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In this lesson, students will investigate the Law of Conservation of Mass.

Grade Level

High school


By the end of this lesson, students will

  • Better understand the Law of Conservation of Mass.

Chemistry Topics

This lesson supports students’ understanding of

  • Law of Conservation of Mass


Teacher Preparation: 60 minutes

Lesson: 60 minutes


  • Erlenmeyer flask (2 per group) (stoppers may be requested by students)
  • Test tube (2 per group) (stoppers may be requested by students)
  • Balance
  • Graduated cylinders (10 mL) (up to three per group)
  • Magnesium
  • Baking soda (sodium hydrogen carbonate)
  • Calcium carbonate
  • Calcium
  • Aluminum
  • Water
  • ~0.1-M solutions of
    • Hydrochloric acid
    • Acetic acid
    • Lead(II) acetate
    • Copper(II) chloride
    • Barium chloride
    • Potassium iodide
    • Potassium carbonate
    • Sodium sulfate


  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • 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.

Teacher Notes

  • The reactions are grouped into two categories: the first group of reactions results in a gas, the second group results in a solid.

  • Lead compounds may not be allowed in your school. In which case, you could have just three options for each part and you can eliminate calcium and water from part I. This reaction’s results aren’t as reliable as the other three in part I, depending on how oxidized the calcium is.

  • Students are left to design their own procedure. They should take the mass of all glassware before and after the mixing of the chemicals. If they choose not to include a stopper, the law will appear to not hold true in the first group of compounds, however once they balance the equations they should recognize that a gas results and was allowed to escape to the atmosphere.

  • Proper balanced equations for each possible reaction:


Magnesium + hydrochloric acid
Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)

Sodium hydrogen carbonate + acetic acid
NaHCO3 (s) + HC2H3O2 (aq) → CO2 (g) + H2O (l) + NaC2H3O2 (aq)

Calcium carbonate + hydrochloric acid
CaCO3 (s) + 2 HCl (aq) → CaCl2 (s) + H2O (l) + CO2 (g)
Calcium + water
Ca (s) + 2 H2O (l) → Ca(OH)2 (aq) + H2 (g)


Lead(II) acetate + potassium iodide
Pb(C2H3O2)2 (aq) + KI (aq) ⇾ KC2H3O2 (aq) + PbI2 (s)
Copper(II) chloride + potassium carbonate
Cu(Cl)2 (aq) + K2CO3 (aq) ⇾ 2 KCl (aq) + CuCO3 (s)
Barium chloride + sodium sulfate
BaCl2 (aq) + Na2SO4 (aq) ⇾ BaSO4 (s) + 2 NaCl (aq)
Aluminum + copper(II) chloride
2 Al (s) + 3 CuCl2 (aq) ⇾ 2 AlCl3 (aq) + 3 Cu (s)

For the Student


In your own words, describe how you know the law of conservation of matter is true in your daily life. Suggest an example that appears to disobey the law, and explain how you know it is true if you think of the example on the microscopic level. Using the equation that describes how water is formed, describe how balancing a chemical equation upholds these conservation laws: H2(g) + O2(g) ⇾ H2O(l).


Does the Law of Conservation of Mass really hold true?


Carry out two reactions, one from each part. Record enough data to prove that mass is conserved in both reactions. Choose one reaction from each section.

I. Magnesium + hydrochloric acid

Sodium hydrogen carbonate + acetic acid

Calcium carbonate + hydrochloric acid

Calcium + water

II. Lead(II) acetate + potassium iodide

Copper(II) chloride + potassium carbonate

Barium chloride + sodium sulfate

Aluminum + copper (II) chloride

When you do the experiment, if you have a liquid and a solid, the liquid should go in the test tube and the solid should go in the bottom of the flask. You can use a scoop to get a small amount of solid in the flask (less than 0.5 g). If you have two liquids, it doesn’t matter which one goes in the flask and which one goes in the test tube.

Results & Observations

Make a data table that holds the data you need to record in this experiment.


Write the balanced chemical equation for the two reactions you carried out. Was the mass exactly conserved in both reactions? Explain why or why not (keeping in mind that you did NOT disprove the law in either trial).

Were the changes you saw in this experiment chemical or physical? Explain how you know.


In two to three sentences, answer the problem.