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TOPIC: Gases Demo 010 Boyle's Law (Qualitative)This is a simple opening demo showing the reciprocal relationship between gas pressure and volume. It is an example of what Boyle called "The Spring of the Air." MATERIALS
Figure. Plastic syringe apparatus for Boyle's law demonstration. PRESENTATION Set out the apparatus and read the volume of the trapped air in the syringe. Take one book and place it on top of the plunger. Read the new volume of the gas. Add a second identical book to the pile and repeat the volume reading. You should be able to repeat this two or three more times to get a set of data relating gas volume in mL against pressure in units of "textbooks." You should be able to graph (with large data dots) the variation of volume against pressure from this experiment. The result should be an approximately P ´ V = constant plot (hyperbola).Alternatively, this can just be used to show that as pressure is applied to the plunger of the syringe, the volume occupied by the gas decreases. When released, the plunger returns to its original position. Air is elastic. Point out the connection to contemporary devices like air-filled automotive shock absorbers. DISCUSSION Boyle's law. It is useful to set out the context for Boyle's investigations in the late 17th century. Remember that the only known gas at that time was air and that this was well before the identification of the components of air, nitrogen, oxygen, carbon dioxide, etc. Air was thought to be an element of some sort. Most educated people thought of air as being without much substance, certanly not capable of supporting the weight of heavy objects. Boyle's study laid the groundwork for the development of all sorts of compressed air machinery and pneumatic devices like automotive shock absorbers. Boyle's investigation of the physical properties of air was done in a different apparatus from that used in this demonstration. The 17th century did not provide the technology to make something like a syringe, plastic or otherwise. Boyle used a glass tube bent in the shape of the letter "J" with the short end sealed and the long end open. By pouring mercury into the open end, it was possible to trap an easily measured quantity of air in the closed end. The pressure on the trapped air was directly proportional to the height of the mercury column; the volume of the gas was proportional to the length of the column of trapped air. Thus it was possible to measure the volume occupied by a fixed sample of air at a measured pressure. The pressure could be changed by adding more mercury to the tube and the resulting new gas volume determined by merely measuring the length of the trapped gas column. With such an apparatus it is possible to obtain a good set of pressure versus volume data at constant temperature and amount of gas. The product of P ´ V is constant to three significant figure accuracy.HAZARDS The author cannot imagine much in the way of hazards for this demonstration. When performing the Boyle's law experiment, try not to drop any heavy books on your foot. REFERENCES D.A.Davenport, J. Chem. Educ., 39, 252 (1962).
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