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Introduction |
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A Solution is a homogeneous mixture of two or more chemically non-reacting substances called components. The composition of the components can be varied with in certain limits. |
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Types of Solutions |
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A solution can be solid, liquid or a gas depending upon the physical state. The various types of solutions are:
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Concentration of a Solution |
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The mass percentage of a component in a given solution is the mass of the component per 100g of the solution. For e.g., if WA is the mass of the component A, WB is the mass of the component B in a solution. |
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Strength of a Solution |
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Strength of a solution is defined as the amount of the solute in gms, present in one litre of the solution. It is expressed as gL-1. |
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Solubility of Gases |
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Gases are completely miscible with each other. Gases also dissolve in liquids and solids. |
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Solid Solution |
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Solid solutions are formed by mixing two solid components. Alloys (mixtures of metals) are the examples of solid solutions. |
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Solubility of Solids in Liquids |
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The extent to which a solute is soluble in a solvent is expressed in terms of solubility. Solubility of a substance may be defined as the amount of solute dissolved in 100gms of a solvent to form a saturated solution at a given temperature. |
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Vapor Pressure of Liquids |
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Vapour pressure is defined as the partial pressure exerted by the vapors above the liquid surface in equilibrium with liquid at a given temperature. |
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Vapor Pressure of Solutions |
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Let us suppose a small amount of a non-volatile solute (sugar) is dissolved in a solvent (water). When evaporation of this solution takes place, only water evaporates as the other component (i.e., sugar) is non-volatile. The vapor pressure of the solution is found to be less than that of the pure solvent. The lowering of vapor pressure in this case can be explained on the basis of reduced tendency of the solvent molecules to go into vapor phase. |
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Vapor Pressure and Raoult's Law |
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In 1886 the French chemist Francois Marie Raoult carried out a series of experiments to study the vapor pressure of a number of binary solutions containing non-volatile as well as volatile non electrolyte solutes. On the basis of the results of the experiment he stated a law known as Raoult's law. |
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Ideal Solution |
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An Ideal solution is the one which obeys Roault's Law at all concentrations and temperature and during the formation of which no change in enthalpy and volume takes place. |
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Solutions With Negative Deviation |
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In this type of deviation, the partial vapor pressure of each component of solution is less than the vapor pressure as expected according to Raoult's law. |
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Solutions With Positive Deviation |
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These types of solutions where DH of mixing is >0 and DV of mixing >0 are found to exhibit positive deviation from Raoult's law. Here solute-solute interactions and solvent-solvent interactions are greater than solute - solvent interactions. |
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Azeotropic Mixtures |
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In case of solutions showing positive deviation from Raoult's law, at one of the intermediate compositions, the total vapor pressure is the highest and the boiling point is the lowest. Similarly in case of solutions showing negative deviations from Raoult's law, at one of the intermediate compositions, the vapor pressure is the lowest and boiling point is the highest. For such solutions the composition in the liquid and the vapor phase is the same. In other words such solutions vaporizes without any change in the composition i.e., they behave as pure liquid. |
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Colligative Properties |
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Certain properties of solutions depend only on the number of particles of the solute (molecule or ions) present in definite volume of the solution and do not depend on the nature of solute. Such properties are called colligative properties. |
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Colligative Properties - Depression in Freezing Point |
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Freezing point of a substance is defined as the temperature at which the vapor pressure of the solid form is in equilibrium with liquid form of the substance. In other words, solid and liquid forms of the substance has same vapor pressure. |
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Colligative Properties - Osmotic Pressure |
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The excess pressure that must be applied to the solution/conc.solution to prevent osmosis. |
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Tonicity of Solutions |
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Cell walls in living organisms are seldom perfectly semipermeable. Usually they permit passage to some of the simpler substances and then these substances fail to register an osmotic pressure contribution on the cell walls. The reduced osmotic pressure of a solution operating on a particular cell membrane is called the tone of the solution with respect to such membrane. |
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Reverse Osmosis and Water Purification |
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The osmosis process can actually be reversed if a pressure larger than the osmotic pressure be applied to the solution. This phenomenon has found uses in water purification, especially the desalination of sea water. |
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Abnormal Molecular Masses |
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The relation between colligative property and molecular mass of the solute is based on the following assumptions:
i) The solution is dilute so that Raoult's law is obeyed.
ii) The solute neither undergoes dissociation nor association in solution. |
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Association of Solute Molecules |
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Certain solutes in solution are found to associate. This eventually leads to a decrease in the number of molecular particles in the solution. |
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Dissociation of Solute Molecules |
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A number of electrolytes dissociate in solution to give two or more particles (ions). Therefore, the number of solute particles, in solution of such substances is more than the expected value. Accordingly, such solutions exhibit higher values of colligative properties. |
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Conclusion |
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Solutions are homogeneous mixtures having great importance in our day to day life. In this chapter we have learnt the various types of solutions, the meaning of vapor pressure, Raoult's Law, Colligative Properties and Abnormal Molecular Masses. |
