Mastering Solutions at a glance:
S O L U T E
SOLVENT SOLID LIQUID GAS
SOLID Alloys Silver amalgam / jellies Hydrogen adsorbed in palladium
LIQUID Salt in water Alcohol in water Aerated drinks
GAS Camphor in air Moisture in air Mixture of gases
Units used to express the strength of solution
Molarity – It is the number of moles of solute present in one litre of the solution.
Molality - It is the number of moles of solute present in one Kilogram of the solvent.
Mole fraction – It is the ratio of the number of moles of one component to the total number of moles of all
the components in the solution.
Normality – It is the number of gram equivalents of the solute present in one litre of the solution.
PPM – It is the number of parts of a component per million parts of the solution.
Formality – It is the number of gram formula weight of the ionic solute dissolved per litre of the solution.
Interconversion formulae:
Molarity = Normality x density = Molarity { + }
Solubility of solids in liquids:
Solubility of solids in liquids depends upon
- Nature of the solute and solvent - temperature.
[Polar substances dissolve in polar solvents and non-polar substances dissolve in non polar solvents.]
Network crystals (diamond), the atoms are held by covalent bonds and are insoluble in all liquids.
Molecular crystals, the molecules are held by weak vander Waals interactions which are soluble in non-polar
solvents. Example: Solid iodine composed of non polar I2 molecules is soluble in CCl4 and slightly soluble in water.
Highly polar solutes and ionic solids are not soluble in non-polar solvents.
Reason: The weak solute-solvent interactions, compared to solute- solute interactions, are not sufficient to
break the crystal lattice.
Ionic solids - The force of attraction between the oppositely charged ions in a crystal give rise to LATTICE
ENERGY (which opposes the tendency of a solute to dissolve).
Larger the lattice energy of the crystal of a solute, the smaller is its solubility.
If ‘F’ is the force of attraction between oppositely charged ions separated by a distance ‘r’ and D is the dielectric
constant of the medium.
The force of attraction increases with increase in the charge.
Force of attraction increases
Uni- univalent electrolyte----bi univalent electrolyte---bi – bivalent electrolye
NaCl BaCl2 BaSO4
For an ionic solid to dissolve, an extra energy is needed to overcome the force of inter-ionic attraction.
If the solvent is water, the ion –dipole interaction is called hydration. During hydration, energy released is
Called Hydration energy.
∆Hsolution = ∆Hlattice + ∆HHydration , the compound dissolves if ∆H = - ve.
Effect of temperature:
Solubility of solid in liquid depends upon the heat of solution of the solute and the nature of the solute and solvent.
Ionic substances ( KCl, KBr,KNO3,NaNO3, NH4Cl, NH4NO3) when dissolved in water involve absorption of
heat—ENDOTHERMIC—solubility of solute increases with increases in temperature.
Ionic substances (Na2CO3, CaO,Li2CO3 ,Ce2(SO4)3,Na2SO4) when dissolved in water involve evolution of
heat --- EXOTHERMIC--- solubility of solute decreases with increase in temperature.
Sodium Chloride dissolves in water with very small absorption of heat. Solubility of sodium chloride is
almost constant with rise in temperature.
For a Plot of solubilities of solute against temperature, the curves obtained are called solubility curves.
The solubility curves are continuous exception to sodium sulphate. The solubility of sodium sulphate
increases with increase in temperature up to 32.4ᴼC and then decreases with increase in temperature.
At 32.4ᴼC, hydrated sodium sulphate changes into anhydrous sodium sulphate. The temperature at
which one form of the substance changes into another form is called TRANSITION TEMPERATURE.
Effect of Solubility of gases in liquid:
Solubility of a gas in a solvent varies with the nature of the gas.
Gases which can be liquefied easily (CO2, NH3 and HCl) are more soluble in common solvents.
Gases which are capable of forming ions in aqueous solutions are more soluble in water than in
other solvents. Example HCl and NH3 are more soluble in water than in benzene.
At constant pressure, the solubility of a gas decreases with increase in temperature.
[Solubility of some sparingly soluble gases, such as H2 and inert gases increases slightly with
increase in temperature in non-aqueous solvents such as alcohol, acetone and hydrocarbons.
Solubility of gases always increases with increase in pressure.
Henry’s law: The mass of a gas dissolved per unit volume of a solvent is proportional to the pressure
of the gas in equilibrium with the solution at constant temperature.
If m is the mass of a gas dissolved per unit volume of a solvent.
P is the pressure of the gas in equilibrium with the solution
m α p or m = kp , K = proportionality constant.
Also, Henry’s law can be defined as
The pressure of a gas over a solution in which the gas is dissolved is proportional to the mole
fraction of the gas dissolved in solution.
If ‘x’ is the mole fraction of the gas in the solvent and ‘p’ is the partial vapour pressure, then
‘x’ α ‘p’ or ‘x’ = K ‘p’ or p = or p = KHx , KH is called Henry’s law constant
‘
Different gases have different values of KH at the same temperature.
Greater the value of KH at a given pressure, the lower is the solubility of the gas in
the liquid.
Limitations of Henry’s Law.
The temperature and pressure of the gas should be moderate.
The gas should not undergo association or dissociation in the solvent.
The gas should be sparingly soluble in the solvent.
Applications of Henry’s law:
Carbonated beverages are bottled under pressure to ensure high concentration of carbon dioxide.
The dissolved gas adds acidic flavour to soft drinks.
The cylinders carried by scuba divers are filled with air diluted with helium.
Reason: When scuba diver swim back to the surface of water. Nitrogen and oxygen dissolved in the
blood at high pressure are suddenly released in the form of bubbles in the blood vessels. To reduce
the toxic effects of high concentrations of nitrogen in the blood. The cylinders contain helium.
At higher altitudes, the partial pressure is low and the concentration of oxygen in the blood and
tissues in the people residing in high altitudes is low which makes the person weak and unable to
think. This condition is called anoxia.
