Law of Conservation of Mass

Active, hands-on chemistry helps Year 9 students see the invisible—gases escaping, sealed containers holding mass—so the abstract law of conservation of mass becomes measurable and memorable. When students feel the mass of a balloon inflating or watch a sealed bag inflate without loss, the law shifts from memorized words to observed truth.

• Year 9
• Science
• 4 activities
• 20 min–45 min

Learning Objectives

1. Explain the law of conservation of mass, stating that matter is neither created nor destroyed during a chemical reaction.
2. Calculate the total mass of products formed in a sealed system given the masses of the reactants.
3. Compare the mass of reactants and products in a controlled experiment to verify the conservation of mass.
4. Analyze experimental data to identify sources of apparent mass change in open systems, such as gas escape.
5. Critique proposed experimental designs for testing the law of conservation of mass, identifying potential flaws.

Inquiry Circle

⏱ 30 min·Pairs

Sealed Reaction Weigh-In: Baking Soda and Vinegar

Students add vinegar to baking soda inside a sealed zip-lock bag or bottle with a balloon. Weigh the setup before and after the reaction. Record masses and discuss why the total stays the same despite gas production.

Prepare & details

Why doesn't a burning piece of wood weigh the same as the ash it leaves behind — where does the rest of the mass go?

Facilitation Tip: During the Sealed Reaction Weigh-In, remind students to zero the balance with the empty sealed container first to avoid systematic error from the bag’s mass.

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

Stations Rotation

⏱ 45 min·Small Groups

Stations Rotation: Reaction Mass Checks

Set up stations with safe reactions: effervescent tablets in water, steel wool and vinegar, and a teacher demo of candle in jar. Groups weigh sealed containers before, during, and after, noting observations.

Prepare & details

How can scientists predict the exact mass of products formed in a sealed reaction using only the mass of the reactants?

Facilitation Tip: In the Station Rotation, assign one student per station to record the initial mass so every pair shares the same starting data.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Inquiry Circle

⏱ 20 min·Whole Class

Prediction Challenge: Whole Class Demo

Display reactant masses on board; students predict product masses then watch teacher perform sealed magnesium combustion. Class compares predictions to measured results and adjusts understanding.

Prepare & details

What experimental evidence would you need to convince a sceptic that mass is always conserved in a chemical reaction?

Facilitation Tip: During the Prediction Challenge, pause after the demo to ask students to predict the mass change aloud before revealing the result.

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·Individual

Data Graphing: Individual Analysis

Provide reaction data sets; students graph reactant vs product masses, identify patterns, and explain conservation in writing.

Prepare & details

Why doesn't a burning piece of wood weigh the same as the ash it leaves behind — where does the rest of the mass go?

Facilitation Tip: After the Data Graphing, have students compare their line slopes to determine if mass change was consistent across trials.

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 approach this topic by anchoring abstract principles in concrete, visible measurements students can trust. Emphasize the difference between open and closed systems early, using the sealed container to make the invisible gases tangible. Avoid rushing to the equation; let students experience the mass equality first, then link it to particle rearrangement. Research shows that when students predict outcomes and then observe matching data, their misconceptions shift more reliably than when they are simply told the law.

What to Expect

Successful learning looks like students confidently predicting and measuring unchanged mass, explaining gas escape in open systems, and balancing equations with mass totals. They should articulate why a burnt log weighs less and connect the idea to their sealed experiments.

Watch Out for These Misconceptions

Common Misconception

During Sealed Reaction Weigh-In, watch for students who expect a mass drop because they see bubbles form in the bag.

What to Teach Instead

Use the sealed bag’s mass as the baseline, then have students feel the bag inflate without measuring a change. Ask them to trace the path of gas particles and note that the bubbles are carbon dioxide held inside the bag.

Common Misconception

During Station Rotation, watch for students who focus only on the visible solids and ignore the liquid or gas phases.

What to Teach Instead

Provide a triple-beam balance and a small sealed vial of water at one station so students weigh liquid and vapor together. Ask them to explain why the vial’s mass doesn’t change even when water evaporates inside it.

Common Misconception

During the Prediction Challenge, watch for students who think new matter forms when magnesium burns.

What to Teach Instead

After the demo, have students use the magnesium ribbon mass and ash mass data to calculate the missing mass. Ask them to explain where the extra mass went using the balanced equation Mg + O₂ → MgO and the concept of gas product formation.

Assessment Ideas

Quick Check

After the Sealed Reaction Weigh-In, present students with a sealed bag containing 3 g baking soda and 20 g vinegar. Ask students to write the expected total mass of products and explain in one sentence using the law of conservation of mass.

Discussion Prompt

During the Station Rotation, pose the question: ‘Why does a log burning in an open fireplace appear to lose mass, even though mass is conserved?’ Facilitate a class discussion where students connect their station data about gases escaping to the open system of the fireplace.

Exit Ticket

After Data Graphing, provide students with the unbalanced equation Fe + O₂ → Fe₂O₃ and masses: 28 g Fe and 12 g O₂. Ask them to balance the equation, then calculate the expected total mass of Fe₂O₃ based on the law of conservation of mass.

Extensions & Scaffolding

• Challenge early finishers to design a sealed system using effervescent tablets and water to test whether mass is conserved when gases are produced.
• Scaffolding for struggling students: Provide pre-labeled diagrams of the baking soda-vinegar reaction showing gas particles and ask them to color code solids, liquids, and gases before measuring.
• Deeper exploration: Ask students to research how the law applies to combustion engines, focusing on how catalytic converters manage mass balance despite gas emissions.

Key Vocabulary

Conservation of Mass	A fundamental principle stating that in a closed system, the mass of the reactants before a chemical reaction is equal to the mass of the products after the reaction.
Reactant	A substance that takes part in and undergoes change during a chemical reaction.
Product	A substance that is formed as a result of a chemical reaction.
Closed System	A system in which matter cannot enter or leave, allowing for accurate measurement of mass changes during reactions.
Open System	A system where matter can be exchanged with its surroundings, leading to apparent mass changes due to gas escape or absorption.