Science · 8th Grade

Active learning ideas

Conservation of Mass

Active learning helps students grasp the conservation of mass because it moves the concept from abstract theory to tangible evidence. When students manipulate materials and measure mass changes themselves, they confront misconceptions with direct experience rather than abstract explanations.

• Common Core State Standards
• MS-PS1-2

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

Activity 01

• Inquiry Circle
• 40 min · Small Groups

Inquiry Circle: Sealed Bag Reaction

Students mass a plastic bag containing baking soda in a small paper cup and vinegar, sealed tightly. They tip the bag to mix the reactants, observe gas production, and mass the bag again without opening it. The class compiles all group data and discusses why the mass did not change and what would happen if the bag were opened.

Explain how the law of conservation of mass applies to chemical reactions.

Facilitation Tip

During the Sealed Bag Reaction, circulate to ensure students seal bags tightly and use digital scales correctly to avoid air leaks that distort results.

What to look for

Provide students with a scenario: 'A student mixed 10g of baking soda with 50g of vinegar in an open beaker. A vigorous reaction produced fizzing and a gas. The remaining liquid weighed 55g.' Ask students to: 1. Calculate the expected mass of products if the system were closed. 2. Explain why the measured mass was less than the sum of the reactants.

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

Activity 02

• Inquiry Circle
• 45 min · Small Groups

Inquiry Circle: Open vs. Closed System Comparison

One group measures the mass of a baking soda and vinegar reaction in an open beaker; another uses a sealed flask with a balloon to capture the gas. Both groups record mass before and after. The class compares results, discusses why the open system appears to lose mass, and writes a claim-evidence-reasoning statement about conservation.

Analyze experimental data to demonstrate that matter is conserved during a reaction.

Facilitation Tip

In the Open vs. Closed System Comparison, emphasize consistent measurement techniques so students notice the impact of system type on mass readings.

What to look for

Present students with a diagram of a sealed flask containing reactants. Ask them to predict whether the mass inside the flask will change after the reaction occurs and to justify their prediction using the term 'closed system' and the law of conservation of mass.

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

Activity 03

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: The Disappearing Chalk

Students watch a demonstration of chalk dissolving in acid, where the beaker appears to lose mass. Partners discuss where the mass went and sketch a sealed-system experimental design on paper that would prove mass was conserved. Groups share their designs and the class identifies the best controls.

Design an investigation to prove that mass is conserved in a closed system.

Facilitation Tip

For the Disappearing Chalk Think-Pair-Share, ask guiding questions like 'Where did the chalk go?' to push students past initial observations toward evidence-based reasoning.

What to look for

Pose the question: 'Imagine you are burning a small log in a fireplace. If you could collect all the ash, smoke, and gases produced, would the total mass be equal to the mass of the original log? Explain your answer, considering whether the fireplace is an open or closed system.'

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

A few notes on teaching this unit

Teach conservation of mass by pairing measurement with explicit discussion of system boundaries. Avoid framing it as just a rule to memorize. Instead, use activities to build evidence, then connect that evidence to the law. Research shows students grasp conservation best when they experience measurable transformations and discuss why some transformations seem to defy the rule.

Successful learning looks like students confidently predicting mass outcomes, explaining discrepancies using system types, and connecting their experimental data to the law of conservation of mass. They should articulate why mass seems to change in open systems but not in closed ones.

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Watch Out for These Misconceptions

• During Collaborative Investigation: Sealed Bag Reaction, watch for students attributing mass loss to gas creation rather than gas escape.

  After the sealed bag reaction, open the bag inside the sealed container to show students the gas is still present, then re-measure to demonstrate mass remains constant. Ask students to explain how opening the system allowed gas to escape and why mass appeared to decrease.

• During Collaborative Investigation: Open vs. Closed System Comparison, watch for students assuming energy release always causes mass loss.

  During the open vs. closed system comparison, measure mass before and after exothermic reactions in both systems. Highlight that while energy is released as heat, mass remains the same, and discuss why energy and mass are related but distinct in everyday chemical reactions.

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Methods used in this brief

• Inquiry Circle
• Think-Pair-Share