CHAPTER VI. General Conditions of Equilibrium.
Chemical Reactions. — In many cases when two substances which react chemically upon each other are brought into contact^ it may be observed that the reaction proceeds gradually. This is particularly evident when there is a visible surface of separation between the two reacting bodies, as, for instance, zinc and sulphuric acid ; that is, when the system is heterogeneous. On the other hand, if there is no surface of separation between the two reacting substances — that is, if they be perfectly mixed or dissolved in each other, either in presence or absence of a third substance (solvent) — physical or chemical methods must be appUed in order to detect any change of the properties of the solution which depends on the chemical composition.
The typical example of such a homogeneous system in which a physical property, easy to be examined, changes, is a solution of cane sugar in wffter to which some acid has been added. Such a solution possesses the power of rotating the plane of polarised light through a certain angle; this power gradually changes as the dextro-rotatory cane sugar is transformed into Isevo-rotatory invert sugar (a mixture of equal parts of dextrose and levulose), according to the equation —
CiaHaaOii + HaO (+ acid) = CeHiaOe + C6Hi206(+ acid).
If the chfimge in the rotatory power of the solution be followed we can estimate how far the reaction has proceeded at each moment.
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