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Chemistry: Solutions and Colligative properties

Topics are arranged as per SYLLABI published by Maharashtra State Board of Secondary and Higher Secondary Education.

Solution is a mixture of two or more components, depending on the sizes of the particles of component of the solution.

Homogenous Solution Heterogeneous solution
Solute & Solvent form uniform homogeneous one phase. Solution consists of two or more phases.
A true Solution is Homogenous Solution. A Coarse Mixture and Colloidal Dispersion is Heterogeneous Solution.
Ex: Solution of NaCl in Water. Ex: Colloidal Solution of Starch.
Solution consists of a solvent and a solute. Based on the physical state of Solute and Solvent, types of solutions are mentioned below:

Mass percentage Volume percentage Mass by volume percentage Parts per million Mole fraction Molarity Molality
w/w% Commonly used in Industrial Chemical Applications. For example, commercial bleaching solution contains 3.62 mass percentage of sodium hypochlorite in water. V/V% Solutions containing liquids are commonly expressed in this unit. For example, 10% ethanol solution in water means that 10 mL of ethanol is dissolved in water such that the total volume of the solution is 100 mL. w/V commonly used in medicine and pharmacy. It is the mass of solute dissolved in 100 mL of the Solution. ppm When a solute is present in trace quantities, it is convenient to express concentration in parts per million (ppm). The concentration of pollutants in water or atmosphere is often expressed in terms ppm or Solutions and Colligative properties x Mole fraction unit is very useful in relating some physical properties of solutions, say vapour pressure with the concentration of the solution and quite useful in describing the calculations involving gas mixtures. M Number of moles of a solute present in one litre of the solution. M Number of moles of a solute dissolved in 1kg of a solvent.
Temperature independent. Useful in special applications. Adv: Used in Stoichiometry. Measure by volume Temperature Independent. Useful in special applications.
Measured by Mass. Must know density to convert to molarity. Disadvantage: Temperature-Dependent. Must know density to find solvent mass. Measure by Mass. Must know density to convert to molarity.
Mass percentage. Volume percentage. Mass by volume percentage. Parts per million. Mole fraction. Molarity. Molality.
Independent of temperature. Depends on temperature. Depends on temperature. Independent of temperature. Independent of temperature. Depends on temperature. Independent of temperature.
Mass Percentage Solutions and Colligative properties
Volume Percentage = Solutions and Colligative properties
Parts Per Million = Solutions and Colligative properties
Mole Fraction of a component Solutions and Colligative properties
Molarity Solutions and Colligative properties
Molality Solutions and Colligative properties
For different solutions Same Molar Concentration Different Non-electrolyte Solutes The colligative Properties have the same value for all
For different molar concentrations Of the same solute The colligative property has greater value for more concentrated solution.
For solutions of different solutes Having same % strength The colligative property has greater value for the solute having least molecular weight.
  • Gases are soluble in water and other liquids and their solubility depends upon the nature of the gases.
  • Non-polar gases like oxygen, have less solubility in polar solvents.
  • Polar gases like CO2, NH3 etc are more soluble in polar solvent like water.
  • Solubility of gases in liquids increase with the increase in pressure and the decrease in temperature.
  • All gases are soluble in water and other liquids.
  • According to Charles Law, the volume of a given mass of a gas increases with the increase in temperature at constant pressure.
  • Volume of Gas dissolved increases with increase in temperature.

  • Solid solution of two or more metals; or metal with one or more non-metals is called solid solution or Alloy.
  • Alloy may or may not be in solid state.
  • In the solid solution, the properties of the metals are improved.
  • A alloy of metal or metals with mercury is called an amalgam. (It may be solid or liquid depending upon the content of mercury in the alloy).
  • Duralumin is alloy of aluminum, copper, magnesium and manganese. It’s light and durable; it is used in the manufacture of aircraft.

The pressure exerted by the vapours of a liquid which are in equilibrium with it at a given temperature is called vapour pressure. Vapour Pressure variation with temperature is given as Solutions and Colligative properties
Solutions and Colligative properties and Solutions and Colligative properties are vapour pressures at temperature T1 and T2 respectively. Solutions and Colligative properties is heat of vaporisation.

