COLLIGATIVE PROPERTIES OF DILUTE SOLUTIONS

 A dilute solution is one in which the amount of the solute is very small in comparison to the amount of the solvent.The dilute solutions show more or less ideal behavior as the heat and volume changes, accompanying the mixing of solute and solvent, are negligible for all practical purposes.

Dilute solutions obey Raoult’s law.

 Dilute solutions containing non-volatile solute exhibit some special properties which depend only upon the number of solute particles present in the solution irrespective of their nature. These properties are termed as colligative properties. The colligative properties are:

 (i) Lowering in the vapour pressure,

 (ii) Elevation in the boiling point,

 (iii) Depression in the freezing point, and

 (iv) Osmotic pressure

Colligative properties are the properties of dilute solutions, that is why these are termed as colligative properties of dilute solutions. These properties are related to one another. Thus, if one is measured, the other can be calculated. The importance of these properties lies in the fact that they provide methods for the determination of molecular masses of dissolved solutes. The results are excellent if the following g three conditions are satisfied.

 (i) The solution should be very dilute.

 (ii) The solute should be non-volatile

 The solute does not dissociate or associate in solution

COMPARISON BETWEEN IDEAL AND NON-IDEAL SOLUTIONS

Ideal solutions	Non-ideal solutions
	Positive deviation from Raoult’s law	Negative deviation from Raoult’s law

1.Obey Raoult’s law at every range of concentration. 2.?Hmix = 0; neither is evolved nor absorbed during dissolution. 3.?Vmix = 0; total volume of solution is equal to sum of volumes of the components. 4.P = pA + pB = pA0XA + pB0XB    i.e., pA = 5.A—A, A—B, B—B interactions should be same, i.e., ‘A’ and ‘B’ are identical in shape, size and character. 6. Escaping tendency of ‘A’ and ‘B’ should be same in pure liquids and in the solution. Examples: dilute solutions; benzene + toluence: n-hexane + n-heptane; chlorobenzene + bromobenzene; n-butyl chloride + n-butyl bromide.

1.Do not obey Raoult’s law. 2.?Hmix>0. Endothermic dissolution; heat is absorbed. 3.?Vmix > 0. Volume is increased after dissolution. 4.pA > pA0XA; pB > pB0XB ∴ pA + pB > pA0XA + pB0XB 5.A—B attractive force should be weaker than A—A and B—B attractive forces. ‘A’ and ‘B’ have different shape, size and character. 6. ‘A’ and B’ escape easily showing higher vapour pressure than the expected value. Examples: acetone + ethanol acetone + CS2; water + methanol; water + ethanol; CCl4 + toluene; CCl4 + CHCl3; acetone + benzene; CCl4 + CH3OH; Cyclohexane + ethanol

1.Do not obey Raoult’s law. 2.?Hmix<0. Exothermic dissolution; heat is evolved. 3.?Vmix <0. Volume is decreased during dissolution. 4.pA < pA0XA; pB < pB0XB ∴ pA + pB < pA0XA + pB0XB 5. A—B attractive force should be greater than A—A and B—B attractive forces. ‘A’ and ‘B’ have different shape, size and character. 6. Escaping tendency of both components ‘A’ and ‘B’ is lowered showing lower vapour pressure than expected ideally. Examples: acetone + aniline; acetone + chloroform; CH3OH + CH3COOH; H2O + HNO3; Choloroform + diethyl ether, water + HCl; acetic acid + pyridine; chloroform + benzene.

RAOULT’S LAW

 According to this law, the partial pressure of any volatile constituent of a solution at a constant temperature is equal to the vapour pressure of pure constituent multiplied by the mole fraction of that constituent in the solution.

 Let a mixture (solution) be prepared by mixing nA moles of liquid A and nB moles of liquid B. Let pA and pB be the partial pressures of two constituents A and B in solution and pA0 and pB0 the vapour pressures in pure state respectively.

 Thus, according to Raoult’s law,

  pA = nA/nA+nB pA0 = mole fraction of A × pA0 = XApA0

 And pB = nB/nA+nB pA0 = mole fraction of B × pB0 = XBpB0

 If the total pressure be P, then

 P = pA + pB

  = nA/nA+nB pA0 + nB/nA+nB pA0

  = XAPA0 + XBPB0

 This law, in fact, is the major deciding factor, whether a solution will be ideal or non-ideal. Ideal solutions obey Raoult’s law at every range of concentration. Non-ideal solutions do not obey Raoult’s law. They show either positive or negative deviation from Raoult’s law. for comparison between ideal and non-ideal solutions a table has been given on next page. (Only binary combinations of miscible liquids have been considered.)

TYPES OF SOLUTIONS

All the three states of matter (gas, liquid or solid) may behave either as solvent or solute. Depending on the state of solute or solvent, mainly there may be the following seven types of binary solutions.

S.No.

Solute

Solvent

Example

1. 2. 3. 4. 5. 6. 7.

Gas Gas Gas Liquid Liquid Solid Solid

Gas Liquid Solid Liquid Solid Liquid Solid

Air Aerated water (CO2 + H2O) Hydrogen in palladium Alcohol in water, benzene in toluene Mercury in zinc amalgam Sugar in water, common salt in water Various alloys

TYPES OF SOLUTIONS

1. True solutions: True solutions are homogeneous system and have the size of dispersed particles less than 1 nm, i.e. 10-9. The particles of solute present in a true solution are either single molecules or irons and are homogeneously distributed throughout the solutions. These particles are invisible and cannot be seen even with a microscope. Due to very small size of dispersed particles, true solutions pass through ordinary filter paper as well as through animal membranes. Sodium chloride, sugar, urea etc. form true solutions in water. 

2. Colloidal solutions: Colloidal solutions are heterogeneous systems and have the size of dispersed particles lying between 1 nm-1000 nm (i.e. 10-9-10-6m). The particles in a colloidal solution are thus larger particles and are referred to as colloidal particles. Although colloidal particles are larger in size, yet they are not large enough to be seen with naked eye. However, they can be seen with the help of an ultra microscope. Colloidal solutions can pass through ordinary filter paper but not through an animal membrane, Gum Arabic, gelatin, glue etc. form colloidal solutions when dispersed in water.

3. Suspensions: Suspensions are also heterogeneous system and have still larger particles. The size of particles present in a suspension is more than 1000 nm (i.e., >10-6m). These particles are either visible to naked eye or can be seen under a microscope. The suspensions neither pass through an animal membrane nor through an ordinary filter paper. Stirred muddy water is an example of suspensions.