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LAB in Conservation of Mass, Measurements. Last updated March 29, 2018.


In this lab, students will perform a simple chemical reaction in a closed system to produce a small amount of gas. They will collect mass measurements for the reactants and the products in order to demonstrate the conservation of mass during the reaction. They will also release the produced gas from the closed system in order to verify that gasses have mass.

Grade Level

Middle or High School

NGSS Standards

  • 5-PS1-1: Develop a model to describe that matter is made of particles too small to be seen.
  • MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
  • MS-PS1-5: 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.


By the end of this lab, students should be able to

  • Differentiate between a closed and open system.
  • Specify between reactants and products in a chemical reaction.
  • Describe the law of conservation of matter/mass.
  • Explain how the law of conservation of matter/mass applies within open and closed systems.

Chemistry Topics

This lab supports students’ understanding of

  • Chemical Reactions
  • Law of Conservation of Mass
  • Observations
  • Measurement


Teacher Preparation: 30-40 minutes

Lesson: 30-45 minutes

Materials (per group)

  • Glass vial w/ lid
  • 1 small piece of tape (masking or scotch)
  • 1/8 tablet of Alka-Seltzer or equivalent tablet.
  • Water
  • Electronic Balance


  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.
  • This lab produces a gas in a closed system, it is extremely important that you only use small amounts of substance that can be contained in the containers. Teachers must monitor your students to ensure they don’t use more than the 1/8th tablet recommended. Larger quantities could create pressures that could rupture the container.

Teacher Notes

  • It will be helpful if the teacher breaks the tablets into appropriately sized chunks for the students (about 1/8 tablet) in advance of the lab. This will help ensure students use the appropriate amount and are not tempted to put larger chunks into their container. As mentioned in the Safety section it is extremely important that only small amounts of substances are used in order to ensure the vial does not break.
  • As an alternative to using a glass vial, you could try performing this using a Ziploc bag. You would need to keep the dry tablet away from water until after the bag is sealed. You can reference the AACT demonstration Comparing Chemical Reactions for guidance about completing the reaction in a Ziploc bag.
  • The procedures will instruct the students to tape the tablet to the inside of the lid so that they can close the system and measure the total mass of all of the reactants before the reaction occurs.
    • Make sure your students understand how to roll tape back on itself to make it 2 sided, and maybe demonstrate this for them.If the tablet falls off prematurely, the data will not be useful.
    • The tape sticks best to a dry lid, so if possible, leave time for lids to dry, or make sure students dry them well between uses.
  • As an optional component, students could repeat the procedure to collect data in a second trial for data comparison.

For the Student



We have discussed the law of conservation of matter and some types of chemical reactions. Now we are going to explore different chemical reactions and determine whether mass is conserved during the reaction, and also if a gas has any mass.

Prelab Questions

  1. Define a closed system:
  2. Define an open system:


  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.


  • Glass Vial/Cap
  • Alka Seltzer (1/8 tablet)
  • Balance
  • Tape
  • Eye Shields
  • Water


  1. Fill your vial 1/3 full with water. Use tape to secure the piece of Alka-Seltzer to the inside of the lid.
  2. Place the bottle, cap & Alka-Seltzer on the balance and mass. Record the total mass in the data table below.
  3. Screw the lid securely and flip the vial upside down so that the Alka-Seltzer is submerged in water.
  4. After the tablet is dissolved, and the reaction has completed, place the unopened vial on the balance and measure its mass, record data.
  5. Determine the change in mass (mass after reaction – mass before reaction) then record your answer in the data table.
  6. Slowly loosen the cap and listen carefully to hear if gas escapes.
  7. Place the vial and cap on the balance, measure the mass, and record it in the data table.
  8. Determine the mass of the gas that escaped (mass after reaction – mass after release).
  9. Optional: Repeat the procedures in order to collect data for a second trial.


Trial 1 Trial 2
Total mass before reaction
Total mass after reaction
Change in mass during reaction
Mass AFTER releasing gas from vial
Mass of the gas escaped from the vial


  1. Does the gas in the experiment have mass? How do you know?
  2. How does this lab demonstrate the law of conservation of mass?
  3. What other household/daily life items do you think we could perform similar measurements with to demonstrate the mass of a gas?
  4. Where in your house could you find a closed system? An open system?
  5. Write a brief summary of what you learned in this lab: