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ExposeExciteIgniteMay2012

Teaching Extension How

Teaching Extension How do we know the globules are silicon? Any of the following methods can provide evidence that the globules are indeed silicon: A. Silicon’s reaction with caustic soda Silicon is insoluble in common acids but will react with alkalis liberating hydrogen gas. This provides a test for silicon. Place a cleaned small silicon globule in a test tube. Add 5M sodium hydroxide solution (caustic soda) and heat gently for 1 – 2 minutes. The characteristic ‘pop’ when a lighted splint is brought to the mouth of the test tube will confirm the presence of hydrogen. Si + 2NaOH + H2O ! Na2SiO3 + 2H2 2H2 + O2 ! 2H2O + energy B. Demonstrating the reverse temperature effect Clamp one cleaned globule of silicon using two crocodile clip wires without plastic insulation. Check that the clips hold on firmly but do not touch each other (Figure 6.10). Figure 6.10 Connect the following items in series to the two wires: ! One battery pack: 2 x AA (3.0V) ! One 3.2 V lamp in a lamp holder ! One ammeter (multimeter) The lamp should light up. Record the ammeter reading. Now heat the silicon in a bunsen flame or blow torch. The lamp should brighten up and the ammeter reading should increase (increased current). Silicon is a semi-conductor. It has a reverse temperature-resistance coefficient (resistance decreases as the temperature increases) since the number of free charge carriers increases with temperature. To compare with a conductor, find a metal such as an iron nail and repeat the process. Metals (conductors) experience an increase in resistance with increasing temperature and thus a decrease in current. 71

Disposal Allow the solids and clay fragments to cool to room temperature. The clay flower pot invariably cracks and should not be re-used. Dispose of all solids in the waste bin. Key Terms Silicon, metal reduction, thermite reaction, metalloid, semi-conductor, activation energy, reverse temperature effect. References 1. Bedford, M (2009) How sand is transformed into silicon chips. www.techradar.com, Dec 2010 2. Grant, R. (June 1979) Some experiments with silicon, Spectrum, 17, pp 19 - 20 3. Shakhashiri, B. Z.; Chemical Demonstrations: A handbook for teachers of chemistry, Vol. 1, The University of Wisconsin Press: Madison, 1983; pp 85 - 89 4. “Values of Chemical Thermodynamical Properties”, CRC Handbook of Chemistry and Physics, 57th Edition. CRC Press 72