Accounting for Atoms

Accounting for Atoms

Middle School

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Learning Objectives

• Define a chemical reaction and identify its reactants and products.

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

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

• Differentiate between reactions that release and those that absorb energy.

• Identify the signs that show a chemical reaction has happened.

Key Vocabulary

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Atom

Smallest unit of an element; keeps the chemical properties of that element unchanged.

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Molecule

Two or more atoms chemically bonded, moving together as one unit of matter.

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Chemical Reaction

Process where atoms break and form bonds, creating new substances with different properties.

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Reactant

Starting substance in a reaction, whose atoms can be rearranged into new products.

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Product

Substance produced by a reaction, containing atoms rearranged into a new chemical arrangement.

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Conservation of Mass

Scientific rule stating total mass stays the same before and after any chemical reaction.

Key Vocabulary

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Substance

A particular kind of matter with uniform properties that can be pure or a mixture.

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Property

A characteristic of a substance that can be observed, such as color or flammability.

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What is a Chemical Reaction?

• A chemical reaction is a process that changes reactants into new products.

• During a reaction, atoms rearrange to form substances with new properties.

• For example, flammable wood burns and turns into non-flammable ash.

• Signs include a color change, gas production, or the release of heat.

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Multiple Choice

What is the main outcome of a chemical reaction?

• New products are formed from reactants.

• Reactants are physically mixed together.

• The state of matter of a substance changes.

• The mass of the reactants is lost.

Multiple Choice

Why do the products of a chemical reaction, like ash from wood, have different properties than the reactants?

• Because the atoms of the reactants have rearranged.

• Because the reactants have simply been mixed.

• Because the reactants have changed their physical state.

• Because the products have absorbed heat.

Multiple Choice

If a scientist mixes two clear liquids and observes that the mixture begins to bubble and change color, what is the most logical conclusion?

• A chemical reaction is likely occurring.

• The liquids are boiling due to high temperature.

• The two liquids are not able to mix.

• One liquid is dissolving into the other.

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The Law of Conservation of Mass

• Matter is not created or destroyed during a chemical reaction in a closed system.

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

• Chemical reactions only rearrange atoms; they don’t create or make atoms disappear.

• Think of it like rebuilding a LEGO house into a car with the same bricks.

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Multiple Choice

Which statement best describes the Law of Conservation of Mass?

• In a closed system, the total mass before a reaction is the same as the total mass after the reaction.

• Chemical reactions cause atoms to disappear.

• The mass of a substance always gets smaller during a reaction.

• New atoms are created during a chemical reaction.

Multiple Choice

According to the Law of Conservation of Mass, what happens to atoms in a chemical reaction?

• They are rearranged into new molecules, but the atoms themselves are not lost.

• They are destroyed and replaced by new atoms.

• They are combined with new atoms from the air.

• They are broken down into smaller, simpler particles.

Multiple Choice

A student mixes two liquids in a beaker and notices the total mass decreases. Which statement best explains this result without violating the Law of Conservation of Mass?

• The reaction occurred in an open system, and some of the products escaped as gas.

• The law does not apply to reactions that produce a gas.

• The mass was converted into heat and light energy.

• The atoms in the reactants were destroyed by the chemical change.

Modeling Chemical Reactions

• Models help us see how atoms behave in chemical reactions.

• We count atoms to show mass is conserved during a reaction.

• Atoms from reactants rearrange to form new product molecules.

Multiple Choice

What is the main purpose of using models for chemical reactions?

• To help visualize how atoms behave and rearrange

• To increase the speed of the chemical reaction

• To change the types of atoms involved in the reaction

• To measure the mass of the reactant molecules

Multiple Choice

During a chemical reaction, what happens to the atoms from the reactant molecules?

• They are destroyed during the reaction

• They are rearranged to form new product molecules

• They remain unchanged in their original molecules

• They are converted into different types of atoms

Multiple Choice

If a model of a chemical reaction correctly shows that mass is conserved, what must be true when comparing the reactants and the products?

• The number and type of atoms are identical on both sides.

• The product side has fewer atoms than the reactant side.

• The product side has different types of atoms than the reactant side.

• The arrangement of atoms is identical on both sides.

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Energy in Chemical Reactions

Exothermic Reactions

• ​Reactions that release energy into the surroundings, usually in the form of heat and light.

• ​​Energy is released because new, more stable chemical bonds are formed during the chemical reaction.

• ​A campfire is a perfect example, releasing warmth and light as the wood burns away.

Endothermic Reactions

• ​Reactions that absorb energy from their surroundings, which causes the immediate temperature to drop.

• ​​Energy is absorbed from the environment to break the existing chemical bonds in the reactants.

• ​A cold pack for injuries is a common example of an endothermic reaction in action.

Multiple Choice

What is the primary difference between an exothermic and an endothermic reaction?

• One releases energy, while the other absorbs energy.

• One forms new chemical bonds, while the other breaks them.

• One involves heat and light, while the other involves only cold.

• One happens in nature, while the other only happens in a lab.

Multiple Choice

What is the underlying reason why an exothermic reaction releases energy into its surroundings?

• Energy is released because new, more stable chemical bonds are formed.

• Energy is absorbed from the surroundings to break existing bonds.

• The reaction causes the immediate temperature to drop.

• The reaction requires a constant input of heat to continue.

Multiple Choice

When a chemical cold pack is activated, it quickly becomes cold to the touch. Based on the principles of energy in chemical reactions, what is the best explanation for this observation?

• It is an endothermic reaction that is absorbing energy to break chemical bonds.

• It is an exothermic reaction that is releasing energy by forming stable bonds.

• It is an exothermic reaction that is absorbing energy to break chemical bonds.

• It is an endothermic reaction that is releasing energy by forming stable bonds.

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Common Misconceptions

Misconception	Correction
Burning destroys matter, making it disappear forever.	Matter is not destroyed, only rearranged into new substances like ash and gas.
Chemical reactions create new atoms.	Reactions rearrange existing atoms into new molecules; no new atoms are ever created.
Bubbles are always a sign of a chemical reaction.	Bubbles can also be from physical changes, like boiling water.
Melting ice into water is a chemical reaction.	It's a physical change. The substance is still water (H2O).

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Summary

• A chemical reaction rearranges atoms from reactants into new products.

• The Law of Conservation of Mass means atoms are never created or destroyed.

• Reactions can release energy (exothermic) or absorb energy (endothermic).

• Models help track atoms, and signs like color change indicate a reaction.

Poll

On a scale of 1-4, how confident are you about explaining the Law of Conservation of Mass to a friend?

• 1 - Not at all

• 2 - A little

• 3 - Mostly

• 4 - Very confident