What are the SOLUTIONS? its types and physical activities
A homogeneous mixture of two or more non-reacting substances is known as
solution. Homogeneity or heterogeneity depends upon particle size and states of matter present in the solution. Every solution is made up of a solvent (present in larger quantity) and one or more solute (present in smaller quantity).
TYPES OF SOLUTION
A. Gaseous Solutions
- (i) Gas in gas
- (ii) Liquid in gas
- (iii) Solid in gas
B. Liquid Solutions
- (i) Gas in liquid
(ii) Liquid in liquid
(iii) Solid in liquid
C. Solid Solutions
- (i) Gas in solid
- (ii) Liquid in solid
- (iii) Solid in solid
Note : In solution chapter we mostly deal with solid in liquid or liquid in
UNITS OF CONCENTRATION
(i) Molarity (M) :
It is the no. of moles of solute present per litre of solution.
mM = millimoles
Molarity changes with temperature of the solution. Increase in temperature decreases the molarity. It is the most convenient method to express concentration of the solution. On dilution molarity decreases.
(ii) Molality (m) :
No. of moles (n) of solute present per kg of solvent.
It is independent of temperature since no volume factor is involved in the equation.
(iii) Mole fraction (x):
It is the ratio of no. of moles of one component to the total no. of moles present in the solution.For a system having two components A and B.
Mole fraction is also independent of temperature.
(iv) In terms of % :
% by weight is independent of temperature while % by vol., % by strength or strength are temperature
VAPOUR PRESSURE AND RAOULT’S LAW
The pressure exerted by the vapours at the free surface of liquid provided system is closed is known as its vapour pressure. The V.P. of a pure liquid is always greater than its solution (In case of non volatile solute).
(a) Raoult’s Law for a solution having non volatile solute.
i.e relative lowering of vapour pressure is equal to the mole fraction of solute.
(b) Raoult’s Law of miscible liquid-liquid solution.
For ideal solution the partial vapour pressure is directly proportional to their mole fraction at constant
temperature. For two components A and B in liquid solution.
Most of the solutions show appreciable deviations from ideal behaviour known as real or non ideal solution.
The characteristic property of these solutions is just the opposite to that of ideal solutions. In some cases
the deviation is +ve while in some cases deviation is -ve.
IDEAL AND NON-IDEAL SOLUTIONS
The solutions which obey Raoult’s law are ideal solutions and those which do not obey Raoult’s law form non-deal solution.
A colligative property of a solution is one that depends on the number of particle dissolved in it.
(a) Relative lowering of V.P. : The relative lowering in V.P. of an ideal solution is equal to the mole fraction of solute at that temperature.
Determination of molecular masses by relative lowering in vapour pressure.
(b) Osmotic pressure : The excess pressure which must be applied on a solution to prevent the passage
of solvent into it through a semipermeable membrane.
Semipermeable membrane → egg membrane;
Chemical Semipermeable membrane → cupric ferrocyanide.
(c) Elevation in boiling point : The property of rise in boiling point when some non volatile solute is added.
We know that the vapour pressure of the solution of the lower than that of the pure solvent and vapour pressure increases with increase in temperature. Hence the solution has to be heated more to make the vapour pressure equal to the atmospheric pressure.
Alternatively, the elevation in boiling point may be explained on the bais of plots of vapour pressure versus temperature as follows :
Vapour pressure of the solvent increases with increase in temperature as shown by the curve AB. As at
any temperature, vapour pressure of the solution is less than that of the solvent, the curve for the solution lies below that of the solvent, as shown by the curve CD. The temperatures at which the vapour pressure
of the solvent and the solution become equal to the atmospheric pressure are T0 and T respectively.
Obviously T > T0
Molal elevation constant or ebulioscopic constant, kb . It is the decrease in boiling point when the molality of the
solution is unity.
(d) Depression in freezing point : The property of decrease in freezing point when some non-volatile solute is dissolved. The depression in freezing point is given by △Tf.
Freezing point : Temperature at which the liquid and the
solid forms of the same substance are in equilibrium and hence have same vapour pressure.
We know that vapour pressure of the solution is less than that of the pure solvent. As freezing point is the
temperature at which the vapour pressure of the liquid and the solid phase are equal, therefore for the solution, this will occur at lower temperature (lower the temperature lower the vapour pressure).
The graph explains this.
Molal depression constant. or cryoscopic constant (kf). It is the decrease in freezing point when the molality of solution is unity.
Kb and Kf are intensive property of solvent and doesnot depend upon solute or solution.
ABNORMAL MOLECULAR MASS AND van’t HOFF FACTOR (i)
Relation between degree of association or dissociation, & van’t Hoff’s factor (i).
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