Science · Year 8

Active learning ideas

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.

• ACARA Content Descriptions
• AC9S8U04

• 30–45 min
• Pairs → Whole Class
• 4 activities

Activity 01

• 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.

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.

What to look for

Present students with a scenario: '10g of magnesium reacts with 10g of oxygen to form magnesium oxide.' Ask: 'If this reaction occurs in a perfectly closed system, what will be the total mass of magnesium oxide produced? Explain your answer using the law of conservation of mass.'

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

Activity 02

• 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.

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.

What to look for

Provide students with a data table from a dissolving experiment (e.g., salt in water). The table shows the mass of water, mass of salt, and total mass before dissolving, and the mass of the solution after dissolving. Ask: 'Does this data support the law of conservation of mass? Justify your answer with specific numbers from the table.'

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

Activity 03

• 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.

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.

What to look for

Pose the question: 'Imagine you burn a small piece of wood in an open fire. The ash left behind has a much smaller mass than the original wood. Does this violate the law of conservation of mass? Why or why not? Consider what else might be involved besides the ash.'

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

Activity 04

• 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.

Explain why atoms are conserved during a chemical change.

What to look for

Present students with a scenario: '10g of magnesium reacts with 10g of oxygen to form magnesium oxide.' Ask: 'If this reaction occurs in a perfectly closed system, what will be the total mass of magnesium oxide produced? Explain your answer using the law of conservation of mass.'

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

A few notes on teaching this unit

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.

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.

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Watch Out for These Misconceptions

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

  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.

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

  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.

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

  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.

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Methods used in this brief

• Problem-Based Learning