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

# Pure substance with illustration

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

- Asked in: 2070 Bhadra, 4 marks
- Video: [Introduction of Pure Substance | Chapter-9 || Engineering Theromodynamics-85 ||](https://www.youtube.com/watch?v=i-V78NvHd_4) — EveryEng - Mechanical
- 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-illustration
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-3

## Question

Define pure substance. Explain with illustration.

## Short answer

A pure substance has homogeneous and invariable chemical composition in all phases (e.g., water in any phase). Air in a single gas phase is considered pure, but a mixture of liquid and gaseous air is not because composition differs between phases.

## Answer

A **pure substance** is a substance that has a **homogeneous and invariable chemical composition** throughout its mass, in any phase or mixture of phases. It may exist in one or more phases, but each phase must have the same chemical composition.

**Illustrations**
- Water in any phase: liquid water, a mixture of ice and liquid water, or a mixture of liquid water and steam is a pure substance, because all phases are H₂O.
- Air (a mixture of gases) is regarded as a pure substance as long as it stays in one gas phase (composition is fixed).
- A mixture of liquid air and gaseous air is **not** a pure substance, since the liquid is richer in oxygen (composition differs between phases, because N₂ and O₂ condense at different temperatures).
- Oil and water, or a mixture of different gases that react, are not pure substances.

```text
 Pure:               Not pure:
 +----------+       +----------+
 | steam    |       | gas air  |  (mostly N2)
 |----------|       |----------|
 | water    |       | liquid   |  (more O2)
 | (all H2O)|       | air      |
 +----------+       +----------+
```

For a simple compressible pure substance, any two independent intensive properties fix the state.

## Check yourself

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

1. A pure substance has:
   A. Only one phase
   B. Homogeneous and invariable chemical composition
   C. Only one element
   D. Constant temperature
   Answer: B. It may exist in several phases. (Key point: Definition)

2. Mixture of ice and liquid water is:
   A. Not a pure substance
   B. A mixture of compounds
   C. A pure substance
   D. Invalid
   Answer: C. All phases are H2O. (Key point: Ice and water)

3. Liquid air with gaseous air is not pure because:
   A. It is too cold
   B. It is a single gas
   C. Pressure varies
   D. Compositions differ between phases
   Answer: D. N2 and O2 condense at different temperatures. (Key point: Liquid-gas air)

4. For a simple compressible pure substance state is fixed by:
   A. Any two independent intensive properties
   B. Temperature alone
   C. Pressure alone
   D. Mass
   Answer: A. Standard state postulate. (Key point: State fixing)

## Related in Fundamental of Thermodynamics & Heat Transfer

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