Properties of Pure Substances Explained: Phase Change, Steam Tables, P-v, T-v and P-T Diagram
Graphical Abstract
Introduction
In thermodynamics, many engineering systems involve substances that undergo phase changes, such as water converting into steam in boilers or refrigerants evaporating in refrigeration systems.
To analyze these systems accurately, engineers must understand the properties of pure substances.
This topic forms the foundation for studying:
Steam power plants
Boilers
Condensers
Refrigeration systems
Air conditioning systems
Heat exchangers
Learning Outcomes
After studying this article, students will be able to:
✓ Define a pure substance.
✓ Explain phase-change processes.
✓ Understand saturated liquid and saturated vapor states.
✓ Read and interpret steam tables.
✓ Explain dryness fraction and quality of steam.
✓ Analyze P-v, T-v, and P-T diagrams.
✓ Apply thermodynamic property data in engineering calculations.
What is a Pure Substance?
A pure substance is a material with a uniform and fixed chemical composition throughout.
Examples:
Water (H₂O)
Nitrogen (N₂)
Oxygen (O₂)
Refrigerant R-134a
Even if a substance exists in multiple phases, it remains a pure substance if the chemical composition is unchanged.
Example:
Water + Steam = Pure Substance
because both are H₂O.
Phases of Matter
A pure substance can exist in three phases:
Solid
Liquid
Vapor (Gas)
Phases of Matter
SOLID ↔ LIQUID ↔ VAPOR
Melting Evaporation
Freezing Condensation
Different phases of a pure substance and possible phase transformations.
Phase Change Process
Consider water at atmospheric pressure.
When heat is added:
Ice → Water → Steam
The temperature does not always increase continuously.
During phase change, heat is absorbed without temperature change.
This energy is called latent heat.
Saturated States
The term saturation refers to the condition at which phase change begins.
Saturated Liquid
A liquid about to vaporize.
Example:
Water at 100°C and atmospheric pressure.
Saturated Vapor
A vapor about to condense.
Example:
Steam at 100°C and atmospheric pressure.
Saturation Temperature
The temperature at which phase change occurs at a given pressure.
Symbol:
Tsat
Saturation Pressure
The pressure at which phase change occurs at a given temperature.
Symbol:
Psat
Heating Water at Constant Pressure
Temperature
Steam Region
/
/
100°C -----------------
Phase Change
----------------------
Water Region
Heat Added →
Temperature remains constant during the boiling process.Latent Heat
Latent heat is the energy required for phase change without temperature change.
Types:
Latent Heat of Fusion
Solid ↔ Liquid
Latent Heat of Vaporization
Liquid ↔ Vapor
Critical Point
The critical point represents the state where liquid and vapor become indistinguishable.
Beyond the critical point:
No distinct liquid phase
No distinct vapor phase
The substance becomes a supercritical fluid.
Triple Point
The triple point is the unique condition where:
Solid
Liquid
Vapor
coexist in equilibrium.
For water:
Temperature = 0.01°C
Pressure = 0.611 kPa
Triple Point and Critical Point
Pressure
Liquid
/\
/ \
Solid / \ Vapor
/ \
----•--------•-----
Triple Critical
Temperature →Steam Tables
Steam tables contain thermodynamic property data for water and steam.
Engineers use steam tables instead of repeatedly performing complex calculations.
Information Available in Steam Tables
Saturation Temperature
Saturation Pressure
Specific Volume
Internal Energy
Enthalpy
Entropy
Common Symbols Used
| Symbol | Property |
|---|---|
| vf | Specific volume of saturated liquid |
| vg | Specific volume of saturated vapor |
| uf | Internal energy of saturated liquid |
| ug | Internal energy of saturated vapor |
| hf | Enthalpy of saturated liquid |
| hg | Enthalpy of saturated vapor |
| sf | Entropy of saturated liquid |
| sg | Entropy of saturated vapor |
Wet Steam Region
During evaporation, water and steam coexist.
