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ExposeExciteIgniteMay2012

combustion of the paper

combustion of the paper as long as the heat reservoir still has absorbance capacity. Water No Water 6. Draw students’ attention to the steam forming and when the water starts boiling, pour yourself a cup of coffee and drink to water’s “invisible safety features”. Focus attention on the unscathed bottom of the pan (Figure 5.7)! I call this cool hot science! Teaching Extension • Place a raw egg in the water in the paper pan and boil for 4 minutes or how about a poached egg? (Figure 5.8) Another eggsothermic process - enjoy a science breakfast! • Alternatively you may use a thin paper cup or paper bag. If the bag leaks, seal it with tape. Figure 5.7 Figure 5.8 Discussion Although this demonstration points to the same phenomena of heat capacity, it is different in that the water is heated to boiling point. So we have an additional “sink” for the flame’s energy - that of latent energy during a phase change. When the water phase change starts (liquid ! gas) the heat capacity is infinite! Let’s sum up: When we boil water it absorbs heat ! to break its intermolecular hydrogen bonds and cause molecular vibrations that dissipate as heat (translational motion) raising its temperature to boiling point and ! to facilitate the escape of molecules to the gas phase (phase change ! latent heat of evaporation). 59

Both these ‘heat sinks’ ensure that the paper’s temperature stays below its ignition temperature of 233! (451"). This is a remarkable outcome as the temperature of the flame is anywhere between 320! and 1600! (Ref. 4) C. Igniting Bubbles on your Hand (safely) [] Many teachers are familiar with the ‘non-burning’ American dollar bill or handkerchief demonstration (Ref. 2). In this demonstration a US dollar bill is set alight but the paper is not consumed as the fuel is mixed with water. This is another great demo on water’s insatiable appetite for heat! In the next demonstration we will rely heavily on water’s magical property of heat absorption and find that it will protect us from the flame’s heat. We will ignite a small quantity of butane bubbles on a gloved hand! It is much safer than it appears and clearly demonstrates the amazing heat capacity water has. Need I say that it is very popular with students of all ages - but do warn them that it is a specialized demonstration and not to be practiced on their own. Figure 5.9 Water will absorb heat from flames and thus protect. When bush fires approach, people sometimes spray buildings with water and cover themselves in wet blankets. Safety / Risk Assessment ! Do not use open flames in the proximity of flammable gases ! Check that you are at a safe distance from flammable material such as paper, curtains, etc. ! Ensure that you wet the gloved hand and exposed body parts thoroughly with water. Follow instructions closely ! Have a fire extinguisher readily available ! Wear safety glasses and a lab coat ! Only use a small volume of bubbles 60