Text 27 A

The Properties of Solutions

Text 27 A The Properties of Solutions - раздел Образование, Part One THE PREDICATE It Is Difficult To Give A Definition Which Tells Clcarly And Briefly How <...

It is difficult to give a definition which tells clcarly and briefly how <r solutions differ from mixtures and compounds, in spite of the fact that solutions arc among the most familiar substances to be found in nature. However, it is often true that the most common concepts wc use are the most difficult to define precisely. A solution is a homogeneous substancc that has, over ccrtain limits, a continuously variable composition. The word "homogeneous" sets a true solution apart from a mcchanical mixture, for mixtures have macroscopic regions which have distinct and different composition and properties. The properties and composition of a solution arc uniform, as long as the solution is not examined at the molecular level. There arc substanccs, however, not to be clcarly classified as solutions or

mixtures. A solution of soap in water has a cloudy appcarancc due to particles which consist of many soap molcculcs collected together. Such a substancc has properties and composition which might be described as cither inhomogcncous or homogeneous depending on the experiment to be done. Therefore, there is no sharp dividing line between mixtures and solutions.

The requirement that solutions have continuously variable composition distinguishes them from most compounds. However, many solid materials wc commonly think of as compounds actually show variable composition. Cuprous sulphide and ferrous oxide arc examples of compounds which might also be thought of as solutions. No matter how carefully wc made our definitions of solution, mixture, and compound, wc must expect to find ccrtain substanccs not to be uniquely classified as one of these. There is no reason to expect nature to be cooperative and producc only substanccs which arc easily classified, and sincc this is the case, the most useful definitions are often the shortest, rather than the most exhaustive.