---
title: "Pure substance and state postulate"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 3
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:pure-substance-state-postulate"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/pure-substance-state-postulate
site: BCT Engineering
---

# Pure substance and state postulate

Fundamental of Thermodynamics & Heat Transfer · Chapter 3: Properties of Common Substances

- Asked in: 2068 Bhadra, 4 marks
- Video: [Pure Substance And State Postulate | Basic Concepts | Engineering Thermodynamics](https://www.youtube.com/watch?v=eP9Fd2N2Jtc) — 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/pure-substance-state-postulate
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-3

## Question

Define pure substance. State and explain 'State Postulate'.

## Short answer

A pure substance has homogeneous, invariable chemical composition. The state postulate says a simple compressible system is fixed by two independent intensive properties; in the two-phase region T and P are dependent, so an additional property like quality is needed.

## Answer

#### Pure substance
A pure substance is one with a homogeneous and invariable chemical composition throughout its mass, whatever the phase. Examples: water (liquid, steam or ice-water mixture), nitrogen, air in a single gas phase.

#### State postulate
The state of a simple compressible system (a system with no electrical, magnetic, gravitational, motion or surface-tension effects) is completely specified by **two independent intensive properties**.

In general, the number of independent intensive properties needed equals the number of relevant quasi-equilibrium work modes plus one (the one is for the heat-transfer mode). A simple compressible system has only the P dV work mode, so 1 + 1 = 2 properties are needed.

**Explanation**
- Properties are *independent* if one can be varied while the other is held fixed. For example, in a single-phase system, T and v are independent, so (T, v) fixes P, u and h.
- In the two-phase region, T and P are *not* independent: P = P_sat(T). So a third property, such as quality x or v, is needed together with T (or P) to fix the state.
- For an ideal gas, P v = R T gives the third property from any two.

Example: for steam at 1 MPa, specifying T = 300 °C fixes v, u, h and s from the tables.

## Check yourself

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

1. A pure substance is one that has:
   A. Homogeneous and invariable chemical composition in all phases
   B. Only one phase present at a time
   C. Constant temperature during any process
   D. Only one chemical element
   Answer: A. Water as ice, liquid and steam is still pure; composition is uniform and unchanging. (Key point: Pure substance)

2. For a simple compressible system the state is fixed by:
   A. Any one intensive property
   B. Two independent intensive properties
   C. Three independent intensive properties
   D. Temperature and pressure always
   Answer: B. One for the P dV work mode plus one for heat transfer gives 2. (Key point: State postulate)

3. Why are T and P not enough to fix the state in the two-phase region?
   A. Because the system is no longer simple compressible
   B. Because P and T are both constant only in the vapour region
   C. They are not independent there, since P = P_sat(T); a property like quality is needed
   D. Because volume cannot be measured in the mixture
   Answer: C. In the dome T fixes P, so one more property such as x or v is required. (Key point: Independence in two-phase)

4. In general, the number of independent intensive properties needed equals:
   A. Number of phases plus two
   B. Number of work modes minus one
   C. Always two regardless of the work modes
   D. Number of quasi-equilibrium work modes plus one
   Answer: D. Each work mode adds one, plus one for heat transfer. (Key point: Number of properties)

## Related in Fundamental of Thermodynamics & Heat Transfer

- [Pure substance with illustration](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Apure-substance-illustration) — Fundamental of Thermodynamics & Heat Transfer, chapter 3