MS-PS1-5: Mass Conservation in Chemical Reactions

MS-PS1-5
Mass Conservation in Chemical Reactions

Middle School

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Learning Objectives

• Use models to show how atoms rearrange during a chemical reaction.

• Explain that atoms and mass are conserved in a chemical reaction.

• Use chemical formulas and equations to model simple chemical reactions.

• Identify that chemical reactions form new substances with different properties.

Key Vocabulary

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Chemical Reaction

A process where atoms of reactants rearrange to form completely new substances called products.

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Reactant

A starting substance that is present at the beginning of a chemical reaction.

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Product

A new substance that is formed as a result of a chemical reaction.

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Conservation of Matter

The principle that atoms are not created or destroyed, only rearranged during a chemical reaction.

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Conservation of Mass

The total mass of the reactants must equal the total mass of the products.

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Chemical Equation

A model using chemical formulas to show what happens during a chemical reaction.

Key Vocabulary

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Model

A model is a representation used to explain things that are not directly observable by us.

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Molecule

A molecule is a distinct group of two or more atoms that are chemically bonded together.

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Precipitate

A precipitate is a solid product that forms from a solution during a chemical reaction.

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Combustion

Combustion is a rapid chemical reaction with oxygen that results in producing both heat and light.

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Atom

An atom is the fundamental and basic unit of any chemical element in the periodic table.

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Properties

Properties are the distinct characteristics of a substance, such as its color, density, or state.

What is a Chemical Reaction?

• Reactants are starting substances that change into new substances called products.

• During a reaction, the atoms of the reactants are simply rearranged.

• Carbon dioxide and limewater react to form a new, milky-white substance.

Multiple Choice

What is the role of reactants in a chemical reaction?

• They are the new substances created in a reaction.

• They are starting substances that change into new substances.

• They are substances that speed up a reaction.

• They are substances that do not change during a reaction.

Multiple Choice

What happens to the atoms during a chemical reaction?

• The atoms of the reactants are destroyed and new atoms are created.

• The atoms of the reactants are rearranged to form new substances.

• The reactants dissolve and disappear completely.

• The reactants combine without changing their atomic structure.

Multiple Choice

When carbon dioxide and limewater are combined, a new milky-white substance is formed. What is the best explanation for this observation?

• The carbon dioxide and limewater simply mixed together.

• The limewater evaporated, leaving a solid behind.

• A chemical reaction occurred, forming a new product.

• The substances changed state from liquid to solid.

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Using Models to Understand Reactions

• Since atoms are too small to see, scientists use simplified models to study them.

• ​These models can be physical, drawings, or symbolic like a chemical equation.

• Models show how atoms in reactants rearrange to form new product molecules.

• They help explain scientific laws, like the law of conservation of matter.

Multiple Choice

What is the primary reason scientists use models to understand chemical reactions?

• Atoms and molecules are too small to be observed directly.

• Models are less expensive than real experiments.

• Chemical reactions happen too fast to see with the naked eye.

• To show that reactants and products are always the same.

Multiple Choice

What is the main function of a scientific model like a drawing or an equation in a chemical reaction?

• They show how atoms are rearranged to form new substances.

• They list all the possible types of chemical equations.

• They explain why atoms are too small to be seen.

• They provide a physical sample of the reactants and products.

Multiple Select

Which of the following are examples of models scientists use to study atoms?

• A physical ball-and-stick model of a molecule

• A drawing showing atom arrangement

• A chemical equation representing a reaction

• A photograph of the products

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Conservation of Atoms

• In any chemical reaction, atoms are only rearranged, not created or destroyed.

• The total number of each type of atom remains exactly the same.

• When methane reacts with oxygen, CH₄ + 2O₂ → CO₂ + 2H₂O, atoms are conserved.

• Each side has 1 carbon, 4 hydrogen, and 4 oxygen atoms.

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Multiple Choice

What does the principle of conservation of atoms state happens in a chemical reaction?

• Atoms are rearranged, but the total number and type of atoms stay the same.

• New atoms are created to form different substances.

• Some atoms are destroyed during the reaction process.

• Atoms are changed into completely new types of elements.

Multiple Choice

Based on the reaction CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, how does the chemical equation show that atoms are conserved?

• The number of each type of atom is the same on both sides of the arrow.

• The number of molecules is identical on both sides of the arrow.

• The reactants and products contain completely different atoms.

• The reaction produces a new substance like a solid or a gas.

Multiple Choice

A student writes the chemical equation H₂ + O₂ → H₂O. Why does this equation fail to show the conservation of atoms?

• The number of oxygen atoms is not the same on both sides of the equation.

• The number of hydrogen atoms is not the same on both sides of the equation.

• The equation does not produce a gas as a product.

• The equation should only have one molecule on the product side.

Conservation of Mass

• Atoms are not created or destroyed in a chemical reaction.

• The total mass of reactants equals the total mass of products.

• Total mass remains constant in a closed system.

Multiple Choice

Which statement best defines the law of conservation of mass?

• The principle that the total mass of reactants equals the total mass of products.

• The principle that atoms can be created or destroyed in a reaction.

• The principle that the mass of products is always greater than the reactants.

• The principle that mass changes depending on the container it is in.

Multiple Choice

What is the underlying reason why mass is conserved in a chemical reaction?

• Because atoms are rearranged in a reaction, not created or destroyed.

• Because the reactants lose some of their atoms during the reaction.

• Because the reaction always produces a gas that escapes.

• Because energy is released, which reduces the total mass.

Multiple Choice

If 56 grams of iron react completely with 56 grams of sulfur in a closed system, what can be concluded about the mass of the resulting iron sulfide?

• The total mass of the products will be 112 grams.

• The total mass of the products will be 56 grams.

• The total mass of the products will be less than 112 grams.

• It is impossible to determine the mass of the products.

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Common Misconceptions

Misconception	Correction
New atoms are made during a reaction.	Atoms are rearranged, never created or destroyed.
Mass can be lost or gained in a reaction.	Mass is conserved in a closed system.
When a substance disappears, it is gone forever.	Atoms rearrange into a new, dissolved substance.
You can change subscripts (e.g., in H	Changing a subscript changes the substance (e.g., H

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Summary

• Reactants rearrange their atoms to form new products with new properties.

• Atoms are conserved, not created or destroyed, so mass is also conserved.

• Chemical equations are models showing how atoms rearrange to balance both sides.

• Signs of a reaction include a color change, precipitate, or producing gas.

Poll

On a scale of 1-4, how confident are you about using models to explain the conservation of mass?

• 1

• 2

• 3

• 4