Properties of Electrolyte Solutions
Most physical properties can be classified as intensive or extensive. Intensive properties are characteristic of the substance and do not depend on the size of the material being studied. Density is an example of an intensive property. Extensive properties are properties that are directly to the size of the material being studied. Mass is an example of an extensive property.
A third category of properties only applies to solutions—colligative properties. Properties can be considered colligative only if their properties are dependent on the amount of solute present in the solution, disregarding the identity of the solute itself. This property is most commonly seen in electrolyte solutions: those solutions that contain dissociated ions.
A simple example of an electrolyte solution is sodium chloride in water. In the presence of water, solid sodium chloride dissociates as it is dissolved, forming an electrolyte solution:
Nonelectrolyte solutions are those in which the solute does not dissociate into ions (like sugar, for example) when dissolved. Electrolytes affect the colligative properties of solutions.
Vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phases (liquid or solid) at a particular temperature. Basically, it is a measure of how much the solvent molecules tend to escape from a liquid or solid phase into the atmosphere. Vapor pressure of a liquid is a colligative property. From an entropic point of view, evaporation is favorable because the entropy of a system will increase given that gaseous molecules occupy a larger volume than molecules in liquid form. However, if the original liquid solvent contains solute, the original entropy is larger, meaning the amount of entropy that can be gained upon liquid entering the gas phase is less. Therefore, there will be less solvent molecules entering the gas phase. This correlates to vapor pressure lowering as solute concentration increases.
To better visualize the effect of solute on the vapor pressure of a solution, consider a pure solvent. This pure solvent has a certain vapor pressure associated with it. Subjected to temperatures below the solvent's boiling point, the molecules changing to the gaseous phase are mostly situated on the top layer of the solution. Now consider a solution composed of both solvent and solute. Some solute molecules will thus occupy space near the surface of the liquid, decreasing the number of solvent molecules near the surface. Therefore, less molecules are changing from the liquid phase into the gas phase—the vapor pressure of the solvent has decreased.
In an ideal solution, the vapor pressure is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. This is detailed by Raoult's law. In equilibrium, the total vapor pressure is
The individual vapor pressure for each component is