overhead projector
edited by DORIS KOLB Bradley University Peoria, IL 61625
A Very Rapidly Growing Silicate Crystal Donald 8. Phillips Eastern Mlchigan University Ypsilanti. MI 48197 Silicate crystals of metallic salts are among t h e easiest of any type t o grow. When small crystals of various salts are dropped into a dilute sodium silicate ("water glass") solution, silicate crystal growth begins almost immediately a n d in a few minutes becomes evident. It is n o t uncommon for several types of these crystals t o grow t o several centimeters overnight. T h e silicate crystal growth t o b e described here is a n extremely fast growth, with t h e crystals often reaching 3-4 cm in length i n 10 s or less. T h i s extremely rapid growth is m a d e even more spectacular when shown b y a n overhead projector. T h e result is a silicate crystal t h a t projects t o b e 34 f t long on the screen. Materials The following materials are needed: several pieces of cobalt(I1) nitrate crystals: [Co(NO&. 6H20] about the size of a small pea or BB 10 mL sodium silicate solution (water glass) 4 0 4 2 Be' 20 mL distilled water 50-mL graduated cylinder 2 125-mm-long test tubes (or whatever is available) forceps overhead projector equipped with a horizontal C'Alyea-type") adantor to oroiect . , throueh the side of the test tube rectsngulnr Plexiglas rontainrr to huld the demonctration teit ruhr cne iurr to put u.ater in the wntainer to i m p w e the projection.)
Directions Place 10 mL of sodium silicate (4042 Be') in the graduated cylinder, add 20 mL of distilled water, and shake to mix. Pour about half of this solution into a test tube. Using forceps, drop a small crystal of cohalt(I1) nitrate herahydrste into the diluted sodium silicate solution. immediatelv on the overhead .oroiector ~. and ,oroiect ,~~ . usmg a hurizontnl ( " A l w a - t y p ~ aa3ptor. "~ !Be rurr to put thr d m . cmmation tuhemwa I'lrx~ylnscvntainer nearly iilled with w t e r ru improve projection.) The cobalt nitrate crystals almost always trap one or more bubbles of air on them. It is the bubble that seems to pull along the silicate crystal as it rapidly grows a tentaclelike crystal. The remaining diluted water glass solution is retained as a back-up in the rare event that a bubble does not catch on the crystal or does not dislodee. ~ . , 1 have nu good rxplanatiun for the ver, rnpd gruwth of the cobalt silirnrerrvstal,otherthan tostresi the imporrance ofnn air bubblr. Cmwrhrtill occurs withwt the preicncr otan air bubhlc. hut not 3s rapidly. The cobalt silicate crystals grow in a generally upward direction, but each in its unique, unpredictable way. The upward growth normally is explained in terms of the lower density of the growing crystal relative to the solution around it. The growing crystal has a semioermeable membrane around it. and selective flow of the water . i n s h thii membrane is the likely rausc for the lower denitg nnd the up\
