Solutions of Nonelectrolytes

1. Solutions of Nonelectrolytes When several nonreacting substances are mixed, three possible types of mixtures may be obtained: a coarse mixture, such as that of salt and sugar; a colloidal dispersion, such as results when fine clay is shaken with water a true solution, obtained when a substance like sugar dissolves in water

2. a coarse mixture the individual particles are readily discernible and may be separated from each other by mechanical means. a colloidal dispersion in a colloidal dispersion the particles are much finer and the heterogeneity is not so readily apparent, the dispersion is, nevertheless, not homogeneous. a true solution in the true solution the constituents cannot be separated from each other by mechanical means, and every part of the solution is found to be like every other part; i.e., a true solution constitutes a homogeneous phase.

3. Unsaturated solution Any solution which contains less than the amount of solute in saturated solution Saturated solution A solution which contains at a given temperature as much solute as it can hold in presence of the dissolving substance is said to be saturated. Supersaturated solution A supersaturated solution can exist only in the absence of dissolving substance and is at best very unstable. Jarring and stirring may, and introduction of solute will, cause the precipitation of excess solute in solution, leading to the formation of a saturated solution.

4. On theoretical grounds it is convenient to subdivide solutions into: (a) solutions of non-electrolytes the solute dissolved in the solvent persists in molecular, uncharged form and exhibits no tendency to dissociate into electrically charged ions. (b) solutions of electrolytes. the solute dissociates to a greater or lesser degree into ions, increasing thereby the total number of particles in solution.

5. Factors Affecting Solubility The extent to which a substance will dissolve in another varies greatly with different substances and depends on the nature of the solute and solvent, In general, compounds of similar chemical character are more readily soluble in each other than are those whose chemical character is entirely different. completely miscible. completely immiscible. partially miscible.

6. the temperature The direction in which the solubility of a substance in a solvent changes with temperature depends on the heat of solution. If a substance dissolves at saturation with evolution of heat, the solubility decreases with rising temperature. On the other hand, if a substance dissolves with absorption of heat, the solubility increases as the temperature is raised. the pressure In general the effect of pressure on solubility is small unless gases are involved.

7. Methods of Expressing Solution Concentration Weight basis (a weight basis are temperature-independent): Per cent or fraction of dissolved substance by weight. Weight of dissolved substance per definite weight of one of the constituents. Weight of dissolved substance per definite total weight of solution. Molality-number of moles of dissolved substance per 1 grams of solvent. Mol fraction.

8. Volume basis (vary with temperature in a manner dependent upon the thermal expansion of the solution): Per cent or fraction of dissolved substance by volume. Weight of dissolved substance per given volume of solution. Molarity-number of moles of dissolved substance per liter of solution. Normality-number of equivalents of dissolved substance per liter of solution.

9. Types of Solutions Although solutions with many components can be prepared, attention will be confined to binary solutions, i.e., solutions containing two components only. Since the solvent and solute may be either gaseous, liquid, or solid, the number of possible types namely: Solution of a gas in a gas All gases are miscible in all proportions, yielding solutions whose physical properties are very nearly additive provided the total pressure is not too high. Under the latter conditions the partial and total pressures are governed by Dalton’s law, the partial and total volumes by Amagat’s law.

10. Solution of a liquid in a gas Solution of a solid in a gas The vaporization of a liquid and the sublimation of a solid into a gas phase may be considered as solution of these substances in a gas. These processes involve first the conversion of the liquid or solid to vapor, and the subsequent solution of the vapor in the gas. Because the vaporization and sublimation pressures of a substance are fixed at any given temperature, the amounts of liquid and solid that can vaporize into a given volume of gas are limited to the amount necessary to establish the equilibrium pressures.

11. Solution of a gas in a solid Solution of a liquid in a solid Solution of a solid in a solid Solution of a gas in a liquid Solution of a solid in a liquid Solution of a liquid in a liquid

12. The Thermodynamic Properties of a Solution The Gibbs free energy, originally called available energyof a body as such: The greatest amount of work which can be obtained from a given quantity of a certain substance in a given initial state, without increasing its total volume or allowing heat to pass to or from external bodies, except such as at the close of the processes are left in their initial condition.

13. The Solution Process • Condition for Equilibrium Between Phases the condition for equilibrium between phases of a pure substance at constant temperature and pressure is that the molar free energy of the substance be the same in all phases. for equilibrium at constant temperature and pressure in a multicomponent system composed of a number of phases, the partial molal free energy of each of constituent must be the same in all the phases.