May 2019 | Resource Feature
My Favorite Demonstrations for Teaching Chemical Reactions
By Stephanie Harry
One of my favorite units to teach is chemical reactions. Chemistry can be a challenging subject, but in my experience, my students seem to enjoy and understand many of the concepts covered in this unit. Since adopting the flipped learning teaching style, I am continuously looking for interactive reinforcement activities to use during class. In this unit in particular, I feel like I am heading in a good direction.
I begin the chemical reaction unit with a series of demonstrations to teach students how to identify evidence that a chemical reaction has occurred, how to write a word equation to explain a chemical reaction, and how to convert a word equation to a balanced chemical equation.
Before beginning the demonstration, I typically ask my students to provide me with a list of the five indicators that can suggest that a chemical reaction may have occurred. The suggested proofs identified by the students are: change in color, formation of a gas, formation of a precipitate, change in temperature, and production of light.
In my experience, although students can often list these five points, not all students have actually observed them in the chemistry setting. For example, I once had a student who thought adding water to red punch, and then seeing the color fade, was an indication of a chemical reaction. Another student assumed that because food changes temperature when placed in the refrigerator, this was a sign that a chemical reaction had occurred.
To overcome these misconceptions, I explain that when we are speaking of change in color or temperature, we are looking for unexpected changes. However, even in doing this, I had felt that I needed to find a way to provide a visual explanation to support my students’ understanding. This thought led me to create a series of seven chemical reaction demonstrations, based on my experience attending different workshops, collaborating with other teachers, and researching the topic online. These days, I perform these demonstrations in the following order:
- Copper metal granules are added to 1M hydrochloric acid.
- Zinc metal granules are added to 1M hydrochloric acid.
- Aqueous solutions of potassium iodide and lead (II) nitrate are combined.
- Aqueous solutions of potassium thiocyanate and iron (III) nitrate are combined.
- Aqueous solutions of potassium iodide and aluminum nitrate are combined.
- Ammonium dichromate is ignited.
- Magnesium ribbon is ignited.
To transition easily from one demonstration to the next, I set up the materials for the seven demonstrations in the following manner:
|Figure 1. Teacher-prepared demonstration materials.|
- Demonstrations 1 and 2:
I place the 1M hydrochloric acid in a test tube, and each metal sample in a beaker.
- Demonstrations 3, 4, and 5: I place one of the aqueous solutions in a test tube and the other solution in a dropper bottle. Alternatively, both reactants can be placed in test tubes.
- Demonstration 6: I place ammonium dichromate in a 1000 mL beaker (or larger) with wire gauze over the opening. Since it will be ignited, I also create a wick using a small piece of paper towel soaked in ethanol. I use tongs to place the wick on top of the ammonium dichromate (like placing a candle into a birthday cake) and then use a burning wooden splint to light the wick.
- Demonstration 7: I place a precut magnesium ribbon in a beaker.
When I am ready to begin the demonstrations, my students and I put on our personal protective equipment and I outline safety precautions prior to each demonstration. Although my students are not handling the chemicals directly, safety is the priority. One reason for this level of caution is that I plan to move around the classroom with the chemicals (as appropriate) for students to get a closer observation of the results.
I often get very specific about safety instructions. Prior to the first and second demonstrations, I inform students that the reactant, 1M HCl, is a corrosive chemical. It is essential that eyes and skin are protected from contact, and the demonstration should be performed in a fume hood or a well ventilated area. Additionally, prior to Demonstration 6, I explain that it will be performed in a fume hood because ammonium dichromate contains toxic chromium (VI). Finally, before we do Demonstration 7, I remind students to not look directly at the magnesium while it is burning in the reaction.
Wear protective gloves, clothing and eyewear for all demonstrations. Wash hands thoroughly after handling.
|1||1M HCl – use only in a hood or well-ventilated area.|
|2||1M HCl – use only in a hood or well-ventilated area.|
|3||Pb(NO3)2 – use only in a hood or well-ventilated area.|
|4||No additional safety requirements.|
|5||No additional safety requirements.|
|6||(NH4)2Cr2O7 – toxic; use only in a hood or in well ventilated area.|
|7||Mg – avoid looking directly at burning magnesium.|
Figure 2. Summary of safety precautions for each demonstration.
