The statement, "Mass is conserved", literally sounds like a foreign language to middle school students. While this law is a fundamental concept in the study of the physical world, it is not one that immediately "makes sense" for students who are used to the magic of Harry Potter and the transfer of information at the click of a finger. As students begin to understand the Law of Conservation of Mass, they begin to realize the elegance of the idea that the universe is finite in terms of the available building blocks of matter - that we indeed, are made of the "stuff of stars".
This lesson is an introduction to the Law of Conservation of Mass in which students are able to develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved (MS-PS1-5). In this case, students use images, chemical equations and physical and chemical changes to analyze and interpret data to provide evidence for this phenomena (SP4) that matter is conserved because atoms are conserved in physical and chemical processes (CCC).
This lesson assumes students have exposure to the basic concepts of atomic structure, chemical formulas and physical and chemical changes. Due to the complex nature of this concept and its support of NGSS cross cutting concepts, this lesson becomes a "touchstone" lesson - one that should be referred back to throughout a unit of study on matter or physical and chemical changes to reinforce the learning of the concept.
In order to ENGAGE students in this lesson, students read this Carl Sagan quote:
In either words or pictures, students respond to this quote by considering this prompt:
What does this quote mean?
The resulting discussion may go in many directions, which is part of the engagement process. The important teaching move here, is to wait for students to make the connection that stars and humans are made of the same types of atoms. The next question becomes:
So, can you be made of an atom that was actually in a star at some point?
At this point, students share a few ideas and the Law of Conservation of Mass is introduced. The following visual helps this introduction:
The EXPLORE stage of the lesson is to get students involved in the topic so that they start to build their own understanding. To help students explore the Law of Conservation of Mass, students complete several activities comprising the Conservation of Mass Investigation.
The Background Information section of this investigation is integral to student understanding. The misconceptions related to the concept, new vocabulary and conceptual steps that must be understood in order to build accurate understanding combine to make this a truly challenging investigation. As students work through this section in groups, students need varying levels of support. This support can include a whole-class lesson, differentiated lessons for groups or individual students, provision of additional resources or use of manipulatives (double pan balance and atom/molecule models). Checks for understanding are also necessary to ensure students are processing the information. These checks for understanding take the form of probing questions such as:
When we are talking about the word conservation, what do we mean?
What has mass in this situation?
What happens during a chemical reaction?
What happens during a physical change?
Do the atoms themselves change in a reaction or do they just change positions?
What are reactants?
What are products?
If there is 36 amu of mass in the reactants side of the equation, how much mass should the atoms in the products side have?
What does the arrow mean in a chemical reaction?
What is the difference between a chemical reaction and physical change?
Once a level of understanding is achieved, students conduct two investigations to measure and compare the mass of a closed system before, during and after a physical change and chemical change takes place to verify the Law of Conservation of Mass. Students use freezer pops and glow sticks to act as the closed systems for the investigation.
Teacher Note: Several items to note:
The EXPLAIN stage provides students with an opportunity to communicate what they have learned so far and figure out what it means. This stage of the lesson presents a great place for a quick formative assessment. Students explain what they have learned by answering several analysis questions located at the end of the investigation using evidence from their investigation to support their claims (Writing Arguments from Evidence). Students may do this individually or collectively with their groups. For a creative alternative, read this section's reflection: Writing Round Robin - Data Analysis Collaboration. For an example of student work using this strategy, view: Conservation of Mass Investigation Student Work.
The EXTEND stage allows students to apply new knowledge to a novel situation. The novel situation in this case is for students to design their own investigation using vinegar and baking soda to collect data that proves the Law of Conservation of Mass. This extension can also serve as a performance assessment to provide evidence that students understand this concept or give students additional practice planning and carrying out their own investigations (SP3). This resource: Conservation of Mass Investigation Extension provides example procedures, data tables and analysis questions to support student investigation. For an example of a student procedure, view: Conservation of Mass Investigation Extension Student Work.
The EVALUATION stage is for both students and teachers to determine how much learning and understanding has taken place. The conceptual rigor involved with this lesson often requires additional instruction on this topic, which can be measured through assessment such as review of analysis questions like these: Conservation of Mass Investigation Analysis Questions.