Scientific Inquiry and the Natural World · 5th Class

Active learning ideas

Conservation of Mass in Reactions

Active learning works because students often assume gases have no mass, making the Law of Conservation of Mass counterintuitive. Hands-on experiments with sealed containers let students confront this misconception directly through measurement and observation rather than abstract explanations. The physical act of weighing before and after reactions builds concrete evidence that supports later abstract reasoning about chemical processes.

• NCCA Curriculum Specifications
• NCCA: Primary - Materials
• NCCA: Primary - Materials and Change

• 20–40 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Inquiry Circle
• 35 min · Small Groups

Sealed Bag Demo: Baking Soda and Vinegar

Provide small groups with a digital scale, ziplock bags, baking soda, and vinegar. Students measure and record reactant masses, seal the bag, initiate the reaction by mixing, then reweigh the entire bag. They compare before-and-after data and explain results in a class share-out.

Explain the Law of Conservation of Mass in the context of chemical reactions.

Facilitation Tip

During the Sealed Bag Demo, remind students to press the bag gently to mix the reactants without breaking the seal, ensuring no mass is lost.

What to look for

Provide students with a scenario: 'You mixed 10 grams of baking soda with 50 grams of vinegar in a sealed bag. The reaction produced gas and new substances. What is the total mass of the products inside the bag after the reaction?' Ask students to write their answer and one sentence explaining their reasoning.

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

Activity 02

• Inquiry Circle
• 25 min · Pairs

Effervescent Tablet Test: Syringe Seal

Each pair seals an effervescent tablet and water in a syringe, measures initial mass, allows reaction, and reweighs. They record observations of gas pressure and mass stability, then graph class data to spot patterns.

Analyze experimental data to demonstrate the conservation of mass.

Facilitation Tip

For the Effervescent Tablet Test, have students practice sealing the syringe before adding water to avoid air leaks that could skew results.

What to look for

During an experiment, ask students to record the mass of reactants before mixing. After the reaction in a sealed container, ask them to record the mass of the products. Then, pose the question: 'Did the total mass change? Explain why or why not, referencing your measurements.'

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

Activity 03

• Inquiry Circle
• 40 min · Small Groups

Steel Wool Reaction: Vinegar Jar

Students weigh steel wool and vinegar separately, combine in a sealed jar, wait for reaction, and reweigh the jar. They predict mass change, test, and adjust predictions based on results during group debrief.

Predict the total mass of products given the total mass of reactants.

Facilitation Tip

In the Steel Wool Reaction, place the jar on the scale only after the lid is secured to prevent accidental mass loss during handling.

What to look for

Present students with two sets of experimental data for the same reaction: one showing equal reactant and product masses, and another showing a slight difference. Ask: 'Which data set best demonstrates the conservation of mass? What might explain the difference in the other data set?'

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

Activity 04

• Inquiry Circle
• 20 min · Whole Class

Prediction Challenge: Whole Class Demo

Display reactant masses on board, have class predict product mass, perform baking soda-vinegar reaction in sealed flask, weigh publicly, and vote on explanations for results.

Explain the Law of Conservation of Mass in the context of chemical reactions.

What to look for

Provide students with a scenario: 'You mixed 10 grams of baking soda with 50 grams of vinegar in a sealed bag. The reaction produced gas and new substances. What is the total mass of the products inside the bag after the reaction?' Ask students to write their answer and one sentence explaining their reasoning.

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

A few notes on teaching this unit

Teachers should approach this topic by starting with simple, safe reactions that produce visible gases but can be contained. Avoid open-system demonstrations at first, as they reinforce the misconception that mass is lost to the air. After students record their data, lead a discussion where they compare predictions and results, using their measurements as evidence. This cycle of prediction, observation, and explanation helps students trust data over intuition.

Successful learning looks like students confidently predicting equal masses before reactions, accurately recording measurements, and explaining the results using evidence from their experiments. They should recognize that closed systems prevent mass loss and connect their data to the law. Class discussions should show students using terms like reactants, products, and conservation correctly when analyzing outcomes.

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Watch Out for These Misconceptions

• During the Sealed Bag Demo, watch for students attributing a decrease in mass to the gas bubbles escaping the bag.

  Use the sealed bag to demonstrate that the total mass stays the same, even when gases are visibly produced. Have students hold the bag and feel the pressure changes, then discuss how the gas is still contained and contributes to the total mass.

• During the Effervescent Tablet Test, watch for students believing the fizzing sound or bubbles indicate new matter was created.

  Use the syringe’s sealed system to show that the mass of the tablet and water equals the mass after the reaction. Ask students to compare the initial and final measurements side by side to highlight that the bubbles are rearranged matter, not new matter.

• During the Steel Wool Reaction, watch for students thinking the rusted steel wool weighs less because it looks different.

  Place the jar on the scale before and after the reaction, leaving it sealed throughout. Have students examine the rusted wool and compare its mass to the original steel wool, reinforcing that the change in appearance does not mean a loss of mass.

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Methods used in this brief

• Inquiry Circle