Class 11 Chemistry Notes Chapter 2 (Chapter 2) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 2 from your Chemistry Lab Manual, which deals with the 'Characterization and Purification of Chemical Substances'. These techniques are fundamental not just for your practical exams but also form the basis for many questions in competitive government exams. Understanding the why and how behind these methods is crucial.

Chapter 2: Characterization and Purification of Chemical Substances - Detailed Notes

1. Introduction

• Most chemical substances, whether synthesized in the lab or obtained from natural sources, are impure.
• Purification: The process of removing unwanted impurities from a substance.
• Characterization: Determining the physical properties (like melting point, boiling point) of a pure substance, which helps in its identification and confirms its purity.
• Purity is essential for studying chemical properties, determining structure, and for various applications (e.g., medicines).

2. Determination of Melting Point (M.P.)

• Principle: The melting point of a solid is the constant temperature at which the solid and liquid phases are in equilibrium at atmospheric pressure. Pure crystalline solids have a sharp and characteristic melting point. Impurities lower the melting point and cause it to melt over a range of temperatures.
• Significance:
  • Identification: Comparing the observed M.P. with known values helps identify the substance.
  • Purity Check: A sharp M.P. (melting within a 0.5-1°C range) indicates purity. A broad melting range indicates impurity.
• Procedure Highlights (Capillary Tube Method):
  • A small amount of finely powdered solid is packed into a thin-walled capillary tube (sealed at one end).
  • The capillary tube is attached to a thermometer.
  • The assembly is suspended in a heating bath (liquid paraffin in a beaker or Thiele's tube). Thiele's tube ensures uniform heating due to convection currents.
  • The bath is heated slowly (1-2°C per minute near the expected M.P.).
  • The temperature range is recorded from when the solid first starts to melt until it completely liquefies.
• Key Points:
  • Packing should be uniform and compact.
  • Heating must be slow and uniform, especially near the melting point.
  • Liquid paraffin is commonly used as it has a high boiling point and is stable.

3. Determination of Boiling Point (B.P.)

• Principle: The boiling point of a liquid is the constant temperature at which its vapour pressure equals the external (atmospheric) pressure. Pure liquids have a sharp and characteristic boiling point at a given pressure. Impurities (usually non-volatile ones) elevate the boiling point. Volatile impurities can raise or lower the B.P. depending on their nature.
• Significance:
  • Identification: Comparing observed B.P. with known values.
  • Purity Check: A constant boiling temperature indicates purity. Boiling over a range suggests impurity.
• Procedure Highlights (Distillation Method / Capillary Method):
  • Distillation Method: Used for larger quantities. Liquid is heated in a distillation flask fitted with a thermometer and condenser. The temperature at which the liquid distils constantly is the B.P.
  • Capillary Method (Siwoloboff's Method): Used for small quantities. A small test tube (ignition tube) containing the liquid and an inverted capillary tube (sealed at the top end) is attached to a thermometer and heated in a bath (similar to M.P. determination). Air trapped in the capillary expands and escapes as bubbles. When heating stops and the bath cools, the temperature at which a continuous stream of bubbles stops and the liquid just begins to enter the capillary tube is the boiling point.
• Key Points:
  • Boiling point depends on external pressure. Lower pressure = Lower B.P.; Higher pressure = Higher B.P. Standard B.P. is measured at 1 atm (760 mmHg).
  • The thermometer bulb should be placed just below the side arm of the distillation flask to measure the temperature of the vapour in equilibrium with the boiling liquid.