I also use a document camera to display each demonstration, which is connected to a classroom Smart Board, allowing all students to easily see the details of the reactions as they occur. Additionally, this serves as a helpful way to display the written information from the student handout next to the results. During the demonstrations, I compile and record information shared by my students in the data table displayed using the document camera and Smart Board.
Before beginning a reaction, I have my students describe the appearance of the reactant(s) in the data table provided on the student handout (Figure 3). I direct them to record details about color, physical state, texture of the reactants, etc.
Appearance of reactant(s)
Proof (List all that apply):
Appearance of product(s)
|1.||Copper metal + hydrochloric acid|
|2.||Zinc metal + hydrochloric acid|
Figure 3. Example of activity data table for students to compete.
As I complete each demonstration, I ask the students whether or not a chemical reaction has occurred. To help them determine their answer, I remind them to consider whether the appearance of any of the reactants changed, and record their answers in the appropriate column. I also remind them that chemical equations are written to explain chemical reactions; so, if there is no chemical reaction, then there is no chemical equation. This helps to remind them that just because there is no observable evidence (change in color, formation of a precipitate, etc.), it doesn’t mean that a chemical reaction has not occurred. Some additional notes that may be helpful:
- In this activity, Demonstrations 1 and 5 will not produce a chemical reaction.
- Five of the reactions will produce indicators of a chemical reaction for students to observe.
|Figure 4. The reaction between potassium thiocyanate and iron (III) nitrate produces a color change.|
- During Demonstration 2, students will observe the formation of gas, as bubbles are produced when zinc metal is added to a test tube containing 1M hydrochloric acid. Additionally, they will notice the change in color of the zinc metal.
- Students should note gas formation in several of the demonstrations, including Demonstrations 2 and 6.
- Demonstrations 3, 4, and 6 will each show a distinctive color change.
- Demonstration 3 will produce a precipitate.
- Students will notice light is produced in Demonstrations 6 and 7.
After all seven demonstrations have been completed, I work with students to develop word equations for any demonstrations that resulted in a chemical reaction.
Word Equations: If a chemical reaction was observed, then complete the word equation below:
1. Copper metal + hydrochloric acid → no reaction
2. Zinc metal + hydrochloric acid → zinc chloride + hydrogen gas
Figure 5. Fill-in-the-blank word equations allow students to indicate the products of the reactions that they observed.
I have found it interesting that after writing the name of the products in Demonstration 3, some students have seen a similarity to Demonstration 4, and were able to correctly predict the products for that reaction. Because these students saw a pattern between the rearrangement of metal and non-metal ions, I introduced them to the term double replacement. This also helped reiterate that all of the elements used in the reactant were present in the product.
In the final component of this activity, students convert the word equations to formula equations. Depending on the level of student understanding, teachers may want to keep the focus on proper formula writing, rather than on actually balancing the equation. My goal at this point in the lesson is to make sure students use their prior knowledge to write the correct formulas for each compound, and place them on the correct side of the chemical equation.
I complete the formula equation for Demonstration 2 with the class and allow the students to complete the other equations on their own. I use this time to move around the classroom and provide individualized help to students, sharing such tips as:
- When completing the formula equation for Demonstration 2, the elemental symbol for zinc is Zn, but the elemental symbol for hydrogen is H2.
- Be careful to properly incorporate symbols, states of matter, and yielding in the equation. I find it easier to share this guidance during this activity, since students have visual examples of the physical state of the reactants and products.
These chemical demonstrations have truly been beneficial in helping students identify indicators of chemical reactions, as they offer great visual examples of the five indicators of a chemical reaction. I have found that this lesson helps minimize my students’ misconceptions and gives them a visual connection to prove that a chemical reaction has occurred. Gathering, recording, and displaying information from students to complete the data table, and allowing students to engage in dialogue about the demonstrations, makes the activity both educational for my students, and more manageable for me.
This activity provides teachers with seven chemical demonstrations that can be easily setup and incorporated into a unit on chemical reactions. Chemical demonstrations are a great way to engage students and create interaction. I encourage you to try it with your own students!
(article cover) Kesu01/Bigstockphoto.com