Page 23 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

2.0 PROPERTIES OF PURE SUBSTANCES

2.1 Introduction

In thermodynamic systems, the working fluid can be in the liquid, steam or gaseous phase.
In this unit, the properties of liquid and steam are investigated in some details as the state

of a system can be described in terms of its properties. A substance that has a fixed

composition throughout is called a pure substance. Pure chemicals (H2O, N2, O2, Ar, Ne,

Xe) are always pure substances. We all know from experience that substances exist in

different phases. A phase of substance can be defined as that part of a pure substance
that consists of a single, homogenous aggregate of matter. The three common phases

for H2O that are usually used are solid, liquid and steam.

When studying phases or phase changes in thermodynamics, one does not need to be

concerned with the molecular structure and behavior of the different phases. However,

it is very helpful to have some understanding of the molecular phenomena involved in

each phase.

Molecular bonds are strongest in solids and weakest in steams. One reason is that

molecules in solids are closely packed together, whereas in steams they are separated by

great distances.

The three phases of pure substances are: -

Solid Phase

In the solid phase, the molecules are;

(a) closely bound, therefore relatively dense; and
(b) arranged in a rigid three-dimensional pattern so that they do not easily deform.

An example of a pure solid state is ice.

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