What is Conservation of Matter?
Lesson 9 of 12
Objective: TSW conduct an investigation and develop a model to demonstrate the Law of Conservation of Matter.
Required Background Knowledge:
In order to be successful in meeting the objectives outlined in this lesson, students should already be proficient in:
- Using a triple beam balance (or other selected tool) to determine mass
- Reading basic chemical equations
- Differentiating between chemical and physical changes
- Understanding the meaning of matter and mass
I begin the lesson by posting the following question on the board:
Assuming the scientists in the video started out with approximately 100 billion molecules in this experiment, about how many molecules do you think they there were in the end? Explain why you think so.
Give each student an index card or sticky note to write down their response. After watching the Elephant Toothpaste video, allow 3-5 minutes for each student to respond. Then have a few students share their thoughts. You can also record the student responses on the board. I always do this, because it allows students to gauge their learning at the end of the lesson (and it also serves as a reminder to me to come back to this as part of the lesson closure!)
Pass out the Conservation of Matter Lab and direct students to the first paragraph. Have students underline the definition of the Law of Conservation of Matter. Ask them what they think this means, and have them record their thoughts in the margin. Explain to the students that the purpose of today's lab is to grasp a better understanding of the Law of Conservation of Matter, and how it applies to what we are currently learning.
PART 1- Closed Container
During the investigation section, students get a chance to perform a series of chemical reactions using vinegar and baking soda. I selected these materials because the chemicals - as well as the reaction that takes place - are safe for this age group to work with. In addition, most students have already performed this some time in their educational career. So while they may get excited to see the foaming and bubbling as the products react chemically, and they can focus more on the Law of Conservation, as opposed to the reaction itself.
The attached lab guide will lead the students throughout the entire investigation, allowing the facilitator to circulate through the room and monitor student activity. They will not be stuck in the front of the room leading the experiment, unable to support student discovery.
Students will begin by measuring quantities of each chemical and placing them separately in a large Ziploc bag, closing it tightly, and then measuring the mass of the bag and all of its contents. They will record the mass on the provided data table.
** I always recommend to my students that they measure at least twice! Many times, they get lost in the excitement of the investigation and rush through this part, causing their data to be incorrect. That will invalidate the entire investigation and make for a lot of misunderstandings in the end! (Better safe than sorry, right?!)
Once they record the mass, the students will then mix the chemicals, causing a reaction in the bag. They will observe what happens, recording their observations within the data table. They will measure the mass again, recording their data on the table. Students should learn that the mass is the same both before and after the chemical reaction. This is when I like to question them about why they think this is so. Most will quickly respond that items in the bag are still there and nothing has disappeared. Some will catch on quickly, aligning their findings to the Law of Conservation of Matter. You may need to do a little additional prompting to get them there.
PART 2 - Open Container
Students will repeat the process above, but with one major difference. Instead of mixing the chemicals in a Ziploc Baggie, they will do it in a large beaker or clear plastic cup. Upon collecting their measurements, they will realize that the mass, in fact, did decrease. Many will not expect this, since the mass was the same during Part 1.
**Keep in mind that the change in mass will be slight, so it is very important to use a balance that is accurate and somewhat sensitive. When I originally taught this lesson, I used basic elementary balances from the math adoption, and they were simply not sensitive enough to pick up on the change. You may want to try this part of the investigation on your own a few times, maybe even altering the quantities of the ingredients, until you are sure your students will be able to clearly determine the decrease in mass.
This becomes a great time to move on to the discussion section, as students will want to know why a change occurred once, but not both times.
Why was there a change in the mass of one chemical reaction, but no the other? Students are going to wrestle with this!
There are a few ways to demonstrate why the decrease in mass during the open container reaction occurred:
1. I like to perform a live demonstration for the students, using an Erlenmeyer, or conical, flask and a deflated balloon. I measure the same amounts of vinegar and baking soda as we used in the investigation. I place the baking soda in the balloon, place the vinegar in the flask, and quickly place a balloon over the flask, allowing the baking soda to fall into the flask and mix with the vinegar. This will cause a reaction, and the balloon will start to inflate, as a result of the carbon dioxide gas trying to escape from the flask. I ask the students with they think is causing the balloon to inflate. Most will know a gas is trying to escape but will not know what kind. I explain to them that the gas is carbon dioxide, then I ask them to hypothesize what they think would happen if the balloon was not on the flask. At this time, most students will respond that the gas will escape into the air. You can then confirm this hypothesis, accounting for the change in the mass when using an open container.
