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F nucleation is a process in which a minimum number

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(f)Nucleationis a process in which a minimum number of atoms, ions or moleculesassociate to give a stable solid.Particle growthis a process by which growth continueson existing nuclei.Precipitation by nucleation results in a large number of smallparticles.Precipitation by particle growth results in a smaller number of large particles.12-2.(a)Digestionis a process in which a precipitate is heated in the presence of the solutionfrom which it was formed (themother liquor).Digestion improves the purity andfilterability of the precipitate.(b)Adsorptionis the process by which ions are retained on the surface of a solid.(c)Inreprecipitation, the filtered solid precipitate is redissolved and reprecipitated.Because the concentration of the impurity in the new solution is lower, the secondprecipitate contains less coprecipitated impurity.(d)Precipitation from homogeneous solutionis a technique by which a precipitatingagent is generated in a solution of the analyte by a slow chemical reaction.Local reagentexcess does not occur and the resultant solid product is better suited for analysis thanprecipitate formed by direct addition of precipitating reagent.(e)Thecounter-ion layerdescribes a layer of solution containing sufficient excessnegative ions that surrounds a charged particle.This counter-ion layer balances thesurface charge on the particle.(f)Mother liquoris the solution from which a precipitate is formed.
Fundamentals of Analytical Chemistry: 9thed.Chapter 123(g)Supersaturationdescribes an unstable state in which a solution contains higher soluteconcentration than a saturated solution.Supersaturation is relieved by precipitation ofexcess solute.12-3.Achelating agentis an organic compound that contains two or more electron-donorgroups located in such a configuration that five- or six-membered rings are formed whenthe donor groups complex a cation.12-4.Relative supersaturationcan be regulated through control of reagent concentration,temperature and the rate at which reagents are combined.12-5.(a)There is positive charge on the surface of the coagulated colloidal particles.(b)The positive charge arises from adsorbed Ag+ions.(c)NO3ions make up the counter-ion layer.12-6.342322CH CSNH +H OCH CONH +H SThe slow hydrolysis of thioacetamide can be used to generate a source of hydrogensulfide gas.Hydrogen sulfide gas is then involved in the equilibria below:+223+223H S+H OH O +HSHSH OH O +S+12-7.Inpeptization, a coagulated colloid returns to its original dispersed state because of adecrease in the electrolyte concentration of the solution contacting the precipitate.Peptization can be avoided by washing the coagulated colloid with an electrolyte solutioninstead of pure water.

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