4. Crystallization

• Principle: Based on the difference in solubilities of the compound and its impurities in a suitable solvent. The impure compound is dissolved in a minimum amount of hot solvent, in which it is sparingly soluble at room temperature but highly soluble at higher temperatures. Impurities are either insoluble (removed by hot filtration) or highly soluble (remain in the mother liquor upon cooling). On cooling the hot saturated solution, the pure compound crystallizes out.
• Steps:
  1. Choice of Solvent: The solvent should dissolve the compound readily when hot but sparingly when cold. It should not react chemically with the compound. Impurities should be either highly soluble or insoluble in it. It should be easily removable (volatile). Common solvents: water, ethanol, methanol, acetone, ethyl acetate, hexane. Sometimes mixed solvents are used.
  2. Preparation of Solution: Dissolve the impure solid in the minimum amount of hot solvent to get a saturated solution.
  3. Hot Filtration: If insoluble impurities are present, filter the hot solution quickly (using a fluted filter paper and pre-heated funnel helps prevent premature crystallization).
  4. Cooling: Allow the hot filtrate to cool slowly and undisturbed. Crystals of the pure compound separate out. Rapid cooling leads to smaller crystals, slow cooling gives larger crystals.
  5. Separation of Crystals: Separate the crystals from the mother liquor (remaining solution) by filtration (usually using a Buchner funnel under suction for better drying).
  6. Washing: Wash the crystals with a small amount of cold solvent to remove adhering mother liquor.
  7. Drying: Dry the crystals (air drying, oven drying below M.P., or in a desiccator).
• Fractional Crystallization: Used to separate two or more solids with different solubilities in the same solvent.

5. Sublimation

• Principle: Some solids, on heating, directly convert into vapour without passing through the liquid phase. On cooling, the vapour directly solidifies. This process is called sublimation.
• Applicability: Used to separate sublimable compounds from non-sublimable impurities.
• Examples of Sublimable Substances: Iodine, Camphor, Naphthalene, Anthracene, Ammonium chloride (NH4Cl), Benzoic acid.
• Procedure: The impure solid is heated gently in a china dish covered with an inverted funnel (often with its stem plugged with cotton) or a watch glass with holes/covered by wet filter paper. The pure sublimable compound deposits on the cooler inner surface of the funnel/watch glass. Non-volatile impurities remain in the dish.

6. Distillation

• Principle: Based on the difference in boiling points of liquids. The liquid is converted into vapour by heating, and the vapour is then condensed back into liquid by cooling.
• Simple Distillation:
  • Applicability: Used to separate:
    • A volatile liquid from a non-volatile impurity (e.g., salt water).
    • Two liquids with a large difference in boiling points (generally > 25-30°C).
  • Setup: Distillation flask, thermometer, condenser (Liebig condenser is common), adapter, receiving flask.
  • Procedure: The impure liquid is heated in the distillation flask. The vapour of the more volatile component rises, passes into the condenser where it cools and liquefies, and is collected in the receiver. The less volatile component/impurity remains in the flask.
• Other Types (Brief Overview):
  • Fractional Distillation: Used to separate two or more miscible liquids with close boiling points (e.g., ethanol-water, components of crude oil). Uses a fractionating column placed between the flask and condenser to provide surfaces for repeated vaporization-condensation cycles.
  • Steam Distillation: Used to purify liquids which are immiscible with water, volatile in steam, and have high boiling points, but decompose at or below their boiling points (e.g., aniline, essential oils). Steam is passed through the heated impure compound; the mixture of steam and volatile organic compound distils at a temperature lower than the boiling point of either component.
  • Distillation Under Reduced Pressure (Vacuum Distillation): Used to purify liquids that decompose at their normal boiling points (e.g., glycerol). Lowering the external pressure lowers the boiling point.

