A Simple Model to Demonstrate Solubility Principles Why is it. that some pairs of liquids are miscible, whereas others are nut? I t ia all too easy t,o snsror this qoeslion by using as examples only extreme cases such as t,he completely miscible water-alcuhol pair on the one hand and t,he immiscible water-oil psk on the ut,her. Telling a student, thab "like dissolves like" may serve x nseful innemot~icf~uction,hnt it ~cartcelyadds to hi unders t s n d i n ~m ~ dappreciation of the many ahemical and physical factors which determine so1oWlit.y -. hehavio~. We have found that a system involving only rudimentary equipment and compounds of very simple structure may be nsefu!ly employed to demonstrate q n i k dramn(.ically x variely of chemical principles related to this problem. At room temperature methanol ("water-like") and peutane ("oil-like") are wmplet,ely mi* cible, but a t reduced temperatures they are esseut,ially immiscible. This may be demonstrated in the following way: A s m d crystal of Malachite Green is dissolved st. room temperature in 50 ml of anl~?,drotrsmethanol in s. 38 X 200 mm test tobe. To this is added wit,h shaking 50 ml of pent,aue. The resulting blue-green salutiou may be seen to be homogeneorts and of one phase. However, if this aolotion is now stoppered end plaoed in s. l h y Ice-acetone bath, it rapidly (1-2 min) and sharply separates into a lower, brighl, blue-green methnnal layer and an opper, waber-white peot,ane layer. Upon warming to mom temperature, a homogeneous solnliun is again produced hy &king the tesl tube. The process may be repeated s m a n y times n.s is desired. The interpretation of this phenomenun may be made at, practically any level uf aophistimtiun that the instrnctor desires. Perhaps the aimplest erplanslioa is thnt 81 (.he higher temperature both the pentane and methand molecules have sulficient kinetic energy to overcome diRerences iu stl,raotiveforces between themselves and like or m>lilccncighhom, whereas at. the lower ternperat.ure they muat select their closest neighbors mma cecefully on tho basis of most favorable at,l.racliveforces. Of comne. m o x rimrous tmstments involvine discussions of such factors a~hvdnuen bmdpolar advent a t the lower t,emperature
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Journal o f Chemiccll Education