2. If students need extra support, or if you prefer not to do a live demonstration, you can always show the Conservation of Matter video. In this video, Eric Gosselin performs the same experiment and uses animations to demonstrate the escape to carbon dioxide gases. This is a pretty cut and dry, direct mode of explaining this process to the students.
This concept is also explained further in the lab guide, accompanied by the chemical equation of the reaction that has occurred. No matter how to you choose to demonstrate the concept, it is very important to reinforce that the carbon dioxide still exists, as explained in the lab guide. It is merely escaping into the atmosphere instead of staying trapped in the bag, as it was in part 1, which verifies the Law of Conservation of Matter. Have students read through this after you demonstrate the process to provide further explanation and deepen understanding.
**This activity can be done during the same day as the investigation, or can be a suitable break point. Stopping at the end of the discussion on day 1 and continuing with the model making on day 2 is an alternative if you teach shorter class periods.
During the modeling portion, the teacher hands out the color paper molecules to each student or student group.
**I prefer to do this in pairs to encourage discussion, and to allow for students to support each other in clearing up any misunderstandings. I give each group the same amount of every molecule. They must decide how many of each they will need and group them accordingly, placing the unneeded ones in a community pile for others.
If you are continuing this portion on day 2, you will want to review the discussion portion from the prior day. Have students refer to the chemical equation on their paper and/or write it on the board. You should then have the students use it as a guide to form the products used in the chemical reaction, and then move those same atoms around to form the reactants.
**A lot of students will want to use additional paper atoms to form the reactants. By doing this, they are actually creating new matter, which is exactly what we don't want them to do! By having them place their unwanted atoms in the community pile before making the transition from product to reactant, they are unable to do so.
Students will be prompted in their lab guide to have their models checked by the teacher before moving on. Because of this, you will want to circulate throughout the room, checking student progress, providing support for the those who need it, and asking clarifying or extending questions, such as:
- Why did you choose to create your model this way?
- How many O, H, N etc atoms will you need to complete the entire reaction? How do you know?
- How did you know to use _____ atoms of Hydrogen? Oxygen? etc. in this specific molecule?
- What type of molecule did you just create?
- What do we know about the Law of Conservation of Matter? How does that apply to your model?
- How would this model look different in Part 1, as opposed to Part 2?
Students reflect on the lab, deepening their understanding of the relationship between the Law of Conservation of Matter to the investigation and the model they have built. To do this, they answer the following questions:
- Describe what happens when the vinegar is poured into the baking soda.
- What evidence do you have to prove that a chemical reaction has taken place?
- What are the reactants used in this investigation? What are the products?
- Has any new matter been created or lost? How do you know?
- What was the difference in the change in mass between part 1 and part 2? How would you explain this?
- Explain how this investigation relates to the Law of Conservation of Matter.
The questions can be used as talking points during a class discussion after students have had ample time to reflect on their own. Students should be assessed on the accuracy and quality of their responses to the questions above. For those who have shown some misunderstanding, you may want to conduct small group instruction, guiding the reconstruction of the model and explaining how the atoms "move" from one side of the equation to the other.
Refer back to the Elephant Toothpaste video and the question that the students considered at the beginning of the lesson. Watch it one more time and ask them to revise their response on the sticky note, based on what they have learned in this lab. As you did in the beginning, you can have a few students share their thinking and refer to the initial guesses to see how they have changed. You can also have them revise the notes they took in the margin of the lab guide at the beginning of the lesson. These can both be used as a quick, informal assessment of student understanding.
For students who have adequately grasped the concept, you can extend the learning and encourage deeper thinking by posing the following challenge (provided on the lab guide):
This lab demonstrated one example of the Law of Conservation of Matter. Design another experiment to demonstrate this law. Describe your experiment, including the materials and procedures, below. What results should one expect to get? Why?
Many will think they need to conduct another experiment, and may even want to. Explain to the students they do not actually have to perform the experiment, but rather write out the materials, procedures, etc needed to conduct it. It may not be a bad idea, however, to provide them with this opportunity, as allowing the students to demonstrate another experiment to prove this law may actually provide a reteaching opportunity to those who need extra support.