Science · 6th Grade

Active learning ideas

Law of Conservation of Mass

Active learning transforms the Law of Conservation of Mass from an abstract rule into something students can see and measure. When students handle real materials and observe reactions themselves, they build lasting understanding of why mass stays the same even when substances change form.

• Common Core State Standards
• MS-PS1-5

• 20–45 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Inquiry Circle
• 35 min · Pairs

Inquiry Circle: Sealed Bag Reactions

Pairs mix baking soda and vinegar inside a sealed ziplock bag placed on a balance. They record mass before sealing and after the reaction completes, observe the bag inflate with carbon dioxide gas, and discuss as a group why the total mass stayed the same despite the dramatic change.

Justify the claim that mass is conserved in a closed system during a chemical reaction.

Facilitation Tip

During the sealed bag activity, circulate to ensure students seal bags completely before adding reactants to prevent accidental mass loss.

What to look for

Provide students with a scenario: 'A student mixed baking soda and vinegar in an open beaker and measured a mass loss.' Ask them to write two sentences explaining why mass appeared to be lost and what they would do to demonstrate conservation of mass.

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

Activity 02

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: Where Did the Log Go?

Students discuss with a partner: a 20-pound log burns overnight and only a pound of ash remains. Where did the other 19 pounds go? They must account for all the matter using the Law of Conservation of Mass before sharing their explanation with the class.

Analyze potential sources of error when trying to demonstrate conservation of mass.

Facilitation Tip

For the think-pair-share on burning logs, provide a picture of a forest fire and ask students to trace where the carbon in the smoke and ash came from.

What to look for

Present students with a simple chemical reaction equation, e.g., 2H₂ + O₂ → 2H₂O. Ask them to calculate the total mass of reactants and products using provided atomic masses, assuming a closed system, and state whether mass is conserved.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 03

• Stations Rotation
• 45 min · Small Groups

Stations Rotation: Open vs. Closed Systems

Stations compare the same type of reaction run in open containers (where mass appears to change) versus sealed containers (where it stays the same). Students measure mass at each station and explain the discrepancy by identifying the component that escaped or entered the open system.

Explain where the atoms go when a log burns and seemingly disappears.

Facilitation Tip

At the open vs. closed systems stations, have students record predictions about mass changes before they start each station so they notice differences between their expectations and results.

What to look for

Pose the question: 'If a log disappears when it burns, where did the mass go?' Facilitate a class discussion where students explain the role of gases and the importance of a closed system in observing mass conservation.

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

A few notes on teaching this unit

Teachers approach this topic by emphasizing measurement over memorization. Start with simple, visible reactions like baking soda and vinegar so students see gas production without mass loss. Avoid demonstrations that rely on open containers at first; save those for later when students can explain why mass seems to disappear. Research shows hands-on measurement builds stronger conceptual foundations than abstract balancing of equations alone.

Successful learning looks like students confidently measuring mass before and after reactions, explaining where products come from, and identifying why closed systems matter. They should connect observations to the idea that atoms rearrange but are never lost.

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Watch Out for These Misconceptions

• During the Collaborative Investigation: Sealed Bag Reactions, watch for students who believe the bag inflates because mass is gained or lost.

  Prompt students to re-zero the balance with the sealed bag in place before adding reactants, then have them observe the mass stays the same after the reaction produces visible gas. Ask them to trace the gas back to the solid reactants to see the atoms simply rearranged.

• During the Think-Pair-Share: Where Did the Log Go?, watch for students who believe burning wood destroys matter.

  Use the rusting example to connect prior knowledge: show students pre-weighed iron wool in a sealed container, let it rust, and reweigh to prove the mass increase comes from oxygen combining with iron, not from nothing.

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Methods used in this brief

• Inquiry Circle
• Think-Pair-Share
• Stations Rotation