This unit focuses on chemical properties and chemical reactions. Students analyze evidence and property changes that allow them to distinguish between chemical and physical reactions.
This unit is also designed to answer the Essential Question, "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?"
In addition, students investigate the Law of Conservation of Mass as they look at how bonds are broken and formed in chemical reactions. This unit is full of hands on labs and station rotations that will engage any middle school student in chemistry!
In this lesson, students observe three awesome demonstrations and look for patterns in their qualitative observations to identify signs of a chemical change for themselves! The demonstrations include Elephant's Toothpaste, Iodine Clock Reaction, and Rocket Engine!
This lesson is designed to connect to the following NGSS and Common Core Standards:
MS-PS1 - 2 Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
CCSS.ELA-LITERACY.WHST.6-8.1.B Support claim(s) with logical reasoning and relevant, accurate data and evidence that demonstrate an understanding of the topic or text, using credible sources.
CCSS.ELA-LITERACY.WHST.6-8.2 Write informative/explanatory texts, including the narration of historical events, scientific procedures/experiments, or technical processes.
Science and Engineering Practices:
The NGSS asks that students develop and/or use a model to predict and/or describe phenomena (SP2). In doing this, students should construct, use, and/or present an oral and written argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon. When students look for patterns in data to develop ideas for the signs of a chemical change based on their qualitative observations, they do just that! (SP7) Therefore, this also means that students analyze and interpret data to provide evidence for phenomena. (SP4)
After watching the demonstrations, students search for patterns in their observations to develop their own ideas of what the signs of a chemical change are. Students thus realize that patterns can be used to predict phenomena. (Patterns)
Ask students, "What are you going to learn today?". Students should respond by saying that they will be answering the Essential Question, "How do particles combine into new substances? And, what evidence can show how the physical and chemical properties of the substances change?" This EQ is posted on my board and on the student's Chemistry Unit Plan.
Explain to students that this lesson represents the first lesson that students work with Skill 5 of the Chemistry Unit Plan, "I can provide evidence to show if a reaction is a chemical or physical change." Have the students turn to their unit plans and silently read the skill. After reading the skill have the students rank their current level of mastery on a scale of 1 to 4 (4 being mastery).
In my classroom, students frequently self-assess their level of understanding on each skill in the unit as we go. As you can see from the image below, this student ranks herself as a "2" to start this lesson. As we work more on this skill in the upcoming lessons, the student will re-assess and update this score.
Explain to the students that scientists not only use evidence to support their claims, but they also search for patterns in their evidence in order to predict phenomena. Scientists analyze and interpret data to explain phenomena they see in the world. Let students know that in today's lesson they will need to use their observation skills as they watch multiple reactions to collect data that they can use to predict if reactions are chemical or physical changes.
For a look at all the lessons that have led my students to this point and where we go from here check out the lessons in these units:
Physical Properties: Molecular Arrangement and Phase Changes: Focuses on Skills 1 - 4 of the Chemistry Unit Plan
This unit is designed to answer the Essential Question, "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?".
It particularly focuses on types of matter, physical properties, phase changes, and factors that affect physical properties. This unit's purpose is so much more than just the content, however. It's focus is scientific literacy. It stresses group discussion, discourse and utilizing text references when engaging in argument. Students utilize reading, writing, and speaking strategies in order to develop scientific literacy. It's scientific literacy immersion!
Chemical Properties and Reactions: Focuses on Skills 4 - 6 of the Chemistry Unit Plan.
This unit is also designed to answer the Essential Question, "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?". This unit focuses on chemical properties and chemical reactions. Students analyze evidence and property changes that allow them to distinguish between chemical and physical reactions. In addition, students investigate the Law of Conservation of Mass as they look at how bonds are broken and formed in chemical reactions. This unit is full of hands on labs and station rotations that will engage any middle school student in chemistry!
Provide students with the Chemical Reactions Un-Notes Sheet. Explain that during each reaction, they need to write down as many observations that they can see during the reactions. As we are trying to determine evidence that will determine if a reaction is chemical or physical, it is important for students to especially note any changes to the properties of the substances involved that they see occurring.
It is also important for students to be aware that the chemical being used in these reactions have safety and health hazards. Students must wear goggles and may not touch any of the products formed during the reactions. It is important for you as a teacher to read all of the safety and health hazards for each chemical used in the demonstrations to ensure that you and the students are safe during these reactions.
In my classroom, on the first day of a new unit or skill, I have students participate in demonstrations or labs in which the students develop their own understanding instead of taking traditional notes. Instead of me telling students the signs of a chemical change, they can discover this for themselves. Students call these "un-notes" because that are so different than the typical notes they are used to taking.
Demonstration #1: The Iodine Clock Reaction
¼ and ½ tsp measuring spoons
3 beakers – 500 ml
Stirring rods or spoons
Mixture of ¼ tsp sodium sulfite and ¼ tsp citric acid
1. Pour ¾ cup water into 1st beaker.
2. Stir in 1 starch pellet until it (sort of) dissolves (it won’t entirely).
3. Stir in ½ tsp sodium sulfite/citric acid mixture until dissolves.
4. In 2nd beaker, pour in ¾ cup water.
5. In 2nd beaker, stir in ¼ tsp sodium iodate until dissolves.
6. Pour beaker 1 into beaker 2.
7. Pour mixture back into beaker 1.
8. Pour mixture into beaker 3.
This one happens "all of a sudden". Let students know that even if it looks like nothing is happening, that they shouldn't take their eyes off it or they will miss it! After the reaction has finished, provide students time to write down their observations.
Demonstration #1: Rocket Engine
Long eye dropper
1. Attach a test tube at an angle on a ring stand over a Bunsen burner. Make sure that the opening of the test tube is not pointed at students.
2. Place a small amount of Potassium Chlorate (about the tip of a scapula) in the bottom of the test tube.
3. Place a small amount of Manganese Dioxide (about the tip of a scapula) on top of the Potassium Chlorate.
4. Using a long eye dropper, place a few drops of glycerin on top of the powders.
5. Light the Bunsen burner. (The burner is only supplying heat, lower the flame so that it is not "engulfing" the bottom of the test tube.)
This one takes a minute to develop. When the smoke starts, it's almost ready to react! After the reaction has finished, provide students time to write down their observations.
Demonstration #3: Elephant's Toothpaste
2 100 mL beakers
30% Hydrogen Peroxide
Saturated solution of Potassium Iodide
Dawn dish detergent
1. Wearing gloves, pour 80 mL of the 30% Hydrogen Peroxide into a large graduated cylinder (you can use a 2 Liter bottle as well).
2. Squeeze about 30 mL of Dawn dish detergent (the amount does not need to be exact).
3. Swirl the two around in the bottom of the container.
4. Drip two different colors of food coloring in the "corners" of the cylinder. This is what will make it look like toothpaste.
5. Pour in 40 mL of the saturated Potassium Iodide solution.
6. Step back!
7. Once the reaction is complete, light the end of a wooden skewer and then blow it out so that there are red embers left at the end. Slowly lower the hot tip into the bubbles. The skewer will reignite!
After the reaction has finished, provide students time to write down their observations.
Provide students with time to complete the Chemical Reactions Un-Notes Page. This page can be challenging. Encourage group dialogue when completing it. Analyzing data to identify patterns is a skill that most middle school students have not yet mastered. Group discussion will be important to help students recognize the patterns in the data.
Students write down a large variety of observations. However, it is important that they are noting the changes in color and temperature as well as the idea that gases are being produced when they see bubbles or smoke.
Patterns: Signs of a chemical change.
After looking at the observations they made in the previous questions, students recognize that all of these reactions included color change, temperature change, and gas production. Thus, they develop the idea that if any reaction demonstrates these changes, they could reason it might be a chemical reaction.
It is important to clarify with students that temperature change can mean that it increases in temperature or decreases in temperature. As these three reactions all "got hotter", students need to connect to the idea that some reactions can cause the surroundings to get colder. Make sure students are not stating that "getting hotter" is a sign of a chemical reaction, but simply that a "change in temperature" can indicate a chemical change.
Venn Diagram: Physical Change vs. Chemical Change
In this diagram, students identify that both the chemical reactions we saw today as well as boiling include a temperature change and gas production. Hopefully, what they notice is that in boiling the gas is still water, but in the reactions today, a new gas was formed. Also, students should note that when we boil water, we add energy but in the reactions today, the reactions produced energy on their own without us adding any heat.
This question makes students really have to think. In addition, without a strong foundation in what is actually occurring during a phase change, it will be challenging for students to complete this diagram. If you are not sure your students have a strong foundation in phase changes, a quick review might be in order!
What is a chemical change?
Every year I have students that get so wrapped up in the "5 signs of a chemical change" that they lose sight in the fundamental difference between a chemical and physical change. Thus, in this question, students should note that boiling results in gas production and a temperature change. If they solely considered this, they would identify that boiling is a chemical reaction. However, it is important that they recognize that the key to something being a chemical reaction is that something new is produced that has different chemical and physical properties than the reactants. Therefore, even though boiling water produces a gas, it is still water, it is simply water in its gaseous state. And, while there is a temperature change in boiling, this occurs as we add energy, it is not due to the breaking or forming of bonds.
After developing this initial definition, it will be important for you in an upcoming lesson to model and demonstrate to students how bonds are broken and formed in chemical reactions. On this first day of my unit, my goal is that students identify that in a chemical reaction new substances are formed and that gas production, temperature change, and color change often are signs that a chemical reaction is taking place.
Have students share the patterns they noticed in their observations with the class. On the board or chart paper, be sure to write down the signs of a chemical change that the students identify. From these demonstrations, students should be able to identify:
Let students know that these are a few of the pieces of evidence scientists look for when determining if a reaction is a chemical or physical change. Let the students know that in the upcoming lessons, they will be looking for these signs in the reactions they observe. In addition, let students know that there will be two more pieces of evidence they will be looking for that we didn't observe today: pH change and solid formation (precipitate).
Next, explain that while these signs of a chemical change can be helpful in making predictions, it is important that students recognize that these pieces of evidence do not always mean a chemical change has occurred. Ask students to share what occurred in both the chemical reactions we saw today and the physical reaction of boiling. Students should mention temperature change and gas production. It is important for students to recognize that while these pieces of evidence they came up with today are helpful predictors of a chemical change, they do not always mean a chemical change occurred.
After considering this idea, have the students develop their own definition of what makes a chemical reaction different from a physical reaction if they can both have temperature changes and gas production. Students should typically say things such as, "In a chemical reaction, new substances are formed.", "In a chemical reaction, gas production means a new gas is produced. In a physical reaction like boiling, the gas produced is the same gas.", and "The temperature change in a physical reaction occurs because we are adding or removing energy. In a chemical reaction, the temperature changes without us adding the energy."
Validate these thoughts and emphasize to the students that in a chemical reaction, new substances with different properties from the reactants are produced. In order to do this, bonds in the reactants are broken and new bonds are formed to create the products. Let students know that over the next week they will be looking more closely at what is occurring during chemical reactions and at the evidence that can be used to predict these reactions.