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.

• 5th Class
• Scientific Inquiry and the Natural World
• 4 activities
• 20 min–40 min

Learning Objectives

1. Demonstrate the conservation of mass by measuring reactant and product masses in a sealed system.
2. Explain the Law of Conservation of Mass using evidence from experimental data.
3. Calculate the expected mass of products in a chemical reaction given the mass of reactants.
4. Analyze discrepancies in experimental data to identify potential sources of error in measuring mass.
5. Compare the total mass of reactants to the total mass of products in a closed system reaction.

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.

Prepare & details

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.

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

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.

Prepare & details

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.

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

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.

Prepare & details

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.

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

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.

Prepare & details

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

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

Teaching This Topic

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.

What to Expect

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.

Watch Out for These Misconceptions

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Assessment Ideas

Exit Ticket

After the Sealed Bag Demo, 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 a liquid. What is the total mass of the products inside the bag after the reaction?' Ask students to write their answer and explain their reasoning using their experiment data.

Quick Check

During the Effervescent Tablet Test, ask students to record the mass of the tablet and water before mixing. After the reaction in the sealed syringe, have them record the mass of the products. Then ask: 'Did the total mass change? Use your measurements to explain why or why not.'

Discussion Prompt

After the Steel Wool Reaction, present two data sets: one showing equal masses before and after, 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?' Have students discuss in pairs before sharing with the class.

Extensions & Scaffolding

• Challenge students to design their own sealed reaction using household materials, then predict and measure the mass before and after the reaction.
• For students who struggle, provide pre-labeled diagrams of the sealed system and ask them to trace the path of matter through reactants and products.
• Deeper exploration: Introduce a reaction with two reactants that produce two products, and ask students to calculate the mass of each product based on the law and their measurements.

Key Vocabulary

Chemical Reaction	A process where substances change into new substances with different properties. In this topic, we observe reactions like baking soda and vinegar.
Reactants	The substances that are present at the beginning of a chemical reaction. Their combined mass is measured before the reaction occurs.
Products	The new substances that are formed as a result of a chemical reaction. Their combined mass is measured after the reaction is complete.
Conservation of Mass	The scientific principle stating that matter cannot be created or destroyed in a chemical reaction. The total mass of reactants always equals the total mass of products.
Sealed System	An experimental setup that prevents matter from entering or leaving. This is crucial for accurately measuring mass changes during reactions, especially those producing gas.