Observing Reactions
Procedure
- Cut out the reaction grid below. Place an acetate sheet over the reaction grid and secure with a paper clip.
- Obtain a set of chemicals (4 Beral pipets and a microcentrifuge tube).
- For each column (Ø), place a drop of the liquid or a piece of the solid on the acetate sheet in the area indicated by the column label at the top of the reaction grid. Do not place any material on the darkened areas.
- For each row (-->), place a drop of the liquid or a piece of the solid on the acetate sheet in the area indicated by the row label at the right side of the reaction grid. (Avoid contamination of the droppers. Wipe the forceps if they touch a solution.) Make careful observations, and record them in the data table provided.
- If this procedure is followed, you will end up mixing two samples at a time and will systematically make all of the possible combinations.
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Safety
Wear safety goggles and aprons in the lab at all times. Unknown chemicals may pose unexpected hazards, so use caution in dealing with them. Wash your hands immediately upon contact with chemicals. Avoid breathing vapors from chemicals.
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TG Closure
Use an overlay on the overhead projector in front of the class. Ask students to provide the observations. After a few minutes there will be considerable heated debate. See if you can then develop two charts such that all students find their results compatible with one or the other.
The discussion can lead to ways in which scientists work out their differences. For example, do students suggest that they return to repeat experiments? Do they want to watch one another repeat experiments?
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TG Discussion
In this experiment all students receive what appear to be identical groups of chemicals. Some students tend to disregard their observations and adopt the majority opinion. This experiment can help to build a student's confidence in making observations.
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TG Disposal
Collect the used chemicals into a single vessel. Treat with 1 M sulfuric acid and allow to stand for several days. Filter the solid lead sulfate, and hold for solid waste disposal. Pour the supernatant liquid into the sink and flush with 10 volumes of tap water per volume of supernatant fluid.
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TG Hazards
1 M HCl is corrosive. Lead nitrate is toxic.
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TG Lab Hints
- The acetate sheet is an inexpensive and readily available alternative to expensive porcelain spot plates. Markings need not be made on the acetate; instead, a suitably-marked reaction grid is placed underneath the acetate. (These grids may be laminated for repeated use.)
- Beral pipets are best for holding solutions. Describe to students the techniques needed to avoid contamination. (A pipet tip should not touch anything except the solution it is being used to deliver. Only one pipet should be out at a time. Pipet tips should not touch plastic sheets.)
- The bromcresol green solution in set two must match the color of the copper sulfate solution in set one.
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TG Materials
Two sets of chemicals which appear to be alike. Half of the students receive one set while the other students receive the other set.
Set One: Beral pipets labeled A to D and a microcentrifuge tube labeled E containing:
- 1 M hydrochloric acid (8.3 mL concentrated HCl diluted in water to 100 mL)
- 0.1 M copper sulfate pentahydrate (2.5 g dissolved in water to 100 mL)
- 0.1 M potassium iodide (1.7 g dissolved in water to 100 mL)
- 0.1 M lead(II) nitrate (3.3 g dissolved in water to 100 mL)
- Mossy zinc metal or zinc chips (in a capped microcentrifuge tube)
Set Two: (A similar group of materials).
- 1 M hydrochloric acid solution (dissolve 8.3 mL of concentrated HCl in enough distilled water to make 100 mL of solution)
- Bromcresol green solution (0.01 g diluted with a 0.001 M sodium hydroxide solution to 100 mL)
- 0.1 M potassium iodide (1.7 g KI dissolved in enough distilled water to make 100 mL of solution)
- 1 M ammonium hydroxide (7.5 mL concentrated ammonia diluted in water to 100 mL)
- Mossy zinc metal or zinc chips (in a capped microcentrifuge tube)
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TG Presentation ?
- What is the best way for observations to be checked?
- This is a question without a specific answer. In this case, switching sets of bottles will be effective. When the instructor uses one of the sets at the overhead projector, students who used the other set will probably be provoked to comment. Some students actually will change their report of what they saw.
- Why do the drops stay in one place on the plastic instead of running all over?
- Surface tension holds the drops in place.
- What happens if the plastic is placed on a tilted table or surface?
- Nothing happens if the tilt is slight; surface tension is strong enough to hold them in place. The drops run if the tilt is pronounced.
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TG Purpose
- To make observations of reactions.
- To organize data in tabular form.
- To draw conclusions from observations.
- To defend conclusions in a debate.
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TG Time
Teacher preparation: 50 minutes
Presentation: 20-30 minutes
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