This mixture is called wet steam.
The state lies between:
Saturated liquid
Saturated vapor
Dryness Fraction (Quality of Steam)
Dryness fraction represents the mass fraction of vapor in wet steam.
Symbol:
x
Formula:
x = Mass of Dry Steam / Total Mass
Range:
0 ≤ x ≤ 1
Interpretation
x = 0
Saturated liquid
x = 1
Dry saturated steam
0 < x < 1
Wet steam
Property Equation Using Dryness Fraction
Specific Enthalpy:
h = hf + x(hg − hf)
Similarly:
v = vf + x(vg − vf)
u = uf + x(ug − uf)
s = sf + x(sg − sf)
Worked Example
A steam mixture has:
hf = 640 kJ/kg
hg = 2740 kJ/kg
Dryness fraction x = 0.85
Find specific enthalpy.
Solution:
h = hf + x(hg − hf)
h = 640 + 0.85(2740 − 640)
h = 640 + 1785
h = 2425 kJ/kg
Answer:
Specific Enthalpy = 2425 kJ/kg
P-v Diagram
The Pressure-Specific Volume diagram is widely used in thermodynamics.
It shows:
Saturated liquid line
Saturated vapor line
Two-phase region
Regions on P-v Diagram
Compressed Liquid Region
Left of saturation dome.
Wet Region
Inside saturation dome.
Superheated Vapor Region
Right of saturation dome.
T-v Diagram
Temperature plotted against specific volume.
Useful for visualizing:
Boiling process
Condensation process
Superheating
P-T Diagram
Pressure versus Temperature representation.
Shows:
Triple point
Critical point
Phase boundaries
Engineering Applications
Boilers
Steam generation analysis.
Condensers
Condensation processes.
Steam Turbines
Steam property calculations.
Refrigeration Systems
Refrigerant phase-change analysis.
Nuclear Power Plants
Steam cycle evaluation.
Common Student Mistakes
Mistake 1
Confusing dry steam with superheated steam.
Mistake 2
Ignoring units while using steam tables.
Mistake 3
Using saturation properties outside the saturation region.
Mistake 4
Incorrect interpretation of dryness fraction.
Examination Questions
Short Answer Questions
Define a pure substance.
What is latent heat?
Define saturation temperature.
Explain dryness fraction.
What is a critical point?
Long Answer Questions
Explain phase-change processes.
Discuss steam tables and their applications.
Explain P-v and T-v diagrams.
Describe critical and triple points.
Numerical Problems
Calculate enthalpy of wet steam using steam table data.
Determine specific volume using dryness fraction.
Find entropy of a steam mixture.
Frequently Asked Questions
Is water and steam together a pure substance?
Yes. Both phases have the same chemical composition (H₂O).
Why are steam tables important?
They provide accurate thermodynamic properties for engineering calculations.
What does dryness fraction indicate?
The proportion of dry steam present in wet steam.
What is the difference between wet steam and superheated steam?
Wet steam contains liquid droplets, while superheated steam exists entirely in vapor form above saturation temperature.
Summary Table
| Concept | Key Idea |
|---|---|
| Pure Substance | Uniform chemical composition |
| Saturated Liquid | Ready to vaporize |
| Saturated Vapor | Ready to condense |
| Dryness Fraction | Quality of steam |
| Steam Tables | Thermodynamic property database |
| Critical Point | Liquid and vapor become identical |
| Triple Point | Three phases coexist |
| P-v Diagram | Pressure-volume relationship |
| T-v Diagram | Temperature-volume relationship |
| P-T Diagram | Phase boundary representation |
Conclusion
Understanding the properties of pure substances is essential for analyzing steam power plants, refrigeration systems, and thermal engineering equipment. Concepts such as saturation, phase change, steam tables, dryness fraction, and thermodynamic diagrams provide the tools engineers need to evaluate and design energy systems effectively.
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