Raoult’s law for the solutions of liquids in liquids."The equilibrium vapour pressure of a volatile solute is linearly proportional to the mole fraction of that component in liquid phase".
Solutions and Colligative properties and Solutions and Colligative properties
Where, A and B are volatile solute and solvent respectively.
Solutions and Colligative properties and Solutions and Colligative properties are the vapour pressure in pure state.
If Solutions and Colligative properties and Solutions and Colligative properties are the mole fractions of the components A and B respectively in the vapour phase then
Solutions and Colligative properties
Solutions and Colligative properties
Raoult’s law for solutions of solids in liquids ie for non-volatile solutions Solutions and Colligative properties
According to Raoult’s law equimolar solutions of all the substances in the same solvent will show same colligative property ie equal elevation in boiling points as well as equal depression in freezing point.
The solution containing n-heptane and ethanol shows non-ideal behaviour with positive deviation from Raoult’s law. This is because the ethanol molecules are held together by strong H-bonds, however the forces between n-heptane and ethanol are not very strong, as a result they easily vaporise showing higher vapour pressure than expected.

  • Freezing point is the temperature at which the liquid and the solid states of a substance are in equilibrium with each other or it may be defined as the temperature at which the liquid and the solid states of a substance have the same vapour pressure.
  • It is observed that the freezing point of a solution is always less than the freezing point of the pure solvent. Thus the freezing point of sea water is low than that of pure water.
  • The depression in freezing point of a solvent is the difference in the freezing point of the pure solvent and the solution.
  • Depressed freezing point of water reduce the temperature of the formation of ice.
  • Depression in freezing point is determined by Beckmann’s method and Rast’s camphor method. Study of depression in freezing point of a liquid in which a non-volatile solute is dissolved in it is called as cryoscopy.

  • The flow of solvent from pure solvent or from solution of lower concentration into solution of higher concentration through a semi-permeable membrane is called Osmosis.
  • Osmotic pressure method is the best method for determination of molecular masses of polymers because for polymer solutions observed value of any other colligative property is too low to be measure accurately.
  • Osmotic behaviour of solutes undergoing association or dissociation is equal to molarity X number of particles produced per formula unit of solute, known as osmolarity.
Exo-Osmosis Endo-Osmosis Reverse Osmosis
The outward osmotic flow of water from a cell containing an aqueous solution through a semi-permeable membrane. The inward flow of water into the cell containing an aqueous solution through a semi-permeable membrane. If a pressure higher than osmotic pressure is applied on the solution, the solvent will flow from the solution into the pure solvent through the semi-permeable membrane.

Solutions having equal molar concentration and hence equal osmotic pressure are called isotonic (or isosmotic) solutions.
  • A 0.91% solution of pure NaCl is isotonic with human Red Blood Corpuscles (RBC).
  • A NaCl solution with concentration < 0.91% is called hypotonic and RBC will swell and even burst in the solution.
  • A NaCl solution with concentration > 0.91% is called hypertonic. RBC shrink in this solution i.e. undergo plasmolysis.

Experimental measurement of osmotic pressure is best carried out by Berkeley and Hartley method. [First experiment was carried by Abbe Nollet in 1748]

Colligative properties such as freezing point depression or boiling point elevation can be used to calculate the molecular weight of a soluble solid. To complete this calculation, the mass of solute and solvent must be known as well as the freezing points/boiling points of the pure solvent and the solution.

Molecular masses can be calculated by measuring any of the colligative properties. The relation between colligative properties and molecular mass of the solute is based on following assumptions.
(1) The solution is dilute, so that Raoult’s law is obeyed.
(2) The solute neither undergoes dissociation nor association in solution.
When the molecular mass of a substance as determined by using colligative properties doesnot come out to be same as expected theoretically, then it is said to show abnormal molecular mass.
Abnormal molecular mass is obtained when the substance in the solution undergoes dissociation or association.
Dissociation results in the increase in the number of particles hence increase in the value of colligative property in the value of colligative property and decrease in molecular mass as all the colligative properties are inversely proportional to molecular mass. Association results in the decreases in the number of particles and hence decrease in the value of colligative property and increase in the molecular mass.
Association results in the decreases in the number of particles and hence decrease in the value of colligative property and increase in the molecular mass.

