TABLE OF CONTENTS The sample referred to in the definition of thermal analysis in Chapter 1, is very often in the solid state, at least at the start of the investigation. The thermal behaviour of liquids can also be studied using special techniques (see below), but gases are not normally the principal reactants in thermal analysis experiments.
The main characteristic feature of the solid state is the relatively ordered arrangement of the constituent atoms, molecules or ions. Just as the concept of an "ideal gas" is useful in describing the behaviour of real gases, the concept of a "perfect solid" or "perfect crystal" is useful as the reference point for real solids. A perfect (crystalline) solid has a completely ordered arrangement of its constituents, while real solids have imperfections of many kinds. When the order present is marginally greater than for liquids, but considerably less than in a perfect crystal, the substance is sometimes referred to as a "non-crystalline solid". When liquids composed of complex molecules or ions (e.g. sucrose or silicates and a vast number of organic polymers) are cooled rapidly a glass may be formed. A glass resembles a solid in many of its physical properties, e.g. rigidity, but differs in that the constituents do not show the regular (lattice) arrangement of a crystalline solid. Glasses are thus examples of non-crystalline solids. They do not melt at a sharply-defined temperature, but soften over a temperature interval. This transition from the rigid glassy...
