Conservation of Mass

Active learning lets students see the Law of Conservation of Mass in action, not just read about it. When they seal reactants in a bag and watch the mass stay the same, the abstract concept becomes tangible and memorable.

• 6th Class
• Scientific Inquiry and the Natural World
• 4 activities
• 25 min–45 min

Learning Objectives

1. Analyze experimental data to calculate the total mass of reactants and products in a closed system.
2. Explain the Law of Conservation of Mass using evidence from hands-on investigations.
3. Predict the mass of products formed in a chemical reaction given the mass of reactants in a sealed container.
4. Compare the mass of a system before and after a chemical reaction to demonstrate that mass is conserved.

Inquiry Circle

⏱ 30 min·Small Groups

Sealed Bag Reaction: Baking Soda and Vinegar

Provide each small group with a zip-lock bag, baking soda, and vinegar. Students measure and record reactant masses, seal the bag after adding vinegar, observe the inflation from gas, then reweigh. Groups compare before-and-after data and calculate differences.

Prepare & details

Explain the Law of Conservation of Mass.

Facilitation Tip: During the Sealed Bag Reaction, remind students to seal the bag tightly before weighing to prevent leaks that could skew results.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 35 min·Small Groups

Steel Wool and Vinegar: Mass Check

Groups place steel wool in a flask, add vinegar, seal with a balloon. Weigh the setup before and after the reaction causes the balloon to inflate. Students predict outcomes, measure, and graph results to verify conservation.

Prepare & details

Analyze experimental data to demonstrate the conservation of mass.

Facilitation Tip: For the Steel Wool and Vinegar experiment, have students shake the container gently to expose all surfaces to vinegar without breaking the seal.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 45 min·Small Groups

Dissolving vs Reacting: Weighing Stations

Set up stations with salt dissolving in water and Alka-Seltzer reacting in sealed bottles. At each, students weigh before, perform change, weigh after, and note if mass conserved. Rotate and compile class data.

Prepare & details

Predict the mass of products formed given the mass of reactants in a closed system.

Facilitation Tip: At the Dissolving vs Reacting stations, place balances near each station so students can immediately weigh changes and discuss differences in real time.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Inquiry Circle

⏱ 25 min·Pairs

Prediction Challenge: Reactant to Product

Present data cards with reactant masses for common reactions. In pairs, students predict product masses, justify using the law, then test one prediction with a demo reaction and verify.

Prepare & details

Explain the Law of Conservation of Mass.

Facilitation Tip: In the Prediction Challenge, ask students to jot down predictions before seeing the reactants to encourage critical thinking before confirmation.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Teaching This Topic

Teaching this topic works best when students do the weighing themselves and discuss why numbers sometimes seem to change. Avoid relying solely on textbook examples, as hands-on trials build stronger conceptual understanding. Research shows that students retain the principle better when they actively confront and correct their own misconceptions through direct measurement.

What to Expect

Students will confidently explain that mass stays constant in closed systems by referencing their own measurements and observations. They will also be able to identify when mass appears to change and why that happens in open systems.

Watch Out for These Misconceptions

Common Misconception

During the Sealed Bag Reaction activity, watch for students who expect the bag to feel lighter after the reaction because they see bubbles form.

What to Teach Instead

Use the sealed bag reaction to show that even though gas forms and the bag inflates, the total mass before and after remains the same when measured on a balance.

Common Misconception

During the Steel Wool and Vinegar activity, watch for students who believe the steel wool gains mass because it changes color or texture.

What to Teach Instead

Have students weigh the sealed container with steel wool and vinegar before and after the reaction to observe that the total mass stays constant, even as the steel wool rusts.

Common Misconception

During the Burning always reduces mass unit, watch for students who generalize that any reaction involving fire or smoke reduces mass.

What to Teach Instead

Use candle-jar trials to show that in a closed system, the ash and trapped gases together equal the original candle's mass, proving conservation.

Assessment Ideas

Quick Check

After the Sealed Bag Reaction activity, provide students with a scenario: '3 grams of baking soda reacts with 10 grams of vinegar in a sealed bag. What is the total mass of the products? Write your answer and explain in one sentence using evidence from the experiment.'

Exit Ticket

During the Dissolving vs Reacting stations, give students a card with a simple reaction (e.g., sugar dissolving in water vs. baking soda reacting with vinegar). Ask them to draw a closed system diagram, label reactants and products, and write one sentence predicting the total mass.

Discussion Prompt

After the Prediction Challenge activity, pose the question: 'How would the total mass change if we did the baking soda and vinegar reaction in an open cup instead of a sealed bag? Discuss with a partner and explain your reasoning using observations from today's experiments.'

Extensions & Scaffolding

• Challenge: Ask students to design their own closed-system experiment to test conservation of mass using household items, then present their method and results to the class.
• Scaffolding: Provide pre-labeled diagrams of the sealed bag reaction for students to annotate with mass measurements at each step.
• Deeper exploration: Introduce the concept of exothermic and endothermic reactions through additional closed-system experiments, measuring mass and temperature changes.

Key Vocabulary

Conservation of Mass	A fundamental scientific law stating that in a closed system, the total mass of the reactants before a chemical reaction is equal to the total mass of the products after the reaction.
Reactant	A substance that is present at the start of a chemical reaction and is consumed during the reaction.
Product	A substance that is formed as a result of a chemical reaction.
Closed System	A system where no matter can enter or leave, allowing for accurate measurement of mass changes during a reaction.
Chemical Reaction	A process that involves rearrangement of the structure of molecules or compounds, typically resulting in the formation of new substances.