This is the fourth lesson in the chemical reaction unit that covers HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions.
The objective of this lesson is to have students model the Law of Conservation of Mass using a ExploreLearning simulation. In the process students will be required to balance chemical equation, exhibiting proficiency of the Science and Engineering Practice of Mathematics and Computational Thinking.
During this lesson students will recognize that energy and matter is an underlying crosscutting concept involved in all chemical reactions. This is not the main focus of this lesson, but as students continue to develop an understanding of chemical reactions throughout the unit they will build on the idea that matter and energy are conserved in chemical reactions.
In the previous lesson students were introduced to the Law of Conservation of Mass and how to count atoms in a chemical equation. To start this lesson I check the previous day’s homework assignment. While I am walking around I have the answer key displayed on the document camera so students can check their work for accuracy and develop any questions about counting atoms. After a few minutes I switch to the second page.
If students have no questions we move on to the ExploreLearning Chemical Equations Prior Knowledge Questions. The purpose of these questions is to activate prior knowledge and get students thinking.
While students are answering these I am preparing a demo that will demonstrate conservation of matter. I place a large candle on a triple-beam balance and measure its mass so that the balance is balanced. I then light the candle and wait for about one minute.
After a couple minutes I ask, “what happens to the mass of the candle, and how can you tell?” “Where did the missing mass go?” Student responses: It was burned and it turned into smoke and heat
I then Blow out the candle, and invert a large jar over the candle. Rebalance the candle and jar, then remove the jar, light the candle, and replace the jar. I let the candle burn for several seconds before running out of air and burning out.
After a minute or two I’ll ask, “what happened this time?” and “why doesn’t the mass of the candle/jar change now?” Student responses: The smoke and heat remained in the jar. Smoke has mass and it was not able to escape.
The demo is meant to model the Law of Conservation of Mass and provide students with a mental image all matter contains mass, even when it can’t be seen such as a gas.
After a brief discussion I have them log onto the computer. I like for them to work individually because any time mathematical computations are involve I believe that working individually maximize comprehension. To make sure they know what they are doing, I give them a brief tutorial of how the Gizmo works.
Once they have an idea of how the Gizmo works I tell them to complete sections A and B. These sections should take 20-25 minutes. As they are working, I am walking around helping some of the kids that are struggling. I will typically let them work through both sections before stopping them, but if there are a significant number of kids that are struggling I will stop the class and work through section A before letting them move on.
This portion of the activity has students count atoms on the reactant side of a chemical reaction. This portion of the activity was very simple for students because they had worked on counting atoms they day before, so this acted as a reinforcement. This skill is important because if they are unable to count atoms in chemical formulas then they will probably not be able to balance a chemical equation. If a student does struggle with counting it tends to be on the more difficult formulas with polyatomic ions and subscripts outside of the parenthesis. These students will require extra practice.
Activity B is the students first exposure to balancing chemical equations. In this activity they are asked to add coeffiencents to the reactants and products, as they change coefficients they will see the number of atoms change. This will allow them to actually count the atoms on both the reactant and product sides and compare the number of atoms. This process helps them visualize both sides of the equation and balance accordingly.
Some students struggle with this, but since this the first time they are exposed to balancing this is to be expected.
I typically use the either ExploreLearning or a PhET simulation once a unit because they both provide an excellent chance for students to model difficult concepts. There are a variety of web based simulations that students can use. I particularly like ExploreLearning over others because it teaches them how to count atoms, balance chemical equations and references the Law of Conservation of Mass. It does an excellent job of teaching kids how to balance in addition to focusing on terminology that they use throughout the semester. Finally, it summarizes balancing by introducing users to the different types of reactions which will be address later in the unit.
If you don’t have access to ExploreLearning, PhET and The Science Spot (Kidzone) have good simulations too.
At the end of class I ask the students to stop what they are doing and answer the 5 assessment questions at the end of the Gizmo on a separate sheet of paper. This is the only part of the activity that I collect because I go over the Gizmo the following period. The 5 assessment questions provide a sufficient snapshot of student understanding which enables me to get quick and effective feedback. I can then use this feedback to adjust my teaching accordingly and decide how much more practice is needed. Typically more will be needed because this is the first time balancing chemical reactions.