Science · 6th Grade · Molecules in Motion · Weeks 1-9

Law of Conservation of Mass

Students conduct experiments to prove that mass is conserved during chemical changes.

• Common Core State Standards
• MS-PS1-5

About This Topic

The Law of Conservation of Mass states that in a closed system, the total mass of the reactants before a reaction equals the total mass of the products after. This law, established through careful experimentation in the 18th century, is one of the cornerstones of chemistry and underpins the logic of all chemical equations. Aligned with MS-PS1-5, 6th graders are expected to provide evidence supporting this law through observation and careful measurement.

The challenge for students is that many common reactions appear to violate this law. Wood burns and only ash remains. An antacid tablet dissolves and the fizzing stops. In each case, students must account for the gases that escaped into the surrounding air. Mass appears lost when gases leave an open system. The critical word is 'closed': in a sealed container, total mass before and after a reaction will always be equal.

Conducting reactions in sealed containers is the most direct instructional approach. When students measure total mass before and after a reaction in a sealed system and find it unchanged, the law becomes observable reality rather than a statement to memorize. Active investigation with careful measurement is the core of this topic.

Key Questions

1. Justify the claim that mass is conserved in a closed system during a chemical reaction.
2. Analyze potential sources of error when trying to demonstrate conservation of mass.
3. Explain where the atoms go when a log burns and seemingly disappears.

Learning Objectives

• Calculate the total mass of reactants and products in a closed system to demonstrate the conservation of mass.
• Analyze experimental data to identify potential sources of error when measuring mass changes during a chemical reaction.
• Explain the transformation of matter, specifically accounting for gaseous products, when a substance like wood undergoes combustion.
• Compare the mass of reactants and products in sealed containers versus open systems to illustrate the importance of a closed system for mass conservation.

Before You Start

Properties of Matter

Why: Students need to understand that matter has mass and occupies space before they can investigate its conservation.

Introduction to Chemical Reactions

Why: Understanding that chemical reactions involve the transformation of substances is necessary to explore how mass behaves during these changes.

Key Vocabulary

Law of Conservation of Mass	A fundamental principle stating that matter cannot be created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Closed System	An environment where no matter can enter or leave, allowing for accurate measurement of mass changes during a reaction.
Reactants	The substances that are present at the beginning of a chemical reaction and that are consumed during the reaction.
Products	The substances that are formed as a result of a chemical reaction.
Chemical Change	A process where one or more substances are altered into new and different substances, often involving the rearrangement of atoms.

Watch Out for These Misconceptions

Common Misconception

Students often conclude that mass is lost when a gas is produced or when something burns.

What to Teach Instead

The sealed-bag investigation is the most compelling direct correction. When students see equal mass before and after a reaction that visibly produces gas, they must revise their thinking. Peer discussions about what happened to the 'missing' mass and where to find it are productive follow-ups.

Common Misconception

Many students think rusting violates conservation of mass because the rusty object seems heavier.

What to Teach Instead

Rusting is actually evidence of conservation of mass: the rust (iron oxide) is heavier than the original iron because oxygen from the air has chemically combined with it. Weighing iron before and after rusting in a sealed container shows that the gain in the iron's mass equals the loss in oxygen from the air inside.

Active Learning Ideas

Inquiry Circle: Sealed Bag Reactions

Pairs mix baking soda and vinegar inside a sealed ziplock bag placed on a balance. They record mass before sealing and after the reaction completes, observe the bag inflate with carbon dioxide gas, and discuss as a group why the total mass stayed the same despite the dramatic change.

35 min·Pairs

Think-Pair-Share: Where Did the Log Go?

Students discuss with a partner: a 20-pound log burns overnight and only a pound of ash remains. Where did the other 19 pounds go? They must account for all the matter using the Law of Conservation of Mass before sharing their explanation with the class.

20 min·Pairs

Stations Rotation: Open vs. Closed Systems

Stations compare the same type of reaction run in open containers (where mass appears to change) versus sealed containers (where it stays the same). Students measure mass at each station and explain the discrepancy by identifying the component that escaped or entered the open system.

45 min·Small Groups

Real-World Connections

• Chemists at pharmaceutical companies use the law of conservation of mass to ensure that the precise amount of active ingredients and byproducts are accounted for in drug manufacturing, maintaining product safety and efficacy.
• Forensic scientists analyze evidence at crime scenes, using principles of mass conservation to reconstruct events and identify substances involved in chemical reactions, such as accelerants used in arson.
• Engineers designing industrial furnaces must consider mass conservation, accounting for all inputs and outputs, including gases, to optimize efficiency and manage emissions.

Assessment Ideas

Exit Ticket

Provide students with a scenario: 'A student mixed baking soda and vinegar in an open beaker and measured a mass loss.' Ask them to write two sentences explaining why mass appeared to be lost and what they would do to demonstrate conservation of mass.

Quick Check

Present students with a simple chemical reaction equation, e.g., 2H₂ + O₂ → 2H₂O. Ask them to calculate the total mass of reactants and products using provided atomic masses, assuming a closed system, and state whether mass is conserved.

Discussion Prompt

Pose the question: 'If a log disappears when it burns, where did the mass go?' Facilitate a class discussion where students explain the role of gases and the importance of a closed system in observing mass conservation.

Frequently Asked Questions

What is the Law of Conservation of Mass?

In a closed system, the total mass of matter before a chemical reaction equals the total mass after. Atoms are rearranged into new substances, but no atoms are created or destroyed in the process, so the total count of atoms stays the same, and therefore so does the total mass.

Why does wood seem to disappear when it burns?

Most of the carbon in the wood combines with oxygen in the air to form carbon dioxide gas, which rises invisibly. The hydrogen in the wood's molecules bonds with oxygen to form water vapor. Only the small mineral content that cannot form gases remains as ash. All that 'disappeared' mass is actually floating away as invisible gases.

How can active learning help students understand conservation of mass?

The most powerful approach is having students measure mass themselves and see that it is conserved. When a student expects mass to decrease after a fizzing reaction in a sealed bag and then sees the balance read the same number, the cognitive surprise is genuinely memorable. Collaborative discussion about where the gas went deepens the understanding that conservation requires accounting for all matter, including gases.

Does the Law of Conservation of Mass apply to nuclear reactions?

At 6th grade, the law applies to all chemical reactions students encounter. In nuclear reactions, a tiny amount of mass converts to energy as described by Einstein's famous equation, but this is well beyond middle school science and the amounts involved are too small to detect without highly specialized equipment.