Sherry M. Burgess
Introduction
A chemical reaction can be defined as a process in which the physical and chemical properties of the original substance change as a new substance with different physical and chemical properties is formed.2 In other words the process in which a material changes into a new substance. The original substance (s) is termed reactant (s). The resulting new substance (s) is termed product (s). Many times a chemical reaction will involve the combination of two substances. Evidences of chemical reactions can include a temperature change (resulting from either the release or intake of energy), bubbling (indicating the formation of a gas) or the combination of both. Many of the elements, especially the gases, were discovered while early chemists were combining materials during experiments. Hydrogen gas was discovered in this fashion. As early as the 1600’s it had been known that a certain gas (later named hydrogen) was formed when certain acids were combined with certain metals. In 1766, Henry Cavendish, an English chemist, proceeded to study this gas in a scientific way. In other words, once he made the gas by combining the acids and metals, and collected the gas; he investigated it by certain procedures. these included weighing the gas for its mass, and examining its inflammability. He found the gas to be very light (much lighter than air). It was also found to be very inflammable. This was done by Cavendish placing a candle or a burning ember inside a jar containing the gas. the result was that the gas would ignite and display a blue flame.
In the following activity the students will observe what happens when vinegar (an acid) is mixed with baking soda. In this reaction the gas released is carbon dioxide. Teacher’s note: the term chemical property is used in the questions. If this is a new term for the students, explain the term during the pre lab discussion. (Chemical property: those characteristics that describe how a substance interacts, or fails to interact, with other substances to produce new substances. Another definition states that it is a property that describes how a substance changes into a new substance.
Activity
Begin by explaining to the class that they will be performing an experiment that will demonstrate a chemical reaction. They will be recording their observations and either answering questions or writing up a lab report at the end of the experiment. Students should work in pairs. Each group should obtain the following items: baking soda, vinegar, one foil cupcake baking cup, one zip lock bag, (a quart size is good), measuring utensils , one candle. Once all materials have been obtained, instruct students to place three teaspoons of baking soda into a zip-lock bag. Next, 25 ml of vinegar is poured into a foil cupcake baking cup. (The cup is large enough to hold the amount of vinegar that will produce a nice amount of carbon dioxide during the reaction, yet short enough to fit inside the bag). The cup is carefully placed inside the bag containing the baking soda without spilling any inside the bag. The bag is then sealed and the contents are gently mixed. Students will write down their observations. These should include what happened when the contents were mixed. They should note that bubbling or foaming took place as well as the bag expanding (from the gas production). Instruct the students to touch the bag as well while the reaction is going on and to record their observation. While the gas is forming the bag should feel cold. This is a result of energy going into the chemical reaction that occurred between the baking soda (sodium bicarbonate) and the vinegar ( a weak solution of acetic acid). The students should also be instructed to notify the teacher once the gas has finished forming. The teacher will then light the candle at each group’s station, and while the student is holding the bag open (still on its side), carefully insert the tip of the flame just at the opening of the bag (barely inside). The candle should go out. The observations can be done in note form, to be formerly written up later or could be done in the form of the following questionnaire. If a lab write up is done, make sure the topics in the questions are addressed in the write up.
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1. What happened when the baking soda and vinegar were mixed?
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2. How did the bag feel when you touched it while the reaction was going on?
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3. What caused the bag to expand? Do you know the name of the material?
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4. What happened when the lighted candle was placed near the opening of the bag?
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5. Some gases have the chemical property of being able to support burning (allow something to burn). Does this gas have that property?
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6. Another chemical property that a gas may have is the ability to burn itself. This is called flammability. Is this gas flammable?
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7. How could you learn more about the material that caused the bag to expand?
An extension of this activity could be for the students to do a mini research project on the gas carbon dioxide. Topics would include how it was discovered, description of its properties both physical and chemical, and its uses. The second research topic could be for them to find out how a fire extinguisher works.