7. Chromatography

• Principle: A technique used to separate components of a mixture based on their differential distribution between two phases: a stationary phase and a mobile phase.
• Types based on Principle:
  • Adsorption Chromatography: Based on differential adsorption of components onto the surface of a solid stationary phase (e.g., alumina, silica gel). (Column Chromatography, Thin-Layer Chromatography - TLC).
  • Partition Chromatography: Based on differential partitioning (distribution) of components between a liquid stationary phase (often supported on a solid) and a liquid or gaseous mobile phase. (Paper Chromatography, Gas-Liquid Chromatography - GLC).
• Paper Chromatography:
  • Stationary Phase: Water molecules trapped/adsorbed in the cellulose fibers of the chromatography paper.
  • Mobile Phase: A suitable solvent or mixture of solvents (eluent).
  • Principle: Partition Chromatography. Components separate based on differences in their partitioning between the stationary water phase and the mobile solvent phase, and also partly due to adsorption on paper. More soluble components in the mobile phase travel faster/further up the paper.
  • Procedure: A spot of the mixture is applied near one end of the paper strip (baseline). The edge near the spot is dipped into the mobile phase in a closed chamber (to saturate the atmosphere with solvent vapour). The solvent moves up the paper by capillary action, carrying the components along at different rates.
  • Rf Value (Retardation Factor): A characteristic value for a substance under given conditions (solvent, temperature, paper type).
    Rf = (Distance travelled by the substance from the baseline) / (Distance travelled by the solvent front from the baseline)
    Rf value is always less than 1. It helps in identifying components by comparing with known Rf values.
• Thin-Layer Chromatography (TLC): Similar principle to paper chromatography, but the stationary phase is a thin layer of adsorbent (like silica gel or alumina) coated on a glass plate or plastic sheet. Generally faster and gives better separation than paper chromatography.
• Column Chromatography: Stationary phase (adsorbent) is packed in a glass column. The mixture is applied at the top, and the mobile phase (eluent) is allowed to flow down. Components separate into bands and are collected sequentially as they elute from the bottom.

Multiple Choice Questions (MCQs)

1. A pure solid substance is characterized by:
   a) A wide melting point range
   b) A sharp and constant melting point
   c) Melting point lower than the impure sample
   d) Solubility in water

2. In the determination of melting point using a Thiele's tube, uniform heating is achieved due to:
   a) Stirring with a glass rod
   b) Convection currents in the heating liquid
   c) Slow heating rate
   d) Use of a capillary tube

3. Crystallization is based on the principle of:
   a) Difference in boiling points
   b) Difference in adsorption
   c) Difference in solubilities at different temperatures
   d) Difference in sublimation rates

4. Which of the following substances can be purified effectively by sublimation?
   a) Sodium chloride
   b) Sugar
   c) Urea
   d) Naphthalene

5. Simple distillation is used to separate two liquids if their boiling points differ significantly, generally by at least:
   a) 1-2 °C
   b) 5-10 °C
   c) 25-30 °C
   d) 0.5 °C

6. Steam distillation is specifically used for compounds that are:
   a) Soluble in water and volatile
   b) Miscible with water and have low B.P.
   c) Immiscible with water, volatile in steam, and may decompose at their B.P.
   d) Non-volatile and stable at high temperatures

7. In paper chromatography, the stationary phase is:
   a) The paper itself (cellulose)
   b) The mobile solvent
   c) Water molecules adsorbed on the paper
   d) Silica gel coated on paper

8. The Rf value in chromatography is defined as:
   a) (Distance travelled by solvent) / (Distance travelled by solute)
   b) (Distance travelled by solute) / (Distance travelled by solvent)
   c) (Distance travelled by solute) x (Distance travelled by solvent)
   d) (Distance travelled by solvent) - (Distance travelled by solute)

9. Glycerol decomposes at its normal boiling point. Which method is most suitable for its purification?
   a) Simple Distillation
   b) Steam Distillation
   c) Fractional Distillation
   d) Distillation under reduced pressure (Vacuum Distillation)

10. During crystallization, if the hot saturated solution is cooled very rapidly, the crystals formed are usually:
    a) Large and pure
    b) Small and pure
    c) Large and impure
    d) Small and less pure (may trap impurities)

Answer Key for MCQs:

1. b
2. b
3. c
4. d
5. c
6. c
7. c
8. b
9. d
10. d (Rapid cooling often leads to smaller crystals and may trap impurities more easily than slow cooling)

Remember to correlate these lab techniques with the concepts you study in theory, especially regarding intermolecular forces, phase transitions, solubility, and vapour pressure. Good luck with your preparation!