Elastic Silica: A Demonstration JAMES W. COLE University of Virginia, Chorlottesville, Virginia
two interesting and informative acRECENTLY counts,'. have been given of the role of silicon,
gloves, hut the writer felt no ill d e c t s after preparing 10 spheres. The knack of producing a ball that will the second most abundant element, in polymeric chem- bounce when dropped will be readily obtained after one istry. A particular aspect is the combination of silicon or two attempts. with oxygen and alkyl groups to obtain complex moleIn a classroom demonstration it is suggested that cules which are plastic to slowly applied forces and several spheres he prepared so that in addition to the elastic to rapidly applied forces. The name "bouncing bouncing or elastic property, the plastic effectmay also putty" has been applied to cover those materials which he observed. A freshly prepared sphere when placed on flow like a viscous liquid under a static load, but re- a flat surface will settle and in a few minutes a flat side will develop. The process may, be repeated several bound like rubber when thrown. During a phase of some silicon research the writer times by reforming the sphere: prepared a material with rubber-like qualities from orProlonged exposure of this form of silica to the atdinary water glass. Mindful of the possibility of a lec- mosphere gives rise to a drying d e c t which causes the ture demonstration on a topic of contemporary interest, mass to become granular and fragile. The elastic propa procedure was devised for readily showing some elastic erty, however, diminishes slowly, and even when the properties of silica. mass is hard, it will bounce some if dropped. Finally I t was observed that addition of an alcohol to com- it becomes brittle and shatters upon hitting a surface. mercial water glass caused a solid to pr&ipitate which The role of alcohol in producing the elastic silica is can be worked with the fingers into a sphere that not clear. It may be a catalyst but there is some evibounces like rubber when dropped. The relative quan- dence that the alcohol becomes a part of the mass and tities of materials do not seem to he critical. A satis- may he in the form of an organosilicon polymer. The factory aggregate may be formed by diluting 10 ml. of solid dissolves fairly readily inwater to form a solution commercial sodium silicate solution ("egg preserving which is alkaline to litmus but which does not precipisolution") in a small beaker to about 20ml. with water, tate silicic acid upon treatment with small or large quanand then adding slowly with stirring about 10 ml. of tities of hydrochloric acid. Other alcohols will produce 95 per cent ethanol. A metal rod with spatula tip is the same effect and even the alcohol amines will proconvenient for stirring. duce elastic silica when added to water glass. In fact a After the mixture has solidified, the solid may be re- sphere produced with triethanolamine seemed to he moved with the fingers, or with the spatula, and pressed more plastic than one prepared with ethanol. into a ball of about one inch in diameter. I t is advisable Other demonstrations of the properties of silica may to have water flowing from a faucet close by to wash the be visualized from the thoyghts expressed here. More excess caustic from the sphere and the fingers. Some particularly the formation of thin films of a silica-like demonstrators may find it desirable to wear rubber character can be produced on metals by painting the surface with a solution prepared by dissolving some of the solid in water and then allowing the water to evaporate. cones at ~ i d l & d ,