  • In 1880 Van’t Hoff introduced a factor I , known as the Van’t Hoff factor, to account for the extent of dissociation or association.
  • Van’t Hoff factor can be defined as the ratio of observed colligative property to normal or theoretical colligative property.
  • It is denoted by the letter 'i'.
  • This factor was introduced to calculate the extent of association or dissociation. Solutions and Colligative properties
  • Van’t Hoff factor (i) > 1 for solutes undergoing dissociation and it is <1 for solutes undergoing association.

  • Normality is the number of milliequivalents of the solute / cc of the solution.
  • Molarity is the number of millimoles of the solute / cc of the solution.
  • Mole Fraction of more volatile component is always greater in the vapour phase than in the solution phase. This is according to a rule known as Konowaloff’s Ruel.
  • Babbitt metals is an alloy of antimony with tin and copper used in machine bearing as antifriction material.
  • Spiegeleisen alloy containing 5-20% Mn in IRON and ferromagnetic alloy containing 70-80% Man and 30-20% iron are used to manufacture rails.
  • Manganin alloy containing 84% Cu, 12% Mn and 4% Ni used in making instruments for electrical measurements since the temperature coefficient of electrical resistance.
  • In case of non-electrolytes (glucose, sugar, urea, etc.) the value of colligative property depends directly on the concentration. If the substance is an electrolyte (NaCl,BaCl2) it dissociates to give two or more ion. Then the value of the colligative property depends upon the concentration of ions.
  • Nernst Distribution Law: A solute distributes itself between two immiscible solvents in such a way that the ratio of the concentrations of the solute in the two solvents is constant at constant temperature provided that the solute does not undergo any dissociation or association in any of the solvents.
  • If two non-reacting solutions of different normalities are mixed, the normality of the final solution can be calculated using the relation: Solutions and Colligative properties.
  • In osmosis, there is a net flow of solvent from solvent to solution or form less concentrated solution to a more concentrated solution through a semi-permeable membrane.
  • Ethylene glycol is usually added to water in the radiator to lower its freezing point. It is called anti-freeze.
  • Demal (D) is another unit for expressing the concentration of a solution. It is equal to molar concentration at 0oC i.e. 1 D represents one mole of the solute present in one litre of the solution at 0o.
  • The rupture of a cell due to inflow of solvent into it is called hemolysis whereas shrinking of the cell due to outflow of the solvent is called crenation or plasmolysis.
  • Hygroscopic (do not change their state ex: glycerol) and deliquescent (dissolve into it ex: NaOH) substances both absorb moisture from the air.
  • The dissolution can be exothermic or endothermic. The effect of temperature on solubility can be predicted by applying Le Chatelier’s principle.
    • Endothermic: NaNO3, KNO3, NaCl, KCl etc dissolve with the absorption of heat. Their solubility increases with increase in temperature.
    • Exothermic: Li2CO3; Na2CO3; H2O etc dissolve with evolution of heat. Their solubility decreased with increase in temperature.
  • The plots of solubility vs temperature is called solubility curves. For Li2CO3; KNO3 etc. it is continues. However in some cases like Na2SO4.10H2O the solubility curves are discontinuous. This is because at transition temperature, one form of the crystalline solid changes into another form and at that point the trend of solubility changes.
  • There are two types of solutions in which both the components are solids:
    • Substitutional Solid solutions: Atoms, ions or molecules of one substance take the place of the other substances in the crystal lattice. Ex: Brass, bronze, steel, etc.
    • Interstitial Solid Solutions: Atoms, ions or molecules of one substance enter into the voids or interstices of the host lattice. Ex: tungsten carbide, etc.
  • Solubility of a gas (mass dissolved) in given volume of liquid at a particular temperature is directly proportional to the pressure of the gas above the liquid. This is called Henry’s Law.
  • The solubility of the gas in a liquid increased with increase of pressure of decrease of temperature.
  • Ionic strength of a solution is a measure of the electrical intensity due to presence of ions in the solution.
  • Mole fraction of the more volatile component is always greater in the vapour phase then in the solution phase.