---
title: "Thermodynamic equilibrium, state and conditions"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 1
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:equilibrium-state-conditions"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/equilibrium-state-conditions
site: BCT Engineering
---

# Thermodynamic equilibrium, state and conditions

Fundamental of Thermodynamics & Heat Transfer · Chapter 1: Introduction

- Asked in: 2075 Baisakh, 4 marks
- Video: [Thermodynamic Equilibrium | Basic Concepts | Engineering Thermodynamics](https://www.youtube.com/watch?v=LH40zR2EPmQ) — ENGINEERING TUTORIAL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/equilibrium-state-conditions
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-1

## Question

Define thermodynamic equilibrium and thermodynamic state. Also explain conditions required for a thermodynamic equilibrium.

## Short answer

Thermodynamic equilibrium requires no unbalanced potentials: mechanical equilibrium (uniform pressure, no motion), thermal equilibrium (uniform temperature, no heat flow), and chemical equilibrium (no reactions or diffusion). Only then are properties single-valued.

## Answer

#### Thermodynamic state
The state of a system is its condition at an instant, described completely by the values of its thermodynamic properties (P, V, T, etc.). When all properties have fixed values, the system is in a definite state. For a simple compressible pure substance, two independent intensive properties fix the state.

#### Thermodynamic equilibrium
A system is in thermodynamic equilibrium when its properties do not change with time and there is no unbalanced potential (force, temperature or chemical difference) inside it or between it and the surroundings. No spontaneous change occurs if it is isolated.

#### Conditions for thermodynamic equilibrium
A system must satisfy all three together:
1. **Mechanical equilibrium:** no unbalanced force, so pressure is uniform and there is no bulk motion within the system (and across the boundary).
2. **Thermal equilibrium:** no temperature difference, so temperature is uniform and there is no heat flow.
3. **Chemical equilibrium:** no chemical reaction or diffusion of species; chemical composition does not change with time.

If any one is not satisfied, the system is not in thermodynamic equilibrium. Properties of a system can be defined as single values for the whole system only when it is in equilibrium.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. For a simple compressible pure substance, how many independent intensive properties fix the state?
   A. One
   B. Three
   C. Two
   D. Four
   Answer: C. Two independent intensive properties completely fix the state of a simple compressible pure substance. (Key point: Thermodynamic state)

2. Which set of conditions must hold together for thermodynamic equilibrium?
   A. Mechanical, thermal and chemical equilibrium
   B. Mechanical and thermal only
   C. Thermal and chemical only
   D. Mechanical and chemical only
   Answer: A. All three must be satisfied simultaneously; failing any one means no thermodynamic equilibrium. (Key point: Three equilibria)

3. What does thermal equilibrium mean inside a system?
   A. Pressure is uniform
   B. Composition is constant
   C. There is no bulk motion
   D. Temperature is uniform and there is no heat flow
   Answer: D. Thermal equilibrium means no temperature difference and hence no heat flow. (Key point: Thermal equilibrium)

4. Why are properties defined as a single value for the whole system only at equilibrium?
   A. Unbalanced potentials would make values differ from place to place or change with time
   B. Properties are path functions
   C. Equilibrium makes mass zero
   D. Properties exist only during a process
   Answer: A. Without equilibrium, pressure or temperature could vary through the system, so no single value describes it. (Key point: Why equilibrium matters)

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