Chemical Solutions

A solution is a homogeneous mixture of two or more substances at molecular or ionic levels. Components of true solution can not be separated by filtration, settling, centrifugation.

Solute may lose its physical state, but solvent retains its physical state. Based on the number of components, solutions may be binary, ternary, quaternary.

A binary solution contains only two components known as solute and solvent.

Solute + Solvent = Solution

The substance present in smaller proportion in binary solution is known as the solute. The solute is called the dissolved component or dispersed component in the solution.

The substance present in larger proportion is called as the solvent. The solvent is called the dissolving component or dispersion medium in the solution.

In case of solid in liquid type solutions, irrespective of their amounts, solid is solute and liquid is solvent.

Based on the physical state, solutions are of 3 types:

  1. Gaseous solution : Solvent is Gas
  2. The liquid solutions : Solvent is Liquid
  3. Solid solutions : Solvent is solid

In any type of solution the solute may be gas or liquid or solid.

Solutions are of 7 types based on the physical states of solute and solvent:

  1. Gas in gas : Mixture of any two gases
  2. Gas in liquid : Soda water
  3. Liquid in liquid : Alcohol in water
  4. Solid in liquid : Sugar in water
  5. Gas in solid : H2 occluded in Pd
  6. Liquid in solid : Amalgams
  7. Solid in solid : Alloys

Liquid in gas and solid in gas are not considered as true solutions as they are not homogenous.

A solution in which water is used as a solvent is known as aqueous solution. A solution in which alcohol is used as a solvent is known as alcoholic solution. A solution in which an organic liquid is used as a solvent is known as non-aqueous solution.

Based on the amount of dissolved solute, solutions are of 3 types:

  1. Saturated solutions : which can not dissolve any more solute. Usually some amount of undissolved solute is present in it. A dynamic equilibrium exists between dissolved solute and undissolved solute.
  2. Unsaturated solutions : which can dissolve some more amount of solute. No excess of undissolved solute exists. No dynamic equilibrium exists.
  3. Super saturated solutions : which contain excess of dissolved solute. No equilibrium exists. These are unstable.

Concentration

The quantity of the solute present in a definite quantity of the solution relative to the solvent is known as the concentration of the solution or strength of the solution.

A solution which contains less quantity of the solute compared to the solvent is known as dilute solution. The strength of a dilute solution is very low. A solution which contains excess solute, in a definite quantity of the solution is known as a concentrated solution. The strength of a concentrated solution is very high.

Weight of the solution = Volume of the solution × density of the solution

W = V × d

The density of the solution depends on temperature of the solution. The units for the density of solution are gram/ml. The ratio between the density of solution and the density of water, both measured at the same temperature is known as relative density of the solution or specific gravity of the solution. 

Solubility

It is the weight of solute dissolved in 100 grams of solvent to form saturated solution.

Molarity

The number of gram moles of the dissolved solute per litre of solution is known as the molarity of the solution. It is represented by M. Units for molarity are moles/litre. The molarity is the most convenient and commonly used method of expressing the concentration of solution.

The molarity of a solution slightly decreases with increase in temperature of the solution, due to increase in volume.

Normality (N)

The number of gram equivalents of the solute dissolved in one litre of solution is known as its normality. Units for normality are gram equivalents per litre. The normality of a solution decreases with increase in temperature of the solution.

Colligative Properties

The properties of dilute solutions which depend on the number of particles (ions or molecules) of the solute dissolved in the solution are called colligative properties. They are:

  1. Relative lowering of vapour pressure (RLVP) of solution
  2. Elevation in the boiling point of the solution (ΔTb)
  3. Depression in the freezing point of the solution (ΔTf)
  4. Osmotic pressure of the solution (π)

Osmosis

The spontaneous flow of the solvent through semipermeable membrane from pure solvent to solution or from a dilute solution to concentrated solution is known as osmosis.