Change of Phase Equilibrium Phase

Equilibrium.

Phase is defined as a substance's several states (solid, liquid, and gas).  

Equilibrium: When a system is in equilibrium, neither its internal energy level nor its state of motion

 change over time.    

The study of equilibrium that exists within the various phases of matter (solid, liquid, and gas) is what

 is meant by the phrase phase equilibrium.  

Figure 1:- Phase diagram  

  Understanding the connections between the various stages is made easier with the help of the phase

 diagram above. It often illustrates how temperature and pressure affect a system's phase as a function

 of each other. The triple point is a point on the graph where all three states coincide, and it differs for

 each component. The liquid and gaseous phases unite to form one phase at critical temperatures and

 pressures, which is where the term "critical point" originates. A supercritical fluid is a combined single

 phase that exists when a temperature reaches the critical point.     

The condition required for the substance to reach the state of phase change equilibrium are: 

➢  Balance in pressure 

➢  Balance in Temperature         

There are different types of Equilibrium:

1.     Liquid-vapor

2.     Liquid- liquid

3.     Solid-solid

4.     Solid-liquid

The main focus out here is on Vapor- liquid equilibrium.

Vapor- liquid equilibrium

The two phases is said to be in vapor-liquid equilibrium, if a pure substance or mixture is in mechanical

 and thermal equilibrium and in which there is no net mass transfer between the two phases. Once the

 vapor-liquid system has achieved this state, it will show up on the macroscopic level not to be

 experiencing any alter in its properties. Vapor-liquid equilibrium is the foremost in designing

 applications such as refining, natural modeling and common process design. For the chemical industry

 like distillation vapor-liquid is one of the basic factor.

A liquid will evaporate when exposed to air and placed in an open container, just like water and alcohol

 do. Only a small percentage of the molecules in the liquid phase will be moving quite quickly at any

 given moment. One of the molecules can totally avoid the attraction of the other molecules and reach

 the gas phase if it is travelling upward and close to the surface. The average energy of the liquid

 molecules falls when the higher-energy molecules escape, which lowers the liquid's temperature. By

 allowing all of the liquid to finally evaporate over the course of enough time, the average molecular

 speed in the liquid is kept constant by heat absorption. The enthalpy of vaporization relates to the heat

 absorbed throughout the entire process. 

We no longer see complete evaporation of the liquid when we put it in a closed container as opposed to

 an open one. The amount of liquid remains constant once a particular partial pressure of a gas has been

 produced by the evaporation of a liquid. This technique is known as liquid vapor pressure. Depending

 on the liquid, it changes and gets bigger as the temperature rises. As long as there is some liquid

 present, the vapor pressure will remain unchanged (constant) regardless of how much is present or how

 large the container is.  The molecules in the liquid are still evaporating from the liquid surface into the

 vapor as we look at them through a microscope. 

Figure 2: Macroscopic view of the molecular interpretation of vapor pressure.  

However, because the molecules are returning to the liquid at the same rate at which they are fleeing

 from it, the volume of vapor will continue to be the same. The molecules of the vapor behave like those

 of any other gas by bouncing and crashing against the sides of the containers. One of the key elements

 in this situation is how the liquid surface functions as a wall. When a molecule strikes the surface of a

 liquid, it usually enters the liquid's interior because it becomes caught and loses its capacity to escape.

 The rate of recapture will be relatively low when the liquid is first poured to the container since the

vapor molecules are quite small. 

There will be greater opportunities for recapture as more molecules vanish. The rates of recapture and

 escape will be balanced when the vapor pressure has been attended to. At that time, there won't be any

 net liquid or gas condensation. The properties of the vapor-liquid system won't alter at the microscopic

 level once it reaches this state. With respect to time, the amount, volume, pressure, temperature, and

 density of both liquid and gas will remain constant. Vapor liquid equilibrium is the state in which a

 system is said to be in or to have attained equilibrium.