Solution and colligative properties

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1. SOLUTION AND COLLIGATIVE PROPERTIES

2. Solution Solutions are homogeneous mixtures of two or more pure substances. One constituent is usually regarded as the SOLVENT and the others as SOLUTES . In a solution, the solute is one which is in smaller quantity. The solvent is one which is in greater quantity.

3. Types of Solution 1 . An unsaturated solution is a solution in which more solute can be dissolved at a given temperature. 2. A saturated solution is a solution containing the maximum amount of solute that will dissolve at a given temperature. 3. A supersaturated solution is a solution that contains more solute than would dissolve in a saturated solution at a given temperature . UNSATURATED SOLUTION more solute dissolves SATURATED SOLUTION no more solute dissolves SUPERSATURATED SOLUTION becomes unstable, crystals form

4. Types of Solution Saturated Solvent holds as much solute as is possible at that temperature. Dissolved solute is in dynamic equilibrium with solid solute particles. Unsaturated Less than the maximum amount of solute for that temperature is dissolved in the solvent . Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

5. Colligative Properties

6. Freezing point depression Elevation in boiling point Lowering of vapour pressure Osmotic pressure

7. Freezing Point Depression in Solutions The freezing point of pure water is 0°C, but that melting point can be depressed by the adding of a solvent such as a salt. The use of ordinary salt (sodium chloride, NaCl) on icy roads in the winter helps to melt the ice from the roads by lowering the melting point of the ice. A  solution  typically has a measurably lower melting point than the pure solvent. The following figures were found in a published report, but have not been checked out in detail. A 10% salt solution was said to lower the melting point to -6°C (20°F) and a 20% salt solution was said to lower it to -16°C (2°F). A more formal treatment of freezing point depression is given by Ebbing. The freezing point depression ΔT f  is a  colligative property  of the solution, and for dilute solutions is found to be proportional to the molal concentration c m  of the solution: ΔT f  = K f c m Where K f  is called the freezing-point-depression constant .

8. Osmotic pressure Osmosis is defined as the flow of water/ solvent   molecules  through a  semipermeable membrane  from a region of low to high  solute   concentration , until  equilibrium  is established.To counter osmotic flow, some  pressure  must be applied to the  solution  in order to prevent pure solvent from going through the semipermeable membrane separating the two liquids; this is known as the  osmotic pressure .The osmotic pressure is the pressure required to counter, not sustain,  osmosis .

9. Boiling Point Elevation The  boiling point  of pure water is 100°C, but that boiling point can be elevated by the adding of a solute such as a salt. A  solution  typically has a measurably higher boiling point than the pure solvent. A treatment of boiling point elevation is given by Ebbing. The boiling point elevation ΔT b  is a  colligative property  of the solution, and for dilute solutions is found to be proportional to the molal concentration c m  of the solution: ΔT b  = K b c m Where K b  is called the boiling-point-elevation constant. Solutions may be produced for the purpose of raising the boiling point and lowering the freezing point, as in the use of  ethylene glycol  in  automobile cooling systems . The ethylene glycol (antifreeze) protects against freezing by lowering the freezing point and permits a higher operating temperature by raising the boiling point.

10. Relative Lowering of Vapour Pressure Vapour pressure is the pressure exerted by the vapours over the liquid under the equilibrium conditions at a given temperature. Now let us take an example of a pure liquid, the surface of the liquid is occupied by the molecules of the liquid. Suppose a non-volatile solute is now added to this pure liquid. Since the solute molecules are non-volatile, the vapour above the solution consists of only solvent (pure liquid) molecules. After adding the solute, the vapour pressure of the  solution  is found to be lower than that of the pure liquid at a given temperature. This lowering in vapour pressure is due to the fact that after the solute was added to the pure liquid (solvent), the liquid surface now had molecules of both, the pure liquid and the solute. The number of solvent molecules escaping into vapour phase gets reduced and as a result the pressure exerted by the vapour phase is also reduced. This is known as relative lowering of vapour pressure. This decrease in vapour pressure depends on the amount of non-volatile solute added in the solution irrespective of its nature and hence it is one of the colligative properties .

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