denotes the phase change of a substance from liquid to vapor. Below the saturated liquid line, the substance is in pure liquid form, and above it is in partial vapor form till enough heat is added to convert it to 100% vapor.

A substance occupies a higher specific volume above its saturated liquid line due to vaporization.

saturated liquid line

What is a Saturation Line

The saturation line is a point in the Temperature Pressure equilibrium diagram beyond which a substance or component in a system is either changing from liquid to vapor or vapor to liquid.

In general parlance, the Saturation line is synonymous with the concepts of Boiling and Condensation. In fact, both boiling and condensation begin with the saturation line as the starting point.

Linguistically speaking, Saturation refers to a state beyond which a particular system cannot accept more of a component: – Like a saturated solution of sugar in water or a sponge saturated with a liquid. If we add more sugar to a saturated solution of sugar in water, no more sugar would be dissolved if we kept the temperature and pressure of water the same. Under such a state, the sugar solution is said to be saturated.

Similarly, water in atmospheric conditions would start to boil at 100 °C. If the pressure of the system is maintained constant, more water will keep boiling off if more heat is added until all water has transformed into vapor, without any change in the temperature of the system. The temperature at which this phenomenon happens is called the boiling point.

On the other hand, if heat is removed from the water-vapor mixture, keeping the pressure constant, the vapor would start condensing at 100 °C, and it is called condensation point.  

These boiling or condensing points vary with pressure for a particular component and can be denoted by what is called a saturated line in a PV or TV diagram, as shown below.

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Image Credit: PV diagram for a saturated line

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Image source: TV diagram for a saturated line

Definition of Saturated Liquid Line

A substance would exist in liquid form below its saturation line and as a mixture of vapor-liquid above it.

A saturated liquid line identifies the different temperature points in a PV diagram and different pressure points in a TV diagram of any substance beyond which it will cease to exist in pure liquid form.

At any point denoted by the saturated liquid line in the TV and or diagram above, if delta heat is added to the liquid at constant pressure, there will be no change in temperature but its gradual expansion and formation of vapor.

Saturated Liquid Line Temperature

Saturated liquid line temperature varies with system Pressure.

The saturation temperature of a substance shall increase with the increase in pressure, and hence the same is shown by a point higher up in the saturated liquid line of the PV/TV phase diagram.

Saturation liquid line temperature is the point where boiling starts when external heat is added, and the temperature remains the same until all the liquid has vaporized to vapor. This is also the temperature where condensation starts and continues till all the vapors are transformed into liquid. If heat is further removed from the system beyond the point of total vapor condensation, the liquid becomes sub-cooled. Thus the temperature of a sub-cooled liquid lies below the saturated liquid line temperature.

Frequently Asked Questions

Q. What is the definition of a Critical point?

Ans: Critical point signifies the point of a pressure-temperature curve at which liquid and its vapor can coexist.

The top limit of the liquid-vapor equilibrium curve is known as a critical point beyond this point liquid and gas are indistinguishable and form a super critical fluid. The pressure and temperature at this point are known as critical pressure and critical temperature, respectively.

If we consider the PV diagram of water, the point where saturated liquid line and saturated vapor line intersect each other is known as Critical point.


Image credit: Liquid vapor Critical point in a pressure temperature phase diagram

Q. What is a Triple point?

Ans: A point where a small change in pressure and temperature may lead to phase change of a substance.

The point in the pressure-temperature phase diagram, where solid, liquid and vapor phases of a pure substance co exist in equilibrium is known as Triple point.