It is difficult to give a definition which tells clearly and briefly how solutions differ from mixtures and compounds, in spite of the fact that solutions are among the most familiar substances to be found in nature. However, it is often true that the most common concepts we use are the most difficult to define precisely. A solution is a homogeneous substance that has, over certain limits, a continuously variable composition. The word “homogeneous” sets a true solution apart from a mechanical mixture, for mixtures have macroscopic regions which have distinct and different composition and properties. The properties and composition of a solution are uniform, as long as the solution is not examined at the molecular level. There are substances, however, not to be clearly classified as solutions or mixtures. A solution of soap in water has a cloudy appearance due to particles which consist of many soap molecules collected together. Such a substance has properties and composition which might be described as either inhomogeneous 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 we commonly think of as compounds actually show variable composition.
Answer the following questions:
1. Why is it difficult to give a definition of a solution?
2. What is usually considered as the main property of solutions?
3. In what way can you characterize a solution of soap in water?
Interactions in electrolyte solutions
In a solution of an electrolyte, it is often necessary to have a detailed knowledge of the species present. New ions or uncharged molecules resulting from interactions in the solution may behave quite differently from the constituent ions of the electrolyte. Some properties of the solutions will be profoundly affected, and the chemist, in order to understand these phenomena, will require to know the nature of the species present.
There are a number of formidable difficulties in the analysis of such systems and, during the past forty years or so, a great deal of work has been done on the problem. The equilibrium properties of electrolyte solutions and the way in which ion-pair and complex formation can be detected and quantitatively studied are of primary importance. Although the application of new physical and chemical methods has produced significant contributions in this field, the information obtained from measurements of a system at equilibrium is to some extent limited, and in studying the phenomenon it is desirable to know the relevant kinetic parameters. Without this understanding, it is sometimes impossible to sketch the actual reaction mechanism by which the system approaches equilibrium.
In general, we may regard the elucidation of the structure of an electrolyte solution as a difficult problem which requires as many independent lines of attack as possible.
A good deal of success in the study of molecules in the gas phase prompted chemists to attempt to build up theories of solutions in an analogous way.
Answer the following questions:
1. Why is it necessary to know the species present in solution?
2. What do the properties of electrolyte solutions depend on?
3. What knowledge helped chemists to build up theories of solutions?
Liquid-vapour equilibrium
A liquid of relatively low boiling temperature, when placed in a container open to the atmosphere, will eventually evaporate entirely. Remembering that molecules in the liquid are “bound” by attractive forces to their neighbours, we might ask why some are able to overcome these forces and leave the liquid spontaneously. The answer lies in a consideration of the possible magnitudes of molecular kinetic energies, for these, as we have already mentioned, range from very low to very high values, and are distributed according to the Maxwell-Boltzmann law. Therefore, even if the average potential energy which binds the molecules to the liquid is substantial, there are always some molecules which have enough kinetic energy to overcome the binding forces and enter the vapour. According to the Maxwell-Boltzmann law, the fraction of the molecules which have kinetic energies greater than some minimum value, the value required for the molecules to leave the liquid, is proportional to the Boltzmann factor. Therefore, the temperature remaining constant, the fraction of liquid molecules with enough kinetic energy to evaporate remains the same, and evaporation continues. The vessel being open to the atmosphere, vapour molecules are swept away, and evaporation continues until no liquid is left.
Answer the following questions:
1. What happens with a liquid if it is placed in an open container?
2. What forces act between the molecules in a liquid?
3. Under what conditions can some molecules leave the liquid?
