PROPERTIES OF PURE SUBSTANCE

1. Introduction

In thermodynamics, a pure substance is a material that has a uniform and fixed chemical composition throughout. It may exist in more than one physical phase, but the chemical composition remains the same.

Examples:

• Water–steam system
• Ice–water–steam mixture
• A homogeneous mixture such as air (treated as a pure substance in thermodynamics)

2. Definition of Pure Substance

A pure substance is defined as:

A substance that has the same chemical composition and chemical structure throughout, regardless of phase changes.

Key points:

• May exist in solid, liquid, or gaseous phases
• Phase changes do not alter chemical composition
• Mixtures with fixed composition can be treated as pure substances

3. Phases of a Pure Substance

A phase is a physically and chemically homogeneous, uniform, and mechanically separable part of a system.

For water:

• Solid phase → Ice
• Liquid phase → Water
• Vapor phase → Steam

A pure substance can exist:

• In a single phase
• In a two-phase mixture (e.g., water + steam)

4. Phase Change Processes

Phase change occurs when a substance transforms from one phase to another at constant pressure and temperature.

Common phase change processes:

• Melting: Solid → Liquid
• Freezing: Liquid → Solid
• Evaporation: Liquid → Vapor
• Condensation: Vapor → Liquid
• Sublimation: Solid → Vapor

5. Phase Change of Water at Constant Pressure

When water is heated at constant atmospheric pressure:

1. Ice temperature rises until 0°C
2. Ice melts at constant temperature
3. Water temperature rises from 0°C to 100°C
4. Water boils at constant temperature (100°C)
5. Steam temperature rises after complete vaporization

6. Important Thermodynamic Terms

(a) Saturation Temperature

The temperature at which a liquid starts boiling for a given pressure.

(b) Saturation Pressure

The pressure at which boiling occurs at a given temperature.

(c) Saturated Liquid

A liquid that is about to vaporize.

(d) Saturated Vapor

A vapor that is about to condense.

(e) Wet Steam

A mixture of saturated liquid and saturated vapor.

(f) Superheated Vapor

Vapor whose temperature is higher than the saturation temperature at a given pressure.

7. Dryness Fraction (Quality)

The dryness fraction (x) is the ratio of mass of dry vapor to the total mass of the wet mixture.$$x = \frac{\text{Mass of dry steam}}{\text{Total mass of wet steam}}$$​

• For dry saturated steam → $x = 1$x=1
• For wet steam → $0 < x < 1$0<x<1

8. Property Diagrams for Pure Substances

(a) Pressure–Volume (P–V) Diagram

• Shows relationship between pressure and specific volume
• Two-phase region appears as a dome
• Left boundary → Saturated liquid line
• Right boundary → Saturated vapor line

(b) Temperature–Entropy (T–S) Diagram

• Useful for analyzing heat transfer
• Area under the curve represents heat transfer
• Widely used in power plant analysis

(c) Enthalpy–Entropy (H–S or Mollier) Diagram

• Used mainly for steam and refrigeration systems
• Convenient for turbine and nozzle analysis

9. Critical Point

The critical point is the state at which the saturated liquid and saturated vapor become identical.

For water:

• Critical temperature: 374°C
• Critical pressure: 22.09 MPa

Beyond this point, the substance becomes a supercritical fluid.

10. Triple Point

The triple point is the condition at which all three phases (solid, liquid, vapor) coexist in equilibrium.

For water:

• Temperature: 0.01°C
• Pressure: 0.611 kPa

11. Thermodynamic Properties of Pure Substances

(a) Specific Volume (v)

$$v = \frac{V}{m}$$v=mV​

(b) Internal Energy (u)

Energy due to molecular motion and intermolecular forces.

(c) Enthalpy (h)

$$h = u + pv$$h=u+pv

(d) Entropy (s)

Measure of molecular disorder and irreversibility.

12. Steam Tables

Steam tables provide thermodynamic properties of water and steam.

Types of steam tables:

• Saturated steam tables (temperature-based & pressure-based)
• Superheated steam tables
• Compressed liquid tables (approximate)

Properties obtained:

• Specific volume
• Internal energy
• Enthalpy
• Entropy

13. Use of Steam Tables in Numerical Problems

Steps:

1. Identify state of steam (wet, dry, superheated)
2. Use appropriate steam table
3. Apply dryness fraction if wet steam
4. Interpolate values if required

14. Applications of Pure Substance Properties

• Steam power plants
• Refrigeration and air conditioning
• Boilers and condensers
• Turbines and compressors

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