Conservation of Mass Lab

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Objective

SWBAT model, identify and balance chemical reactions demonstrating the Law of Conservation of Mass.

Big Idea

Students conduct an investigation that shows mass is conserved during the chemical reaction between sulfuric acid and steel wool (iron).

NGSS Standard Alignment

Performance Expectation/DCI

This lesson is aligned with 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.  In this lesson students will examine the Law of Conservation of Mass in a chemical reaction, showing the total mass of the reactants equals the mass of the products.

Science and Engineering Practices (SP)

This lesson will cover all aspects of the 5E model using a guided inquiry approach.  The Explore portion of the lesson has students observe a demonstration and plan & carry out an investigations (SP 3) that illustrates the Law of Conservation of Mass. 

The Expain and Elaborate portion of the lesson has students analyzing & interpreting data (SP 4) using mathematics & computational thinking (SP5) during the investigation.

After analyzing the data students will be required to construct an Explanations (SP6) why matter is conserved.  This will come in the form of answering analysis questions and writing a conclusion about the lab activity.

Crosscutting Concepts (XC)

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 part of chemical reactions.

 

Engage

10 minutes

As students walk in they know it is lab day, so they know to get goggles and an apron for the lab.

This lesson starts with a pre-lab activity called Learning goal inventory (LGI).  This learning strategy comes from Page Keeley’s, Science Formative Assessment: 75 Practical Strategies for Linking Assessment, Instruction, and Learning.  The purpose of this strategy is to activate thinking about a topic that has a specific learning goal.  I chose to do this activity during this lab because conservation of matter is explicitly mentioned in DCI:  PS1.2 and is the overarching goal for HS-PS1-7.

Students have been exposed to the Law of Conservation of Mass prior to this lesson, however, it has not been a formal topic of conversation.  Prior to this lesson the concept has only been embedded in instruction to teach other concepts, such as balancing chemical equations.

Implementation

Prior to starting the lab I will pass out the LGI form and ask students to read the bold learning goal that is stated on the paper:  Learning Goal: 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.  

After reading the goal I instruct them to take a few thoughtful moment to fill in the four boxes below the learning goal.  I tell them that this will require some though and anything is better than nothing.  I let them know that it needs to be completed as part of the lab to receive 5 lab points.  I collect it after they are done and while they are working on the lab I read them to assess prior knowledge.  If it was completed they will receive full credit.

The goal of students completing this prior to the lab is to raise awareness about what the focus of the  learning goal of the activity is.  A key principle of learning is that students understand what a learning target is.  This also allows me to address the learning target from a student-centered perspective and address what they know and don’t know about the topic.  This can be used to direct my instruction throughout the unit by having them re-answer the questions after the activity and assessing what level of growth was attained.

Below are several examples of student work that illustrates what they have learned throughout the unit and their understanding of the Law of Conservation of Mass. The following pieces of student work are good examples of how students can answer the questions differently but still demonstrate an understanding of HS-PS1-7.  

 

Explore

20 minutes

Since this lab requires minimal supplies and no computer probeware (such as PASCO or Venier) I have students work in groups of two.  While they were working the pre-lab I handed out the Conservation of Mass Lab.  At this point in the school year the students understand how to weigh materials and perform labs safely, so I provide them little instruction on how to perform this lab.

However, I do instruct students to read through the procedure with their lab partners before starting. I also make sure that all students have written a close-ended question and a hypothesis before proceeding into the lab investigation.  Students often forget to complete these and lose points because of it.  A simple reminder alleviates any problems.

A perfectly acceptable question would be, "Is mass conserved during the chemical reaction of iron and sulfuric acid.  This question is simple and very testable.  Some students will try to over think the question and create a open-ended question such as, "how does the total mass of the reactants change to equal the mass of the products after a chemical reaction?"  There is nothing wrong with  this question and they will receive full credit; however, an open-ended question like this would require further research and is outside the scope of our investigation.  I would simple instruct the student to change the question to a close-ended question.

Since students are working in groups of two, two sets of the lab materials are necessary at each table.  Because this lab requires very little set-up and can be accomplished by the students in a short period of time, the only materials I put out for them are the steel wool, balloons and sulfuric acid.  The rest of the material students know where to locate.  This also requires them to take care of their own clean-up.

The following instructions can be viewed on the lab handout.  

Lab Materials:   balance, funnel, balloon, 125ml Flask, 100 mL beaker, 1 M sulfuric acid

Safety:  Wear goggles and apron at all times.  Handle sulfuric acid with extreme care.  Do not dump waste in the sink without instruction.

Procedure:

Part A:

  1. Weigh a 125-mL flask.
  2. Pour about 20 mL of 1 M sulfuric acid into the beaker
  3. Weigh the beaker to get the mass of the sulfuric acid.
  4. Weigh an egg sized piece of steel wool.
  5. Carefully place the steel wool into the beaker and allow the chemical change to fully react.  To be sure it is completed, carefully swirl the chemicals and observe.  Measure and record the total mass of the products and containers using the same balance you used before.  

Part B:

  1. Weigh a 125-mL flask. 
  2. Pour about 25 mL of sulfuric acid into flask using the funnel to make certain the neck of the flask does not get wet. 
  3. Weigh the flask with the sulfuric acid.  
  4. Weigh the balloon and record the mass.
  5. Weigh an egg sized piece of steel wool.
  6. Carefully place the steel wool into the flask.
  7. Place the balloon over the mouth of the flask, 
  8. DO NOT REMOVE THE BALLOON!
  9. To be sure it is completed, carefully swirl the chemicals and observe. 
  10. Measure and record the total mass of the products, flask, and balloon. 
  11. Remove the balloon, observe what occurs, and reweigh the products without the balloon.

 Once the groups get started step A and B only takes 15 minutes.

As they are performing the lab in the early stages, my main objective is to ensure that they are handling the sulfuric acid with care and weighing things properly.  If they don’t weigh stuff properly the results will be misleading.  From my experience most students do this correctly, but it’s best to double-check with all groups.

As groups are finishing up I remind them that they need to clean their stations.  If their stations are not clean they will lose points.  I also tell them to put the starting and finishing mass of the reaction A and B on the board.  Once every group has provided their data on board I will remove any outlying data (that clearly looks like the lab was done improperly) and take a class average. This data will be put in their data table and will provide a better results (by alleviating potential experimental error) for students to see that mass is conserved before and after the reaction.  

Explain

10 minutes

Before students answer the analysis questions, I lead a brief discussion to summarize the class data. I typically focus attention to the group data shortly before everyone has completed the lab and cleaned-up their lab stations.

Upon completion of the investigation (procedure A and B) students should answer the following analysis questions on a separate sheet of paper:

  1. What evidence do you have that a chemical reaction took place when steel wool was added to the acid?
  2. What happened to the mass when the experiment was performed in an open beaker? Try to explain the result.
  3. Why did you not get the same result when you used the closed bottle?
  4. In Part A, was there a difference between the mass of the reactants and the mass of the products? Why?
  5. Study your results from Parts B. Even though both reactions were covered, mass was not necessarily conserved in each experiment.  Propose a theory explaining this discrepancy (this difference).
  6. In which Part(s) was/were the Law of Conservation of Mass verified?  Describe your evidence.
  7. What possible errors occurred during your experiment?
  8. Why is it better to base results on class data, instead of individual data?

Students' answers to these questions (see lab key for quality answers) should provide an explanation that matter was conserved or not conserved.  If matter was not conserved, they need to provide reasoning why this result occurred (question 7).  To ensure that the learning goal for the lesson (SP1.B) was achieved students should answer the analysis questions based on the class data that was put on the board.  See student work

Elaborate (homework)

This portion of the lesson has students complete the conclusion and balancing portion of the lab. I tell students that they need to look at the rubric at the end of the lab and assess how many points each of these sections are worth.  I say this because sometimes students struggle to reflect on a lab after they have left the classroom.  With this being said, I reinforce the importance of the conclusion and balancing problems by drawing attention to the rubric, and asking students to think about the learning goal.

To bring the learning goal to students' attention I ask someone to volunteer what they learned during the lesson. Most students realize that mass is conserved during a chemical reaction and that it could only be measured with a closed system (trial B) because the gas in trail A was able to escape into the air.  They also realized that there was a variety of ways that experimental error could have occurred, most commonly by not putting the balloon on quickly and not taking precise measurements of the steel wool and acid.

I follow this by giving them a due date of two days later.  I give two days so students can ask for help, and the next class can begin with a discussion about what was learned during this lab.

After performing this lab I would change a couple of little things.  First I would use a 2 or 3 M sulfuric acid, instead of 1 M because it would yield quicker and more abundant results.  However, the only problem with a stronger concentration is that safety would be more of an issue.  Students would perhaps be able to see a greater difference in mass.  Second I would encourage students to leave the ball of steel wool more loosely packed therefore the acid would have more surface area to react.  Having a too tightly packed ball of steel wool does not allow for enough acid to react for a noticeable change in mass.