CHAPTER 3aCHAPTER 3a

PROPERTIES OF PURE SUBSTANCES

PROPERTIES OF PURE SUBSTANCES

CONTENTSCONTENTS

Pure Subtances

Phases of a Pure Substances

Phase Change Processes

Property Diagrams

Property Tables

The Ideal Gas Equation

Compressibility Factor

Define a pure substance

Explain the phase change processes

Sketch P-v and T-v diagrams for a particular process

At the end of the lesson, you should be able to:

LESSON OBJECTIVESLESSON OBJECTIVES

4

PURE SUBSTANCEPURE SUBSTANCE

Pure substance: A substance that has a fixed chemical composition throughout.

Air is a mixture of several gases, but it is considered to be a pure substance.

Nitrogen and gaseous air are pure substances.

A mixture of liquid and gaseous water is a pure substance, but a mixture of liquid and gaseous air is not.

PHASES OF A PURE SUBSTANCEPHASES OF A PURE SUBSTANCE

The molecules in a solid are kept at their positions by the large springlike inter-molecular forces.

In a solid, the attractive and repulsive forces between the molecules tend to maintain them at relatively constant distances from each other.

6

PHASE-CHANGE PROCESSES OF PURE SUBSTANCES

PHASE-CHANGE PROCESSES OF PURE SUBSTANCES

At 1 atm and 20°C, water exists in the liquid phase (compressed liquid: not about to vaporize).

At 1 atm pressure and 100°C, water exists as a liquid that is ready to vaporize (saturated liquid: about to vaporize).

Let us consider the following system undergoes heating process at constant pressure.

As more heat is transferred, part of the saturated liquid vaporizes (saturated liquid–vapor mixture: liquid and vapor coexist in equilibrium).

At 1 atm pressure, the temperature remains constant at 100°C until the last drop of liquid is vaporized (saturated vapor: about to condense).

As more heat is transferred, the temperature of the vapor starts to rise (superheated vapor: not about to condense).

T-v diagram for the heating process of water at constant pressure.

PHASE-CHANGE PROCESSES OF PURE SUBSTANCES (cont’d)

PHASE-CHANGE PROCESSES OF PURE SUBSTANCES (cont’d)

The constant pressure heating process (state 1 - 5) can be illustrated in the following figure known as the T-v Diagram

Saturation state: State at which a phase change begins or ends

Consider repeating this process for other constant pressure lines as shown below. (Where are the saturated liquid and vapour line?)

Critical point: The point at which the saturated liquid and saturated vapor states are identical.

SATURATION TEMPERATURE AND SATURATION PRESSURESATURATION TEMPERATURE AND SATURATION PRESSURE

The temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature.

Water boils at 100C at 1 atm pressure.

Saturation temperature Tsat: The temperature at which a pure substance changes phase at a given pressure.

Saturation pressure Psat: The pressure at which a pure substance changes phase at a given temperature.

Where are the saturation temperatures for P1 and P2?

PROPERTY DIAGRAMS:

T-v Diagram of Pure Substance

PROPERTY DIAGRAMS:

T-v Diagram of Pure Substance

Where are the saturation pressures for T1 and T2 ?

PROPERTY DIAGRAMS:

P-v Diagram of Pure Substance

PROPERTY DIAGRAMS:

P-v Diagram of Pure Substance

Sketch T-v and P-v diagram for the following processes, withrespect to saturation lines. Show initial and final states.

1. A piston cylinder device contains (by mass) 50% liquid water and50% water vapour in equilibrium at 800 kPa (Tsat=170.4°C). Heatis transferred at constant pressure until temperature reaches350°C.

2. Superheated water vapour at 15 bar, T=400°C is allowed to coolat constant volume until it becomes saturated water vapour.

3. A piston cylinder device contains compressed liquid water. Heat isadded to the system at constant pressure until the entire liquidis vaporized.

EXERCISESEXERCISES

Extending the Diagrams to Include The Solid Phase

Extending the Diagrams to Include The Solid Phase

P-v diagram of a substance that contracts on freezing.

P-v diagram of a substance that expands on freezing (such as water).

Triple line: All 3 phases of a pure substance coexist in equilibrium (substance have same P & T but different v)

Sublimation:: Passing from the solid phase directly into the vapor phase.

At low pressures (below the triple-point value), solids evaporate without melting first (sublimation).

P-T diagram of pure substances.

At triple-point pressure and temperature, a substance exists in three phases in equilibrium.

For water, Ttp = 0.01°C

Ptp = 0.6117 kPa

PHASE DIAGRAMPHASE DIAGRAM

P-v-T surface of a substance that contracts on freezing.

P-v-T surface of a substance that expands on freezing (like water).

The P-v-T surfaces present a great deal of information at once, but in a thermodynamic analysis it is more convenient to work with two-dimensional diagrams, such as the P-v and T-v diagrams.

PVT SURFACEPVT SURFACE

Class TakeawayClass Takeaway

Indicate the following states of a pure substance on P-v and T-vdiagrams:

(a) Saturated liquid line

(b) Saturated vapour line

(c) Critical point

(d) Triple point line

(e) Saturation temperature lines at 1 and 10 bar

(f) Constant pressure lines at 100oC and 200oC