Conservation of Mass in Changes

Active learning works for conservation of mass because students must handle materials, record precise measurements, and witness changes firsthand. When they see mass stay the same despite visible reactions or dissolutions, the abstract law becomes concrete and memorable.

• Year 8
• Science
• 4 activities
• 30 min–45 min

Learning Objectives

1. Analyze experimental data to identify quantitative evidence supporting the law of conservation of mass.
2. Explain how the rearrangement of atoms during a chemical change accounts for the conservation of mass at the atomic level.
3. Predict the mass of products formed in a closed system, given the mass of reactants, applying the principle of mass conservation.
4. Compare the conservation of mass in physical changes (e.g., dissolving) versus chemical changes (e.g., reaction).
5. Design a simple experiment to demonstrate the conservation of mass in a closed system, identifying potential sources of error.

Problem-Based Learning

⏱ 30 min·Small Groups

Sealed Bag Reaction: Baking Soda and Vinegar

Students measure and record the mass of a zip-lock bag containing baking soda, then add vinegar through a straw and seal immediately. After the reaction, they reweigh the bag and compare to initial mass. Groups discuss sources of error like incomplete sealing.

Prepare & details

Explain why atoms are conserved during a chemical change.

Facilitation Tip: During the Sealed Bag Reaction, remind students to seal the bag fully before mixing to prevent any gas escape and to record masses immediately after adding reactants.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 45 min·Pairs

Dissolving Challenge: Salt Evaporation

Pairs weigh a beaker of water, add measured salt, stir to dissolve, then evaporate over a hot plate while monitoring mass changes. They reweigh the recovered salt and calculate percent recovery. Extend by predicting masses for different salt amounts.

Prepare & details

Analyze experimental evidence supporting the law of conservation of mass.

Facilitation Tip: For the Dissolving Challenge, have students weigh the dry salt and water separately, then together in a sealed container, to emphasize that mass doesn’t vanish upon dissolution.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 40 min·Small Groups

Prediction Stations: Reactant Products

Set up stations with reactant masses listed for reactions like magnesium combustion in sealed cans. Students predict product masses, perform if safe, or simulate, then verify with class data. Rotate stations to cover physical and chemical examples.

Prepare & details

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

Facilitation Tip: At Prediction Stations, ask students to predict masses for each reactant-product pair before they measure, then compare predictions to actual results to confront initial assumptions.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 35 min·Whole Class

Whole Class Demo: Candle in Jar

Demonstrate burning a candle in an inverted jar over water; measure initial and final masses of setup. Students record observations, predict outcomes beforehand, and analyze why mass conserves despite apparent loss. Follow with group hypothesis testing.

Prepare & details

Explain why atoms are conserved during a chemical change.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teach this topic by balancing hands-on measurement with guided reasoning. Start with simple closed-system activities to build trust in the law, then contrast them with open systems to highlight where mass might seem lost. Avoid rushing to the conclusion—instead, let evidence from their own balances shape understanding. Research shows students grasp conservation better when they connect macroscopic observations to particle-level models, so pair weighings with diagrams of atoms rearranging, not appearing or disappearing.

What to Expect

Students will confidently explain that mass remains constant in closed systems, using their own data to justify claims. They will distinguish between open and closed setups and describe how gases contribute to total mass when contained.

Watch Out for These Misconceptions

Common Misconception

During the Sealed Bag Reaction, watch for students attributing any mass change to gases escaping through gaps in the bag.

What to Teach Instead

Before sealing, ask students to double-check the bag’s closure and to record the mass of the sealed system before mixing. After the reaction, revisit the sealed bag and discuss how the increase in pressure indicates gas is contained, not lost.

Common Misconception

During the Dissolving Challenge, watch for students assuming the salt disappears when it dissolves, leading them to predict a decrease in total mass.

What to Teach Instead

Have students recover the salt by evaporating the water and weighing the dried salt to show it retains its original mass. Ask them to compare the mass of the dried salt to the mass of the dissolved solution to confront the misconception directly.

Common Misconception

During Prediction Stations, watch for students treating new substances as having different total mass because they look or feel different.

What to Teach Instead

Ask pairs to write balanced equations under each prediction, linking the reactants’ total mass to the predicted product’s mass. Then, during the reaction, have them verify that the mass of the products matches their balanced equation, reinforcing that atoms rearrange but do not change in number.

Assessment Ideas

Quick Check

After the Prediction Stations activity, present students with a sealed-system scenario: '5g of baking soda reacts with 10g of vinegar in a sealed bag. What will be the total mass of the products after the reaction? Explain using the law of conservation of mass.' Collect responses to assess whether they apply the law correctly to gas-producing reactions.

Exit Ticket

After the Dissolving Challenge, provide students with a data table showing the mass of water, salt, and solution before and after dissolving. Ask: 'Does this data support the law of conservation of mass? Justify your answer with specific numbers from the table before submitting your exit ticket.'

Discussion Prompt

During the Whole Class Demo: Candle in Jar, ask: 'The ash left after burning the candle has less mass than the original candle. Does this violate the law of conservation of mass? What else might be involved besides the ash?' Use student responses to assess their understanding of open systems and the role of gases in mass conservation.

Extensions & Scaffolding

• Challenge: Ask students to design their own closed-system experiment using household materials to test conservation of mass, then present their method and results to the class.
• Scaffolding: Provide a partially completed data table for the Dissolving Challenge with missing values, guiding students to calculate expected totals before they measure.
• Deeper exploration: Introduce the concept of limiting reactants by having students vary the amount of baking soda in the Sealed Bag Reaction and predict how the final mass will change if one reactant is exhausted.

Key Vocabulary

Conservation of Mass	The principle that matter cannot be created or destroyed in an isolated system, so the mass of the system remains constant over time.
Reactants	The substances that are present at the start of a chemical reaction and are consumed during the reaction.
Products	The substances that are formed as a result of a chemical reaction.
Closed System	A system in which no matter can enter or leave, allowing for the accurate measurement of mass changes during reactions or physical changes.
Atom Rearrangement	The process in chemical reactions where atoms of the reactants break their existing bonds and form new bonds to create products, without atoms being lost or gained.