---
title: "Phase Equilibrium | Heat and Mass Transfer"
description: "Phase Equilibrium is the condition where phases coexist without net change, letting you predict evaporation, condensation, and separation in Heat and Mass Transfer."
canonical: "https://fiveable.me/heat-mass-transfer/key-terms/phase-equilibrium"
type: "key-term"
subject: "Heat and Mass Transfer"
unit: "Unit 9"
---

# Phase Equilibrium | Heat and Mass Transfer

## Definition

Phase equilibrium is the state where two or more phases of a substance coexist with no net change in composition or amount. In Heat and Mass Transfer, it is the baseline for predicting evaporation, condensation, and separation processes.

## What It Is

Phase equilibrium in Heat and Mass Transfer is the condition where a substance’s phases, such as liquid and vapor, can exist together at steady balance. Nothing is “stuck” forever, molecules still move across the interface, but the rate leaving one phase matches the rate returning to it, so there is no net change overall.

For a pure substance, this shows up most clearly at a specific temperature and pressure. If liquid water and water vapor are at equilibrium in a closed container, some molecules are evaporating while others are condensing at the same time. The total amounts stay constant because the two transfers cancel each other out.

That balance is not random. It depends on temperature, pressure, and the substance itself. Raise the temperature, and more molecules have enough energy to escape into the vapor phase. Change the pressure, and you shift how easily the vapor phase can exist. That is why phase equilibrium sits behind phase diagrams, saturation pressure, and boiling behavior.

In mass transfer problems, phase equilibrium tells you what the interface “wants” to do before you calculate how fast the transfer can actually happen. The equilibrium condition gives the limiting composition at the surface, while diffusion, convection, or interfacial area control the rate at which the bulk fluid moves toward that condition. So equilibrium is the target state, and mass transfer is the path toward it.

A common mistake is to treat equilibrium as “nothing is happening.” More accurately, the two opposite phase-change rates are equal. That distinction matters in distillation, evaporation, and crystallization, where the interface may be active even when the system is at equilibrium. It also matters when you compare a real system to the equilibrium case, because the difference between the two tells you the driving force for transfer.

## Why It Matters

Phase equilibrium is the reference point for almost every phase-change problem in Heat and Mass Transfer. If you are modeling evaporation from a liquid surface, condensation on a cold wall, or vapor-liquid separation in a column, you first need to know what the equilibrium state at the interface should be.

This term also tells you whether a process is limited by thermodynamics or by transport. For example, a surface may be at saturation, but the bulk gas may still be far from equilibrium, so mass keeps moving by diffusion. That gap between actual conditions and equilibrium conditions is the driving force in many homework and exam problems.

You also see phase equilibrium when temperature or pressure changes shift the state of a material. A phase diagram becomes useful because it maps where equilibrium between solid, liquid, and vapor is possible. That makes it easier to predict boiling, freezing, condensation, and crystallization before you start setting up a rate equation.

In design problems, equilibrium data often feed directly into separation calculations. If you know the equilibrium relation for a species, you can estimate how much vapor forms, how much liquid remains, or what surface concentration to use in a diffusion model. Without that step, the rest of the mass transfer setup is usually off.

## Connections

### Phase Diagram

A phase diagram shows where phase equilibrium exists for a substance as temperature and pressure change. It is the visual map you use to see whether the material should be solid, liquid, or vapor at a given condition. In homework, this often tells you which region a state point falls in before you calculate phase change or mass transfer.

### Vapor Pressure

Vapor pressure is the pressure of a vapor when it is in equilibrium with its liquid at a given temperature. It is one of the main numbers used to judge whether evaporation or condensation will happen. If the surrounding pressure is lower than the vapor pressure, evaporation is easier; if it is higher, condensation becomes more likely.

### [Saturation Pressure](/heat-mass-transfer/key-terms/saturation-pressure)

Saturation pressure is the pressure at which a substance’s liquid and vapor phases are in equilibrium at a specific temperature. It is closely tied to boiling and condensation calculations in heat and mass transfer. When a problem says a fluid is saturated, you are usually being told that the phase balance is at the equilibrium limit.

### [Raoult's Law](/heat-mass-transfer/key-terms/raoults-law)

Raoult's Law connects phase equilibrium in mixtures to the vapor pressure of each component. It gives a way to estimate how much of each species appears in the vapor above a liquid solution. That makes it useful in distillation and other separation problems where you need the equilibrium composition at the interface.

## On the AP Exam

A quiz problem will usually ask you to identify the equilibrium condition at an interface, choose the right saturation value, or use equilibrium data to set a boundary condition before solving for mass flux. You might see a liquid-vapor system and need to decide whether the surface concentration equals the saturation concentration, or whether the bulk phase is still far enough from equilibrium to drive transfer.

In a problem set, you often use phase equilibrium as the first step before applying diffusion or convection equations. For example, if a liquid surface is exposed to air, you may use equilibrium to find the vapor concentration right at the interface, then use that value to calculate the concentration profile or mass transfer rate away from the surface. If the problem includes a phase diagram or saturation pressure table, the real task is usually choosing the correct equilibrium state and then linking it to the rate calculation.

## Phase Equilibrium vs Vapor Pressure

Phase equilibrium is the general balance condition between phases, while vapor pressure is one specific equilibrium pressure for a liquid-vapor pair. If you are describing the whole state where phases coexist, use phase equilibrium. If you are naming the pressure of the vapor in equilibrium with the liquid at a given temperature, use vapor pressure.

## Key Takeaways

- Phase equilibrium means phases coexist with no net change, even though molecules are still crossing the boundary in both directions.
- In Heat and Mass Transfer, phase equilibrium gives the surface condition you use before solving a transfer-rate problem.
- Temperature and pressure control where equilibrium is possible, which is why phase diagrams and saturation data matter so much.
- The difference between the actual state and the equilibrium state is usually the driving force for mass transfer.
- A surface can be at equilibrium while the bulk fluid is not, so equilibrium does not mean the whole system has stopped changing.

## FAQs

### What is phase equilibrium in Heat and Mass Transfer?

It is the condition where two or more phases of a substance, such as liquid and vapor, exist together with no net change over time. Molecules still move across the interface, but the forward and reverse rates match. In Heat and Mass Transfer, that condition is the starting point for evaporation, condensation, and separation calculations.

### How is phase equilibrium different from vapor pressure?

Phase equilibrium is the broader balance condition between phases. Vapor pressure is the equilibrium pressure of the vapor above a liquid at a given temperature. In other words, vapor pressure is one value that comes out of phase equilibrium for a liquid-vapor system.

### Why does phase equilibrium matter in mass transfer problems?

Because it tells you the interface composition or state before you calculate how fast material moves. The actual rate depends on diffusion, convection, and area, but the equilibrium condition sets the boundary value. If you skip that step, your concentration gradient or flux calculation usually starts from the wrong place.

### What is a simple example of phase equilibrium?

Liquid water and water vapor sealed in a closed container can reach equilibrium at a fixed temperature and pressure. Some water molecules evaporate while others condense at the same rate. The amounts of liquid and vapor stay constant even though molecular motion continues.

## Related Study Guides

- [9.4 Mass Transfer with Phase Change](/heat-mass-transfer/unit-9/mass-transfer-phase-change/study-guide/RO68JAkR8q3d6ADE)
- [7.3 Diffusion with Chemical Reaction](/heat-mass-transfer/unit-7/diffusion-chemical-reaction/study-guide/kYRclZB0Q4rNPrTp)

## About This Document

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