ME2320_CHAPTERIII - CH III ME232 Thermo I Properties of...

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CH. III ME232 Thermo I Properties of Pure Substances 30 PROPERTIES OF PURE SUBTANCES 1. PURE SUBSTANCE A Pure Substance has a homogeneous and invariable chemical composition. It may exist in more than one phase, but the chemical composition is the same in all phases. Thus, liquid water, a mixture of liquid water and water vapor (steam), or a mixture of ice and liquid water are all pure substances. On the other hand, a mixture of liquid air and gaseous air is not a pure substance since the composition of the liquid phase is different from that of the vapor phase. Sometimes, a mixture of gases, such as air, is considered as a pure substances as long as there is not change of phase. 2. PHASES IN A PURE SUBSTANCE There are three principal phases: solid, liquid, and gaseous . However, a substance may have several phases within a principal phase, each with a different molecular structure. Thus, a phase is identified as having a different molecular arrangement that is homogeneous throughout and separated by the others by easily identifiable boundary surfaces. The two phases of iced water are perhaps the clearest example. For example, carbon may exist as graphite or diamond in the solid phase. Helium has two liquid phases; iron has three solid phases. Ice may exist at seven different phases at high pressures. Solid phase is the phase that presents the strongest bonds between molecules. In solids, the molecules are arranged in a three – dimensional pattern called lattice that is repeated throughout. Due to the very small separation between molecules in a solid, the attractive forces between molecules are large and the molecules are kept at fixed positions. Molecules in a solid cannot move with respect to each other, however, they oscillate continuously around about their equilibrium positions. The degree of these Pure substances Water Nitrogen Helium Carbon dioxide
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CH. III ME232 Thermo I Properties of Pure Substances 31 oscillations depends on the degree of energy contained by the solid, that is, the temperature of the solid. The molecular spacing in liquid phase is not totally different to that in solids. However, in liquids the molecules are not fixed relative to each other; that is, in liquids the molecules can rotate and also translate freely. The distance between molecules generally becomes smaller as the liquid solidifies being water a notable exception. In gas phase the molecules are far apart from each other and the molecule order is inexistent. Gas molecules move about at random, continually colliding to each other and with the walls of the container in which they are enclosed. Molecules in gas phase are at a considerable higher energy level than they are in the liquid or solid phases. Therefore, the gas must release a large amount of its energy before condensing or solidifying.
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