72 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
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example of a reaction of this type. In the chemical equations representing reactions belonging to this class the ordinary sign of equality is used. There are theoretical reasons for believing that reactions in a homogeneous system never take place absolutely completely. According to the theory an equilibrium is always established ; but in many cases the reaction proceeds so nearly to completion that by the chemical or physical methods at present available it is im- possible to detect the presence of the substances represented on one side of the equation.
Complete reactions occur during chemical change in a heterogeneous system. The simplest case of such a trans- formation is the change of the state of aggregation of a substance. Water under normal pressure (760 mm. Hg) passes completely into ice if the temperature be lower than 0^ and the opposite change takes place completely if the temperature be higher. If the pressure be 760 mm. ice and water can only be in eqidlibrium at 0°. That point at which an equilibrium may exist (temp. = 0°, press. = 760 mm.) is called the transition point of the system. In this special case, and in general when gases do not take part in the equilibrium, and when the pressure exerts but little influence, it is customary to state that the transition point of the system, ice ^ water : is 0°. According to Eeicher's determination {1) the transition between monoclinic and rhombic sulphur takes place at 956°. The system —
NaaSO4.10H2O +MgS04.7H20 ^ NaaMg(S04)2.4H20 + 13HaO
CrjBt. Glauber M\i. Cryttt. Epeom salt Gryst Mtrmkanite. Water.
has a transition point (determined by various methods) which Kes at about 21*5°. If crystals of Glauber salt be mixed with crystals of Epsom salt below 21-5° no change takes place; but if this mixture be heated to above 21 "5° it is transformed into astrakanite and water.
Systems in which no gases occur (and in which solu- tions play only an unimportant part) are termed condensed systems by van't Hoflf, who, with his pupils, has studied the
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