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.

• 6th Grade
• Science
• 3 activities
• 20 min–45 min

Learning Objectives

1. Calculate the total mass of reactants and products in a closed system to demonstrate the conservation of mass.
2. Analyze experimental data to identify potential sources of error when measuring mass changes during a chemical reaction.
3. Explain the transformation of matter, specifically accounting for gaseous products, when a substance like wood undergoes combustion.
4. Compare the mass of reactants and products in sealed containers versus open systems to illustrate the importance of a closed system for mass conservation.

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.

Prepare & details

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.

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

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.

Prepare & details

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.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

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

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.

Prepare & details

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.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

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

Teaching This Topic

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.

What to Expect

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.

Watch Out for These Misconceptions

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Assessment Ideas

Exit Ticket

After Station Rotation: Open vs. Closed Systems, ask students to write a short paragraph explaining which station surprised them most and why, using evidence from their measurements to support conservation of mass.

Quick Check

During Collaborative Investigation: Sealed Bag Reactions, ask students to calculate the expected product mass using atomic masses provided in their data tables, then compare their calculated mass to their measured mass to confirm conservation.

Discussion Prompt

After Think-Pair-Share: Where Did the Log Go?, facilitate a class discussion where students use particle diagrams to explain where the atoms in smoke and ash came from during combustion, linking their ideas to the Law of Conservation of Mass.

Extensions & Scaffolding

• Challenge: Have students design their own closed-system experiment using household materials to prove mass conservation, then present their method and results to the class.
• Scaffolding: Provide pre-labeled diagrams of particle arrangements before and after reactions to help students visualize why mass stays constant.
• Deeper exploration: Introduce the concept of endothermic and exothermic reactions by having students measure temperature changes while proving mass is conserved in sealed containers.

Key Vocabulary

Law of Conservation of Mass	A fundamental principle stating that matter cannot be created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Closed System	An environment where no matter can enter or leave, allowing for accurate measurement of mass changes during a reaction.
Reactants	The substances that are present at the beginning of a chemical reaction and that are consumed during the reaction.
Products	The substances that are formed as a result of a chemical reaction.
Chemical Change	A process where one or more substances are altered into new and different substances, often involving the rearrangement of atoms.