Solutions

SYNOPSIS

• Solutions are homogeneous mixtures of two or more components.
• Solute: The substance present in a smaller amount in a solution.
• Solvent: The substance present in a larger amount in a solution.
• Binary solutions: Solutions which contain two components.
• Types of Solutions:
  Gaseous Solutions
  
  

  Solute	Solvent	Common Examples
  Gas	Gas	Mixture of oxygen and nitrogen gases
  Liquid	Gas	Chloroform mixed with nitrogen gas
  Solid	Gas	Camphor in nitrogen gas

  
  Liquid Solutions
  

  Solute	Solvent	Common Examples
  Gas	Liquid	Oxygen dissolved in water
  Liquid	Liquid	Ethanol dissolved in water
  Solid	Liquid	Glucose dissolved in water

  
  Solid Solutions
  

  Solute	Solvent	Common Examples
  Gas	Solid	Solution of hydrogen in palladium
  Liquid	Solid	Amalgam of mercury with sodium
  Solid	Solid	Copper dissolved in gold

• Solubility: It is defined as the amount of solute dissolved in 100 g of solvent to form a saturated solution.

• Solubility of a solid in a liquid:
  

  Effect of temperature: The solubility of a solid in a liquid increases with increase in temperature if dissolution is endothermic.
  If dissolution is exothermic, the solubility of a solid in liquid decreases with increase in temperature.
  Effect of pressure: The solubility of a solid in a liquid is not affected significantly by pressure because solids and liquids cannot be compressed.b

• Saturated solution: A solution in which additional solute cannot be dissolved at the same temperature is called a saturated solution.

• Unsaturated solution: A solution in which more amount of solute can be dissolved at the same temperature.

• Supersaturated solution: A solution which contains more mass of the dissolved solute than the saturated solution at the same temperature and pressure.

• Solubility of gases in a liquid: 
  Factors affecting the solubility of a gas in a liquid:

  1. Nature of gas
  2. Nature of liquid
  3. Temperature
  4. Pressure

• Henry’s Law: 
  The solubility of a gas in a liquid is directly proportional to the pressure of the gas.

• Vapour pressure of solutions of solids in liquids:
  Raoult’s Law:
  The partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction.

• Ideal solutions:
  Solutions which obey Raoult’s law over the entire range of concentration.
  Ideal solutions have two important properties:
  

  1. Enthalpy of mixing of the pure components to form the solution is zero.
  2. Volume of mixing is zero.

• Non-ideal solutions:
  Solutions which do not obey Raoult’s law over the entire range of concentration.

• Azeotropic mixtures:
  Some liquid solutions of two (or more) components have the same composition in solution as well as in the vapour phase. Such mixtures are azeotropic and known as azeotropes.

• Colligative properties:
  These are properties of the solution which depend on the number of solute particles present in the solution irrespective of the nature of solute and solvent.

• Relative lowering of vapour pressure of solvent:
  The vapour pressure of a solvent in a solution is less than that of a pure solvent. According to Raoult’s law, the relative lowering in the vapour pressure of dilute solution is equal to the mole fraction of solute (non-volatile) present in the solution.

• Elevation of boiling point:
  The difference in boiling point of solution and pure solvent is called the elevation of boiling point. It is directly proportional to the number of moles of solute in a given amount of solvent.

• Depression in freezing point:
  The difference between the freezing point of a pure solvent and solution is directly proportional to the number of moles of solute in a given amount of solvent.

• Osmosis:
  Osmosis is the tendency of a solvent to move through a thin porous (semi-permeable) membrane from a dilute solution to a more concentrated solution.

• Isotonic solutions:
  Two solutions are said to be isotonic when they exert the same osmotic pressure because they have the same molar concentration.

• Isosmotic solutions:
  When two isotonic solutions are separated by a semi-permeable membrane, no osmosis occurs and the two isotonic solutions are called isosmotic solutions.

• Abnormal molecular mass:
  When the molecular mass calculated with the help of a colligative property is different from the theoretical molecular mass, it is called abnormal molecular mass.

• Association:
  When the number of solute particles is less than the expected number, the solute molecules associate and the process is known as association.

• Dissociation:
  When the number of solute particles is more than the expected number, the solute particles dissociate and the process is known as dissociation.

• Degree of dissociation:
  The ratio of the number of molecules dissociated to the total number of molecules.

• van’t Hoff Factor:
  The ratio of the experimental value of the colligative property to the theoretical value.