 Solubility
The maximum amount of solute dissolves in given amount of the solvent at
particular temperature is called solubility.(1mark)
Solubility of a solid in a liquid
When a solid solute is added to solvent, solute particles get mixed with solvent
and are called dissolution. As the concentration of the solution increases, some
solute particles get separated out from solution, the process is called
crystallization.
At certain stage, rate of dissolution and crystallization become equal hence there
is dynamic equilibrium between the solid solute and the solute particles in the
solution phase.
SOLUTE + SOLVENT
Dissolution
SOLUTION
Crystallization
At this stage, concentration of solute in solution remains constant. So that no
more solute dissolves in the solvent, such solution is called saturated solution.
Saturated solution
A solution in which no more solute can be dissolved at the same temperature
and pressure.
Unsaturated solution
A solution in which some more solute can be dissolved at the same temperature
and pressure.
Factors affecting the solubility of solid in a liquid
1. Nature of solute & solvent 2.Temperature 3.Pressure(2mark)
1. Nature of solute and solvent
Solubility is based on the principle “Like dissolves like”
1.Polar(ionic)solutes dissolves in polar solvents
Ex: NaCl, KCl, KNO3 dissolves in water
2. Non-polar solute dissolves in non-polar solvents
Ex: naphthalene, camphor, anthracene dissolves in benzene, toluene, ether etc
NaCl does not dissolve in benzene. Since NaCl is polar & benzene is non-polar.
2. Effect of temperature
Effect of temperature can be explained by Le-Chatelier’s principle. Dissolution
process may be exothermic or endothermic.
If the dissolution is endothermic (∆H=+ve), then its solubility increases with
increase in temperature.ie forward reaction is favoured.
(2mark)
Ex: NaCl, KNO3, KCl, NH4Cl dissolves in water
If the dissolution is exothermic (∆H=-ve), then its solubility decreases with
increase in temperature. ie backward reaction is favoured.
(2mark)
Ex: Solubility of sodium carbonated, lithium sulphate dissolves in water.
3. Effect of pressure(1mark)
There is no effect of pressure on the solubility of a solid in a liquid solvent
(because solids & liquids are in-compressible)
Solubility of gas in liquid
Gases dissolved in liquids form homogeneous solution.
Ex: CO2 dissolves in water to form soda water.
Factors affecting solubility of gas in liquids
1. Nature of gas & liquid
Polar gases dissolve in polar liquids
Ex: NH3, SO2, HCl, CO2 dissolves in water
Non-polar gases dissolve in non-polar liquids
Ex: H2, Cl2, N2, O2 dissolves in benzene, toluene
2. Effect of temperature(2marks)
Generally solubility of all gases in liquids decreases with increase in temperature
because it is an exothermic process (∆H=-ve), then according to Le-Chatelier’s
principle the backward reaction is favoured. This is because increase in
temperature increases the kinetic energy of dissolved gas molecules. As result,
the gas molecules escape from solution phase.
Ex: solubility of oxygen in water decreases with increase in temperature, so that
aquatic species are more comfortable in cold water than in warm water.
3. Effect of pressure(1mark)
Solubility of gas liquid increases with increase in pressure.
Henry’s law(2mark)
This law states that “at constant temperature, the solubility of gas in
liquid is directly proportional to partial pressure of the gas present
above the surface of the liquid or solution”. OR
It also states that” at constant temperature, the partial pressure of gas
in liquid is directly proportional to mole fraction of a gas in liquid”.
xαp or
pαx
p=kHx
Where P= partial pressure of gas
(Mathematical form)
kH= Henry’s constant
x=Mole fraction of gas(solubility)
Significance of kH (1mark)
kH=p
x
Higher the value of kH lower is the solubility and vice- versa
Applications of Henry’s law(2marks)
1) In the production of carbonated beverages-in order to increase the
solubility of CO2 in cold drinks, beer etc, the bottle are sealed under
high pressure. When the bottle is opened under normal atmospheric
pressure, the pressure inside the bottle falls to atmospheric pressure &
the excess CO2 bubbles out of the bottle causing effervescence.
2) At high altitudes-the partial pressure of oxygen at high altitudes is
less than the ground level. These results in low concentration of
oxygen in the blood & tissues of the peoples leads to a disease called
anoxia.
3) In scuba diving- during scuba diving, when the diver breaths in
compressed air from the supply tank,more nitrogen dissolves in the
blood & other body fluids because the pressure at that depth is far
greater than the surface atmospheric pressure. Hence nitrogen is
mixed with helium as it is less soluble in blood than nitrogen.