212 ELEMENTARY GENERAL SCIENCE CHAP. 



pended material from a liquid, either for the isolation of the solid 

 or the purification of the liquid. 



Solvent Power of Hot and Cold Water. We may now 

 find whether temperature has any influence on the solvent 

 power of water, that is, does hot water dissolve more or less of a 

 solid than cold water 1 



EXPT. 203. Place a quantity of powdered nitre in water 

 and allow it to stand for some time with frequent vigorous 

 shaking, so that a cold saturated solution is formed. Now 

 heat the solution and see whether more solid dissolves 

 or not. 



Do the same experiment with other solids and satisfy yourself 

 of the truth of the following statement, that in general hot 

 water dissolves more of a solid than cold water, or in other words, 

 that the solvent power of a liquid increases with rise in tem- 

 perature. 



Crystallisation. EXPT. 204. Allow a hot saturated, or 

 nearly saturated, solution of nitre obtained as above to cool 

 slowly, and observe that the nitre separates out from the solu- 

 tion as clear glassy solids, which may be observed to increase in 

 size as the solution cools. Examine a few of these and see that 

 they are bounded by plane surfaces. 



These are known as crystals and their formation as crystallisation. 

 Crystallisation usually occurs when a dissolved solid separates 

 out from its solution, either by cooling of the solution, or by the 

 evaporation of the solvent; and the slower the evaporation, or more 

 gradual the cooling, the larger and more perfect are the crystals 

 obtained. This may be easily verified by rapidly cooling, in a 

 stream of cold water, a hot saturated solution of nitre. We 

 shall also find later that crystals may also be formed during the 

 solidification of a melted solid, especially when it is allowed to 

 cool and solidify slowly and undisturbed. 



Precipitation. EXPT. 205. Dissolve some lead acetate in 

 distilled water ; add to it a solution of salt. Observe the form- 

 ation of a white powder in the previously clear liquid. 



Such precipitation always happens if to a solution we add some 

 material capable of converting the dissolved substance into a new 

 product which is insoluble in water, and which therefore separates 



