Solution : A perfectly homogeneous mixture of two or more components is called solution.
Solute : The component which is present in lesser amount or whose physical state is changed during the formation of solution is called solute.
Solvent : The component which is present in larger amount and determines the physical state of the solution is called solvent.
Types of solution : Depending upon the nature of solute and solvent, solutions are classified as follows :
Gaseous solutions : Solutions in which gas acts as solvent are known as gaseous solutions.
Liquid solutions : Solutions in which liquids are present in larger amount.
Solid solutions : Solutions in which solids are present in larger amount.
Different methods for expressing concentration of solution :
Solubility : Maximum amount of substance that can be dissolved in a specified amount of solvent at a specified temperature is called its solubility.
Factors affecting solubility of a solid in a liquid :
Nature of solute and solvent : Polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents. (i.e., like dissolves like).
Effect of temperature :
≫ If the dissolution process is endothermic (ΔsolH > 0), the solubility increases with rise in temperature.
≫ If dissolution process is exothermic (ΔsolH < 0), the solubility decreases with rise in temperature.
Effect of pressure : Pressure does not have any significant effect on solubility of solids in liquids as these are highly incompressible.
Factors affecting solubility of a gas in a liquid :
Effect of pressure : Henry’s law states that “the partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution”
p = KH x where, KH is the Henry’s law constant and is different for different gases at a particular temperature.
≫ Higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid.
Effect of temperature : As dissolution is an exothermic process, then according to Le Chatelier’s principle, the solubility should decrease with increase of temperature.
Raoult’s law : It states that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution. p1 = p°1 x1 and p2 = p°2 x2 where p°1 and p°2 are vapour pressures of pure components 1 and 2 respectively, at the same temperature.
Dalton’s law of partial pressures :
≫ If y1 and y2 are the mole fractions of the components 1 and 2 respectively in the vapour phase then, p1 = y1 Ptotal and p2 = y2 Ptotal
Raoult’s law for solid-liquid solutions : It states that relative lowering in vapour pressure of a solution containing a non-volatile solute is equal to the mole fraction of the solute.
p° = vapour pressure of pure solvent
ps = vapour pressure of solution
x2 = mole fraction of solute.
Ideal and non-ideal solutions :
Azeotropes : The mixtures of liquids which boil at constant temperature like a pure liquid and possess same composition of components in liquid as well as vapour phase are called constant boiling mixtures or azeotropic mixtures.
≫ Minimum boiling azeotropes : They formed by those liquid pairs which show positive deviations from ideal behaviour e.g., ethanol-water mixture.
≫ Maximum boiling azeotropes : They are formed by those liquid pairs which show negative deviations from ideal behaviour e.g., nitric acid-water mixture.
Colligative properties : Properties which depend only on the number of solute particles dissolved in a definite amount of the solvent and not on the nature of the solute are called colligative properties.
Relative lowering of vapour pressure : When a nonvolatile solute is dissolved in a solvent, vapour pressure of the solution is lower than that of the pure solvent which is known as lowering of vapour pressure. Relative lowering of vapour pressure is equal to the mole fraction of the solute in the solution.
Elevation in boiling point : The boiling point of a solution containing a non-volatile solute is always higher than the boiling point of the pure solvent. This increase in boiling point is termed as elevation in boiling point.
Kb is called boiling point elevation constant or molal elevation constant or Ebullioscopic constant, having unit K kg mol–1.
Depression in freezing point : The freezing point of a solution containing a non-volatile solute is always less than the freezing point of the pure solvent. This decrease in freezing point is termed as depression in freezing point.
Kf is known as freezing point depression constant or molal depression constant or Cryoscopic constant, having unit K kg mol–1.
Osmosis and osmotic pressure : The movement of solvent molecules from less concentrated solution to more concentrated solution through semipermeable membrane is termed as osmosis. The hydrostatic pressure which develops on account of osmosis is called osmotic pressure or the excess pressure that must be applied on the solution to prevent osmosis is called osmotic pressure.
≫ Two solutions having same osmotic pressure at a given temperature are called isotonic solutions.
≫ If one solution is of lower osmotic pressure, it is called hypotonic with respect to the more concentrated solution. The more concentrated solution is said to be hypertonic with respect to the dilute solution.
≫ If a pressure higher than the osmotic pressure is applied on the solution, the solvent will flow from the solution into the pure solvent through the semipermeable membrane and the process is called reverse osmosis. It is used in desalination of sea water.
Abnormal molecular mass : When the molecular mass of a substance determined by any of the colligative properties comes out to be different than the expected value, the substance is said to show abnormal molecular mass.
≫ Abnormal molecular masses are observed when the solution is non-ideal (not dilute) or the solute undergoes association or dissociation.
van’t Hoff Factor : It is defined as the ratio of the experimental value of the colligative property to the calculated value of the colligative property.
≫ If i > 1, solute undergoes dissociation in the solution and if i < 1, solute undergoes association in the solution.
≫For substances undergoing association or dissociation in the solution, the various expressions for the colligative properties are modified as follows :