CHAPTER 3 PROPERTIES OF PURE SUBSTANCES

OBJECTIVE

1. Introduce the concept of a pure substance.
2. Discuss the physics of phase-change processes.
3. Illustrate the P-v, T-v, and P-T property diagrams and P-v-T surfaces of pure substances.
4. Demonstrate the procedures for determining thermodynamic properties of pure substances from tables of property data.
5. Describe the hypothetical substance “ideal gas” and the ideal-gas equation of state.
6. Apply the ideal-gas equation of state in the solution of typical problems.
7. Introduce the compressibility factor, which accounts for the deviation of real gases from ideal-gas behavior.
8. Present some of the best-known equations of state.

3.1 PURE SUBSTANCE

• A substance that has a fixed chemical composition.
• A mixture of various chemical elements or compounds as long as the mixture is homogeneous, which is in the same phase either is in solid, liquid or gas.

3.2 PHASES OF A PURE SUBSTANCE
Solid

• Molecules are arranged in 3D pattern that is repeated throughout.
• Attractive forces of molecules on each other are large and keep the molecules at fixed positions.
• Molecules cannot move relative to each other but oscillate about their equilibrium positions.
• At high temperature, the velocity reach the point where the intermolecular forces are overcome.
• Molecules break away and melting process is began. 

Liquid

• Molecules no longer at fixed positions as they can rotate and translate freely.
• Intermolecular forces are weaker than solids and stronger than gas.
• Distance between molecules generally slight increase as solid turn liquid.

Gas

• Molecules are far apart from each other and order is nonexistent.
• Molecules move randomly and colliding with each other and the wall of container.
• Low density and very small intermolecular forces.
• The energy level of molecules are higher than solid and liquid.
• They are needed to release a large amount of energy before they can condense or sublime.

3.3 PHASE CHANGE PROCESSES OF PURE SUBSTANCES

  

   

- Compressed liquid/ subcooled liquid - when water exists in liquid phase and it is not about to vaporizes

- As more heat is transferred, heat addition will cause some of the liquid to vaporize, that is a phase-change process from liquid to vapor is about to take place. A liquid that is about to vaporize is called saturated liquid.

- During a boiling process, the only change is a large increase in the volume and a steady decline in the liquid level as a result of more liquid turning to vapor.

- The vaporization process is continued until the last drop of liquid is vaporized, any heat loss from this vapor will cause some of the vapor to condense

- A vapor that is about to condense is called saturated vapor. 

- Saturated liquid- vapor mixture is occurred since the liquid and vapor phases coexist in equilibrium.

- " The temperature at which water starts boiling depend on the pressure, therefore if the pressure is fixed, so is the boiling temperature."

- Saturation temperature is the temperature at which a pure substance changes phase at a given pressure

- Saturated pressure is the pressure at which a pure substance changes phase at a given temperature

3.4 PROPERTY DIAGRAMS FOR PHASE-CHANGE PROCESSES
  T-V Diagram
  P-V Diagram

  P-T Diagram


3.5 PROPERTY TABLES
Formula that involve: