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Notes - General Methods Of Alkyl Halides

General methods of preparation of Alkyl Halides             (1) From alkanes (i) By halogenation: $\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,$ (Excess) +$C{{l}_{2}}\xrightarrow{hv}\underset{\text{Ethyl}\,\text{chloride (Major}\,\text{product)}}{\mathop{{{C}_{2}}{{H}_{5}}Cl}}\,+HCl$ $\underset{\text{Propane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{3}}}}\,\overset{C{{l}_{2}}}{\mathop{\xrightarrow[UV\,light]{}}}\,\underset{1-\text{Chloropropane (45 }\!\!%\!\!\text{ )}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Cl}}\,\,\underset{\text{2- Chloropropane (55 }\!\!%\!\!\text{ )}}{\mathop{\underset{\,Cl}{\mathop{\underset{|}{\mathop{+\ C{{H}_{3}}CHC{{H}_{3}}}}\,}}\,}}\,$ This reaction proceed through free radical mechanism. Note: r Order of reactivity of ${{X}_{2}}$ for a given alkane is, ${{F}_{2}}>C{{l}_{2}}>B{{r}_{2}}>{{I}_{2}}$. r The reactivity of the alkanes follows the order: $3{}^\circ alkane\text{ }>~2{}^\circ alkane\text{ }>~1{}^\circ alkane$. (ii) With sulphuryl chloride: $R-H+S{{O}_{2}}C{{l}_{2}}\overset{hv}{\mathop{\xrightarrow[Organic\,peroxide{{(R'C{{O}_{2}})}_{2}}]{}}}\,R-Cl+S{{O}_{2}}+HCl$               Note: r In presence of light and trace of an organic peroxide the reaction is fast.   (2) From alkenes (Hydrohalogenation) $\underset{\text{But}-\text{2}-\text{ene}}{\mathop{C{{H}_{3}}-CH=CH-C{{H}_{3}}+HBr}}\,\xrightarrow{{}}\underset{\text{2-Bromobutane}}{\mathop{C{{H}_{3}}C{{H}_{2}}-\underset{Br\,\,\,\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,H-}}\,C{{H}_{3}}}}\,\xrightarrow{{}}$Electrophillic addition. Note: r Addition of HBr to alkene in the presence of organic peroxide take place due to peroxide effect or Kharasch's effect.             r This addition take place by two mechanism, Peroxide initiates free radical mechanism. Markownikoff?s addition by electrophillic mechanism. r From alkyne we cannot obtain mono alkyl halide. r The order of reactivity of halogen acids is, $HI>HBr>HCl$.             (3) From alcohols (i) By the action of halogen acids Groove?s process $\underset{\text{Alcohol}}{\mathop{R-OH}}\,+H-X\underset{300{}^\circ C}{\mathop{\xrightarrow{Anhy.\,ZnC{{l}_{2}}}}}\,\underset{\text{Haloalkane}}{\mathop{RX}}\,+{{H}_{2}}O$              Note: r             The reactivity order of $HX$ in the above reaction is: $HI>HBr>HCl>HF$.\ r Reactivity order of alcohols $3{}^\circ >2{}^\circ >1{}^\circ >MeOH$. r $2{}^\circ$and $3{}^\circ$ alcohols undergo $S{{N}^{1}}$; where as $1{}^\circ$ and MeOH undergo $S{{N}^{2}}$mechanism. r Concentrated HCl + anhy.$ZnC{{l}_{2}}$ is known as lucas reagent. (ii) Using $\mathbf{PC}{{\mathbf{l}}_{\mathbf{5}}}$and$\mathbf{PC}{{\mathbf{l}}_{\mathbf{3}}}$: $C{{H}_{3}}C{{H}_{2}}OH+\underset{\begin{smallmatrix} \text{Phosphorus } \\ \text{pentachloride} \end{smallmatrix}}{\mathop{PC{{l}_{5}}}}\,\xrightarrow{{}}\underset{\text{Chloroethane}}{\mathop{C{{H}_{3}}C{{H}_{2}}Cl}}\,+\underset{\begin{smallmatrix} \text{Phosphorus} \\ \text{Oxychloride} \end{smallmatrix}}{\mathop{POC{{l}_{3}}}}\,+HCl$ $3C{{H}_{3}}C{{H}_{2}}OH+PC{{l}_{3}}\xrightarrow{{}}\underset{\text{Chloroethane}}{\mathop{3C{{H}_{3}}C{{H}_{2}}Cl}}\,+\underset{\text{Phosphorus}\,\text{acid}}{\mathop{{{H}_{3}}P{{O}_{3}}}}\,$   Note: r Bromine and iodine derivatives cannot be obtain from the above reaction, because $PB{{r}_{5}}$ or $P{{I}_{5}}$ are unstable. r This method gives good yield of primary alkyl halides but poor yields of secondary and tertiary alkyl halides. (iii) By the action of thionyl chloride  (Darzan's process) $C{{H}_{3}}C{{H}_{2}}OH+SOC{{l}_{2}}\xrightarrow{\text{Pyridine}}C{{H}_{3}}C{{H}_{2}}Cl+S{{O}_{2}}+HCl$   Note: r Reaction takes place through $S{{N}^{2}}$ mechanism.             (4) From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation) (Free radical mechanism)  $R-\underset{O\,\,\,}{\mathop{\underset{||}{\mathop{C}}\,-}}\,O-Ag+Br-Br\underset{\text{Decarboxylation}}{\mathop{\xrightarrow{CC{{l}_{4}}}}}\,R-Br+C{{O}_{2}}\uparrow +AgBr\downarrow$ Note: r The reactivity of alkyl group is $1{}^\circ >2{}^\circ >3{}^\circ$ r Not suitable for chlorination because yield is poor. r In this reaction iodine forms ester instead of alkyl halide and the reaction is called Birnbourn-Simonini reaction, $2R-COOAg+{{I}_{2}}\xrightarrow{{}}RCOO{R}'+2C{{O}_{2}}+2AgI$.             (5) From alkyl halide (Halide exchange method): $R-X+NaI\underset{\text{Reflux}}{\mathop{\xrightarrow{\text{Acetone}}}}\,R-I+NaX(X=Cl,\,Br)$   Note: r Alkyl fluorides cannot be prepared by this method. They can be obtained from corresponding chlorides by the action of $H{{g}_{2}}{{F}_{2}}$or antimony trifluoride. $2C{{H}_{3}}Cl+H{{g}_{2}}{{F}_{2}}\to \underset{\text{Methyl fluoride}}{\mathop{2C{{H}_{3}}F+H{{g}_{2}}C{{l}_{2}}}}\,$             (6) Other method (i) $ROH\xrightarrow{KI,{{H}_{3}}P{{O}_{4}}\,\,\,\,\,\,}$ (ii) $ROH\underset{\text{Rydon method}}{\mathop{\xrightarrow{{{X}_{2}}+{{(PhO)}_{3}}P}}}\,$ (iii) Dihalide $\underset{HCl}{\mathop{\xrightarrow{Zn-Cu\,\,\,\,}}}\,$$R-X$ (iv) $RMgX\xrightarrow{{{X}_{2\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}$

Horizontal Projectile

Horizontal Projectile   A body be projected horizontally from a certain height ?y? vertically above the ground with initial velocity u. If friction is considered to be absent, then there is no other horizontal force which can affect the horizontal motion. The horizontal velocity therefore remains constant and so the object covers equal distance in horizontal direction in equal intervals of time.   (1) Trajectory of horizontal projectile: The horizontal displacement x is governed by the equation $x\text{ }=\text{ }ut\,\,\,\,\Rightarrow \,\,t=\frac{x}{u}$  ?. (i)               The vertical displacement y is governed by $y=\frac{1}{2}g{{t}^{2}}$     ?. (ii) (since initial vertical velocity is zero) By substituting the value of t in equation (ii) $y=\frac{1}{2}\frac{g\,{{x}^{2}}}{{{u}^{2}}}$   Sample problems based on trajectory Problem 66. An aeroplane is flying at a constant horizontal velocity of 600 km/hr at an elevation of 6 km towards a point directly above the target on the earth?s surface. At an appropriate time, the pilot releases a ball so that it strikes the target at the earth. The ball will appear to be falling [MP PET 1993] (a) On a parabolic path as seen by pilot in the plane (b) Vertically along a straight path as seen by an observer on the ground near the target (c) On a parabolic path as seen by an observer on the ground near the target (d) On a zig-zag path as seen by pilot in the plane Solution: (c)         The path of the ball appears parabolic to a observer near the target because it is at rest. But to a Pilot the path appears straight line because the horizontal velocity of aeroplane and the ball are equal, so the relative horizontal displacement is zero.   Problem 67. The barrel of a gun and the target are at the same height. As soon as the gun is fired, the target is also released. In which of the following cases, the bullet will not strike the target (a) Range of projectile is less than the initial distance between the gun and the target (b) Range of projectile is more than the initial distance between the gun and the target (c) Range of projectile is equal to the initial distance between the gun and target (d) Bullet will always strike the target         Solution: (a) Condition for hitting of bullet with target initial distance between the gun and target $\le$Range of projectile.   Problem 68. A ball rolls off top of a staircase with a horizontal velocity u m/s. If the steps are h metre high and b mere wide, the ball will just hit the edge of nth more...

Bonding and hybridisation inorganic compounds

Bonding and hybridisation in organic compounds   Bonding in organic compounds.   The organic compounds are carbon compounds consisting of one or more carbon atoms. Carbon must form only covalent bonds, i.e., it should share its valency electrons with other atoms. According to the modern concept, a covalent bond is formed between two atoms if there is an overlapping of an atomic orbital of one atom with an atomic orbital of another atom. The overlapping is possible by two ways, (1) End to end overlapping: This type of overlapping is possible between $s-s,\,s-{{p}_{x}}$ and ${{p}_{x}}-{{p}_{x}}$atomic orbitals. The molecular bond formed is termed as sigma $(\sigma )$ bond. (2) Sidewise or parallel or lateral overlapping: Such overlapping is possible between $p-p$ atomic orbitals. The molecular bond formed is termed as $pi\,(\pi )$ bond.
$\sigma$-Bond   $\pi$-Bond
Formed by End to End overlap of AO?s.   Formed by lateral overlap of $p$-orbitals.
Has cylindrical charge symmetry about bond axis.   Has maximum charge density in the cross-sectional plane of the orbitals.
Has free rotation   No free rotation, i.e., frozen rotation
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Cell constant and Electrochemical cells

Cell Constant and Electrochemical Cells   “Electrochemical cell or Galvanic cell is a device in which a spontaneous redox reaction is used to convert chemical energy into electrical energy i.e. electricity can be obtained with the help of oxidation and reduction reaction”. (1) Characteristics of electrochemical cell:  Following are the important characteristics of electrochemical cell, (i) Electrochemical cell consists of two vessels, two electrodes, two electrolytic solutions and a salt bridge. (ii) The two electrodes taken are made of different materials and usually set up in two separate vessels. (iii) The electrolytes are taken in the two different vessels called as half - cells. (iv) The two vessels are connected by a salt bridge/porous pot. (v) The electrode on which oxidation takes place is called the anode (or – ve pole) and the electrode on which reduction takes place is called the cathode (or + ve pole).                 (vi) In electrochemical cell, ions are discharged only on the cathode. (vii) Like electrolytic cell, in electrochemical cell, from outside the electrolytes electrons flow from anode to cathode and current flow from cathode to anode. (viii) For electrochemical cell,                                                  ${{E}_{cell}}=+ve,\,\,\,\Delta G=-ve.$ (ix) In a electrochemical cell, cell reaction is exothermic.   (2) Salt bridge and its significance (i) Salt bridge is U – shaped glass tube filled with a gelly like substance, agar – agar (plant gel) mixed with an electrolyte like KCl, KNO3, NH4NO3 etc. (ii) The electrolytes of the two half-cells should be inert and should not react chemically with each other. (iii) The cation as well as anion of the electrolyte should have same ionic mobility and almost same transport number, viz. $KCl,\,KN{{O}_{3}},\,N{{H}_{4}}N{{O}_{3}}$etc. (iv) The following are the functions of the salt bridge, (a) It connects the solutions of two half - cells and completes the cell circuit. (b) It prevent transference or diffusion of the solutions from one half cell to the other. (c) It keeps the solution of two half - cells electrically neutral. (d) It prevents liquid – liquid junction potential i.e. the potential difference which arises between two solutions when they contact with each other. Note : q Salt bridge can be replaced by a porous partition which allows the migration of ions without     intermixing of solution.   q $KCl\,(aq)$ cannot be used as a salt bridge for the cell, $Cu(s)\left| CuS{{O}_{4}}(aq) \right|\,\left| AgN{{O}_{3}}(aq) \right|\,Ag(s)$. Because $AgCl$ is precipitated as follows, $AgN{{O}_{3}}+KCl\xrightarrow{{}}\underset{(\text{ppt}\text{.})}{\mathop{AgCl}}\,\downarrow +KN{{O}_{3}}$          (3) Representation of an electrochemical cell (i) The interfaces across which a potential difference exists are shown by a semicolon (;) or a single vertical line ($|$). For example, the two half- cells of the following electrochemical cell can be represented as follows, $Zn(s)+C{{u}^{2+}}(aq)\xrightarrow{{}}Z{{n}^{2+}}(aq)+Cu(s);$   $Zn$ ;$Z{{n}^{2+}}$ or $Zn|Z{{n}^{2+}}$ and $C{{u}^{2+}}$;$Cu$ or $C{{u}^{2+}}|Cu$ These indicate that potential difference exists at the $Zn$ and $Z{{n}^{2+}}$ ions interface, and similarly at the $C{{u}^{2+}}$ and $Cu$ more...

Adsorption and Adsorption isotherm Adsorption. (1) Definition : The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in to higher concentration of the molecules on the surface is called adsorption. (2) Causes of adsorption : Unbalanced forces of attraction or free valencies which is present at the solid or liquid surface, have the property to attract and retain  the molecules of a gas or a dissolved substance on to their surfaces with which they come in contact. Example : (i) Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water, giving $N{{H}_{4}}OH$ solution of uniform concentration. (ii) If silica gel is placed in a vessel containing water vapours, the latter are adsorbed on the former. On the other hand, if anhydrous $CaC{{l}_{2}}$ is kept in place of silica gel, absorption takes places as the water vapours are uniformly distributed in $CaC{{l}_{2}}$ to form hydrated calcium chloride $(CaC{{l}_{2}}.\ 2{{H}_{2}}O)$. Some basic terms which are used in adsorption
Interface : Any surface is a plane which separates any two phases in contact with each other. The plane which separates any two phase is generally called an interface between the two phases. Adsorbate and Adsorbent: The substance which gets adsorbed on any surface is called adsorbate for example, if a gas gets adsorbed on to the surface of a solid, then the gas is termed as the adsorbate. The substance on the surface of which adsorption takes place is called adsorbent. Adsorbent may be a solid or a liquid metal powders. Powdered charcoal, animal charcoal silica powder etc. are commonly used as adsorbents.
Desorption :  The removal of the adsorbed substance from a surface is called desorption. Absorption :  When the molecules of a substance are uniformly distributed throughout the body of a solid or liquid. This phenomenon is called absorption.
Sorption :  The phenomenon in which more...

Catalyst and Catalysis

Catalyst and Catalysis Catalysis.              Catalysis.              “Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.” ‘or’             “A catalyst is a foreign substance the addition of which into the reaction mixture accelerates or retards the reaction.”
• Berzelius (1836) introduced the term catalysis and catalyst.
• Ostwald (1895) redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as catalysis.”
Types of catalysis.             Catalytic reactions can be broadly divided into the following types, (1) Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.             (i) Oxidation of sulphur dioxide into sulphur trioxide with oxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process. $2S{{O}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{NO(g)}2S{{O}_{3}}(g)$             The reactants, products and catalyst all are in gaseous state i.e. same phase.             (ii) Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid .             $C{{H}_{3}}COOC{{H}_{3}}(l)+{{H}_{2}}O(l)\xrightarrow{HCl(l)}C{{H}_{3}}COOH(l)+C{{H}_{3}}OH(l)$             (iii) Hydrolysis of sugar is catalysed by H+ ions furnished by sulphuric acid. ${{C}_{12}}{{H}_{22}}{{O}_{11}}(l)+{{H}_{2}}O(l)\xrightarrow{{{H}_{2}}S{{O}_{4}}(l)}{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)+{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)$      (2) Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below.             (i) Oxidation of sulphur dioxide into sulphur trioxide in the presence of platinum metal or vanadium pentaoxide as catalyst in the contact process for the manufacture of sulphuric acid. The reactants are in gaseous state while the catalyst is in solid state.  $2S{{O}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{Pt(s)}2S{{O}_{3}}(g)$             (ii) Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in Haber’s process.             ${{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow{Fe(s)}2N{{H}_{3}}(g)$             (iii) Oxidation of ammonia into nitric oxide in the presence of platinum gauze as a catalyst in Ostwald’s process.                                     $4N{{H}_{3}}(g)+5{{O}_{2}}(g)\xrightarrow{Pt(s)}4NO(g)+6{{H}_{2}}O(g)$             (iv) Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.             $\text{Vagetable}\,\text{oils}(l)+{{H}_{2}}(g)\xrightarrow{Ni(s)}\text{Vegetable}\,\text{Ghee}(g)$   (3) Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below.             (i) Decomposition of ${{H}_{2}}{{O}_{2}}$in presence of colloidal platinum. $2{{H}_{2}}{{O}_{2}}(l)\xrightarrow{Pt}2{{H}_{2}}O(l)+{{O}_{2}}(g)$             (ii) Decomposition of $KCl{{O}_{3}}$in presence of manganese dioxide. $2KCl{{O}_{3}}(s)\underset{{{270}^{o}}C}{\mathop{\xrightarrow{Mn{{O}_{2}}(s)}}}\,2KCl(s)+3{{O}_{2}}(g)$             (iii) Oxidation of ammonia in presence of platinum gauze. $4N{{H}_{3}}(g)+5{{O}_{2}}(g)\underset{{{300}^{o}}C}{\mathop{\xrightarrow{Pt(s)}}}\,4NO(g)+6{{H}_{ & 2}}O(g)$             (iv) Oxidation of sulphur dioxide in presence of nitric oxide. $2S{{O}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{NO(g)}2S{{O}_{3}}(g)$             (v) Oxidation of sulphur dioxide in presence of platinised asbestos or vanadium pentaoxide. $2S{{O}_{2}}(g)+{{O}_{2}}(g)\underset{or\,Pt(s)}{\mathop{\xrightarrow{{{V}_{2}}{{O}_{5}}(s)}}}\,2S{{O}_{3}}(g)$             (vi) Oxidation of hydrochloric acid into chlorine by Deacon’s process in presence of $CuC{{l}_{2}}$. $4HCl(g)+{{O}_{2}}(g)\underset{{{450}^{o}}C}{\mathop{\xrightarrow{CuC{{l}_{2}}(s)}}}\,2C{{l}_{2}}(g)+2{{H}_{2}}O(g)$             (vii) Formation of methane in presence of nickel.$CO(g)+3{{H}_{2}}(g)\xrightarrow{Ni(s)}C{{H}_{4}}(g)+{{H}_{2}}O(g)$             (viii) Synthesis of ammonia by Haber’s process in presence of a mixture of iron and molybdenum. ${{N}_{2}}(g)+3{{H}_{2}}(g)\underset{450-{{500}^{o}}C}{\mathop{\xrightarrow{Fe(s)\,\And \,Mo(s)}}}\,2N{{H}_{3}}(g)$             (ix) more...

Colloids, Emulsion, Gel and Their Properties With Application

Colloids, Emulsion, Gel and Their Properties With Application Colloidal state.              (1) The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. Bhatnagar and others.             (2) Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment paper.             (i) Crystalloids : They have higher rate of diffusion and diffused from parchment paper. Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.             (ii) Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper. Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.             (3) The above classification was discarded i.e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.             (4) The colloidal state depends on the particle size. If is regarded as intermediate state between true solution and suspension.
• True solution : In true solutions the size of the particles of solute is very small and thus, these cannot be
detected by any optical means and freely diffuse through membranes. It is a homogenous system.
• Suspension : The size of particles is large and, thus it can be seen by naked eye and do not pass through
filter paper. It is a heterogeneous system.     The size of different solutions are sometimes expressed in other units also as given below: Size (diameter) of particles in particles in different units
True solutions Colloids Suspensions Relation
$<{{10}^{9}}m$ $<1nm$ $<10{\AA}$ $<1000pm$ ${{10}^{9}}mto\text{ }{{10}^{7}}m$ 1 nm - 100 nm $10\text{ }{\AA}1000\text{ }{\AA}$ $1000pm{{10}^{5}}pm$ more...

Chemical Analysis of Organic Compounds

Chemical Analysis of Organic Compounds Purification and Characterisation of organic compounds              The study of organic compounds starts with the characterisation of the compound and the determination of its molecular structure. The procedure generally employed for this purpose consists of the following steps :             (1) Purification of organic compounds             (2) Qualitative analysis of organic compounds             (3) Quantitative analysis of organic compounds             (4) Determination of molecular mass of organic compounds             (5) Calculation of Empirical formula and Molecular formula of organic compounds             (6) Determination of structure of organic compounds by spectroscopic and diffraction methods               (1) Purification of organic compounds:  A large number of methods are available for the purification of substances. The choice of method, however, depends upon the nature of substance (whether solid or liquid) and the type of impurities present in it. Following methods are commonly used for this purpose,             (i) Simple crystallisation    (ii) Fractional crystallisation (iii) Sublimation (iv) Simple distillation             (v) Fractional distillation   (vi) Distillation under reduced pressure (vii) Steam distillation             (viii) Azeotropic distillation (ix) Chromatography (x) Differential extraction  (xi) Chemical methods             (i) Simple crystallization: This is the most common method used to purify organic solids. It is based upon the fact that whenever a crystal is formed, it tends to leave out the impurities. For crystallization, a suitable solvent is selected (a) which dissolves more of the substance at higher temperature than at room temperature (b) in which impurities are either insoluble or dissolve to an extent that they remain in solution (in the mother liquor) upon crystallization, (c) which is not highly inflammable and (d) which does not react chemically with the compound to be crystallized. The most commonly used solvents for crystallization are: water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc.                     Examples: (a) Sugar having an impurity of common salt can be crystallized from hot ethanol since sugar dissolves in hot ethanol but common salt does not.             (b) A mixture of benzoic acid and naphthalene can be separated from hot water in which benzoic acid dissolves but naphthalene does not.             Note:q  Sometimes crystallization can be induced by adding a few crystals of the pure substance to the concentrated solution. This is called seeding.             (ii) Fractional crystallization: The process of separation of different components of a mixture by repeated crystallizations is called fractional crystallization. The mixture is dissolved in a solvent in which the two components have different solubilities. When a hot saturated solution of this mixture is allowed to cool, the less soluble component crystallises out first while the more soluble substance remains in solution (mother liquor). The mother liquor left after crystallization of the less soluble component is again concentrated and then allowed to cool when the crystals of the more soluble component are obtained. The two components thus separated are recrystallized from the same or different solvent to yield both the components of the mixture in pure form.               Fractional crystallization can more...

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