Fundamental Facts and Concepts
Experience teaches us, that under given external conditions a material is in a certain state. This state is defined by certain properties. At 200 K the compound H2O is solid, at 300 K liquid and at 400 K gaseous. Properties, which are characteristic for a
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Fundamental Facts and Concepts
1.1
General Experience teaches us, that under given external conditions a material is in a certain state. This state is defined by certain properties. At 200 K the compound H20 is solid, at 300 K liquid and at 400 K gaseous. Properties, which are characteristic for a state of a material, are called intensive properties. Among them is the molar volume. As one can see from Fig. 1.1, the molar volume of water is different at 200, 300 and 400 K. Even without a change in phase, a variation of the external condition, here the temperature, can change the state of the system. The molar volume at 300 K is different from that at 277 K. As is well known, water has its highest density at Fig.l.l Molar volume of water as a function of temperature at normal pressure (1)
42.10 3, - -- - - - - - - - - - , 38 '10 3
Solid
I
I
~ L"d IqUi
17
200
300
400
Temperature [K]
B. Predel et al., Phase Diagrams and Heterogeneous Equilibria © Springer-Verlag Berlin Heidelberg 2004
500
2
1 Fundamental Facts and Concepts
277 K and differs in this respect, clearly, from water at 300 K; this is of great importance for the heat balance of the oceans. Quantities, which fix a certain state, are called state variables. In addition to the temperature T, they are the pressure p and the composition c. Only in special cases, which are not discussed here, can other state variables be significant. Whether a change of state really takes place or not during the time of observation due to the change in the external conditions, depends on the kinetics of the process, which reorders the atoms or molecules of the material, as for instance ice ~ water. In solids, especially at low temperatures, the rate of reaction is so slow, that long times are needed to attain the required state. If one has attained the state corresponding to the given p, T and c, then no change will occur during further waiting. This is the equilibrium state, or stable state of the material for the chosen state variables. As long as the equilibrium state is not reached, the material is in a metastable state. In technology, metastable states can, under certain conditions, be of greater importance than stable states. The equilibrium state is independent of the path the system takes to reach it. Water at 300 K has the same properties, independently if it was obtained by melting ice of 200 K, or condensing water vapor of 400 K. The state variables, the properties of the material in equilibrium, are independent of the path used to attain them. One and the same material can appear in different forms, called phases. Phases are in a macroscopic sense homogeneous. Water, ice and water vapor are different phases. The concept of phase is not always connected to a form of aggregation. A solid material can, depending on the state variables, appear in different crystal structures (modifications), which are also defined as phases, as they are, as required by their definition, macroscopically homogeneous. For example we mention the two forms of ice, which appear
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