What I love about this lab:
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brush up on TBB skills
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following directions skills
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making observations
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observing a chemical reaction
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formation of a gas
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mass of a gas
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an endothermic reaction, bottom of flask gets pretty cold!
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reading a chemical equation – shows that same elements are present before and after the reaction, no new elements are introduced or formed.
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formation of new molecules, the atoms are rearranged to form new substances that were not in the flask before.
The one thing that is not very clear to the kids is that the mass of the system before the reaction and after the reaction should be the same. Their results consistently show a loss of about 1 gram (on average) without releasing the gas from the balloon. When the gas was released, there was an additional loss of about 1 gram.
For this experiment, there are 2 factors that can make the final mass different. The first factor may be leakage: gas, liquid, or both escaped during the active part of the chemical reaction. The seal formed where the balloon meets the flask may not be 100% airtight, even with the kids holding it. But if leakage does not happen, the second factor is buoyancy. When the balloon fills up with CO2, the volume of the balloon increases, therefore displacing the air around it. NSTA published a study about this and had results to show that there was a relationship between the volume of the balloon and the decrease in mass for the closed system.
Here is an excerpt from the published article: “Because the apparent loss of mass is not due to leakage, this relationship clearly demonstrates a buoyant effect. Archimedes’ principle states that any object submersed in a fluid will displace a volume of fluid equal to the volume of the object and the mass of the object will be reduced or “buoyed up” by an amount equal to the mass of the fluid displaced. Hence, when measuring the mass of an object in air (a fluid) on a balance, the mass that is measured is the conventional or apparent mass of the object. The true mass of the object would be the mass that would result if the object were massed in a vacuum.” (Science Scope, 2008)
A better example to show conservation of mass may be to place an ice cube or two in a small plastic snack bag, and to take timed measurements as the ice cube melts. (In a humid classroom, condensation forming on the outside of the bag could alter results.) Also, if using baking soda and vinegar, using a soda bottle without the balloon and capping it would keep the volume the same.
Overall, the kids enjoyed this experiment and it covered a lot of concepts that we studied. If you have done either of these experiments, please share your results!
Notebook:
