Law of Conservation of Mass
Physical Science/Chemistry. Recommended for grades 9-12.
Teacher Prep: 20-30 min
Student Activity: 30 min
Safety Gloves Required
Safety Goggles Required
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Essential Question
How can the law of conservation of mass be demonstrated for physical and chemical changes?
Investigation Objectives
- Explain the law of conservation of mass for physical and chemical changes.
- Demonstrate the law of conservation of mass mathematically.
- Recognize the difference between physical and chemical changes.
Next Generation Science Standards* (NGSS)
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Science and Engineering Practices
Developing and Using Models
- Develop a model based on evidence to illustrate the relationships between systems or components of a system.
Disciplinary Core Ideas
PS1.A: Structure and Properties of Matter
- The law of conservation of mass models how reactants are broken down and reformed to make products so total mass remains constant.
Crosscutting Concepts
Energy and Matter: Flows, Cycles, and Conservation
- The law of conservation of mass tracks the movement of atoms within a system.
Safety & Disposal
Wear gloves and goggles and practice safe laboratory procedures.
Calcium carbonate, the precipitate, should be collected from all groups and disposed of in the manner prescribed by local, state and federal guidelines. Do NOT flush it down the sink.
Procedures
- Place the beaker or cup on the balance, demonstrate how to zero or tare balances, record the mass in grams, and then zero or tare the beaker.
- If ice cubes are the only type of ice available, wrap cubes in a towel and break them into pieces with a hammer so they will fit into the tube.
- Remove the cap from the test tube. Fill the test tube half full of chipped ice. Replace the cap.
- Prepare enough volume of each solution for the total number of lab groups. Add an additional 10 to 20% more volume in case students make mistakes.
- Weigh the capped test tube in the beaker or cup on the balance. Record the mass in table A.
- See Carolina’s Solution Preparation Manual for instructions on preparing molar solutions.
- Place the test tube on your lab table and continue to make observations until the ice has melted.
- Demonstrate how to zero or tare balances.
- Reweigh the capped tube in the beaker or cup after the ice has melted. Record the mass in table A.
- Review and discuss with students what physical and chemical changes are and what type of evidence they may observe.
- To determine the change in mass, take the mass of the water with tube and substract the mass of the ice with tube.
- Keep ice in a well-insulated cooler to minimize melting.
- Record the change in mass in table A.
- Emphasize that caps must be on the tubes securely. No leaks. Remind students that the mass of the beaker or cup should not be included in the recorded mass.
Procedure B: CaCl2 and Na2CO3
- Place the beaker or cup on the balance and zero or tare the balance.
- Have all solutions prepared, labeled, and in a central location.
- Using a clean graduated cylinder, measure 1.5 mL of calcium chloride solution (CaCl2).
- Instruct students that M is molarity, which is a unit of concentration. Demonstrate how to pour liquids.
- Pour the calcium chloride into a test tube.
- Wipe off the outside of the tube with a paper towel to remove any excess. Place the tube in the beaker or cup on the balance.
- This is a good time to introduce the vocabulary and concepts of reactants and products.
- Using a clean graduated cylinder, measure 1.5 mL of sodium carbonate solution (Na2CO3).
- Emphasize to students that the graduated cylinder must be clean or a reaction will take place in the cylinder.
- Pour the sodium carbonate into the test tube.
- Wipe off the tube with a paper towel to remove any excess. Place the tube in the beaker or cup on the balance.
- Record the mass of both tubes with solutions in table B.
- If students spill a solution, wipe it up immediately with a paper towel and dispose of the paper towel in the trash.
- Pour the calcium chloride into the test tube that contains sodium carbonate and record your observations in table B.
- If students spill a solution, they will need to start over.
- After combining the calcium chloride and sodium carbonate, reweigh both test tubes in the beaker or cup. Record the mass in table B.
- To determine the change in mass, take the nal mass of the test tubes and subtract the beginning mass of the test tubes. Record the change of mass in table B.
Data and Observations
See the student worksheet. Answers will vary but should be close.
Analysis & Discussion
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Use the data to explain whether mass was conserved in procedures A and B.
Mass should be conserved in both procedures. The initial mass was the same as the final mass.
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Translate your explanation above into a mathematical statement.
mass before = mass after
mass(i) = mass(f)
mass(reactants) = mass(products) -
Create a particle diagram for each process.
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Identify the processes above as a chemical change or physical change, and state the observations used to make the identification.
A. Physical: Water did not change its identity, only its state of matter.
B. Chemical: Two new products and a precipitate were formed.
*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.
