GEORGE L. GILBERT Demson Un~vers~ty Granv~lle.Ohlo 43023
The Preparation of Various Kinds of Glass Submitted by:
Checked by:
Richard F. Jones Sinclair Community College Dayton, Ohio 45402 Erwin Boschmann Indiana Uniuersity-Purdue University Indianapolis, Indiana
Materials and Equipment Meeker burner, gadoxygen handtorch, crucible lids, -eranhite . disks...nine . stem trianeles. fire brick. asbestos hoard or thick paper, ring stand, cob& giass plates'or glassblowing glasses, silicon dioxide or white sand, sodium carbonate, calcium oxide, aluminum oxide, sodium borate, horic acid, red lead oxide, cobaltous oxide, and potassium carbonate.
glass is made. Since glass is a mixture rather than a compound with definite compositions, an almost endless number of glasses can be made. Glass does not have a definite melting point as compounds do hut instead softens over a range of temperatures. This permits the craft of glassblowing. Finally, since glasses do not have a crystalline structure, they may be considered to be supercooled liquids. Graphite disks about 1 cm thick can be cut from large araohite electrodes. All heating should be done in a hood or wkll-ventilatedroom because i f the possibility of toxic fumes being evolved. Cobalt glass plates (or less effectively, glassblowing glasses) mask the bright sodium flare and allow the demonstrator and spectators a better view of glass formation. Such eye protection is recommended for 1) and 2) but is a must for 3) and 4).
a
References
Procedure Preweigh and thoroughly mix the compounds before the demonstration. The following boron glass can he formed using a Meeker burner: 1) 16.8 g horic acid and 3.6 g sodium borate. 2) 15.2 g red lead oxide and 7.4 g boric acid. Place about 2 g of each mixture in a crucible lid or small evaooratina dish mounted on a pipe stem triangle and heat witha Meeier hurner until the mixture is degassed and a glass is formed. The crucible lid with its shallow lip allows better visibility for alarger number of students but the small evaoorating.dish allows a larger hatch to be made. The following silicon glasses can be formed using a gas/ oxygen handtorch: 3) (a Christmas tree ornament glass) 13.6 g silicon dioxide, 7.5 g sodium carbonate, 1.4 g calcium oxide, and 0.6 g aluminum oxide. 4) (a Pyrex glass) 8.0 g silicon dioxide, 6.4 g sodium carbonate, 2.8 g boric acid, 0.2 g aluminum oxide, and 0.1 g potassium carbonate. Mound about 2 g of each mixture on a graphite disk placed on a fire brick. Using a gasloxygen handtorch, direct a gentle flame onto the mixtures from above. As the mixture melts, increase the flame to speed the degassing and fusion to form a glass. Pure silicon dioxide cannot be fused under these conditions.
The following demonstration is a new and exciting approach to an older demon st ratio^.'^^ In fact it is comparable to the famous Thermite demonstration and somewhat safer. The demonstration to a large degree runs counter to human experience. We are taught and experience that carbon dioxide is the ultimate weapon in extinguishinga fire; however, anyone who has fought a magnesium fire will know such is not true. Thermodynamics predicts clearly that the reaction
Comments and Cautions Fused quartz (silicon dioxide or sand) would he the best glass for many purposes because of its low coefficient of expansion, resistance to chemical attack, and optical properties, but the high temperature required to fuse quartz prevents i t from being widely used. The oxides of other elements are used primarily as fluxes to lower the temperature of formation and to give the glass different properties. The degassing during the heating is caused by horic acid decomposing to horon oxide by losing water and from waters of hydration being lost from the starting materials. Show and discuss the different properties of the glasses depending upon their composition such as preparation and working temperatures, coefficient of expansion, and optical properties. Small amounts (about 1%)of cobaltous oxide can be added to the original mixtures to illustrate how colored
gives AH = -193.5 kcal mol-' of carbon and should proceed provided the kinetics allow it, which it does. The demonstration is very dramatic and involves some hazard. Two blocks of dry ice are needed, with a minimum size of about 12 cm X 12 cm X 7 cm, with one set of large smooth matching surfaces. The depth is somewhat criticaisince the mamesium could burn throuah a thinner block. In one block n nugh ca\.ity abo~tr2-3 cm d r q , and alwut 4 rm in diameter should be made u,irh thr aid (La scwwdriver. One sho~lklhokl it firmly in scooping fashion and carefully gouge a cavity taking small bites of dry ice each time to prevent cracking the block. Fill the cavity with magnesium turnings and place the dry ice on a large asbestos sheet. Remove combustible items since some hot burning magnesium turnings are expelled.
450 1 Journal of Chemical Education
A Demonstration of Burning Magnesium and Dry Ice Submitted by:
Checked by:
J e r r y A. Driscoll T h e University of U t a h Salt Lake City 84112 George Wollaston Clarion State College Clarion, Pennsylvania
