Reactions

Reactions

Middle School

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Learning Objectives

• Identify the reactants and products in a chemical reaction.

• Explain how atoms rearrange to form new substances with different properties.

• Describe the law of conservation of mass in chemical reactions.

• Differentiate between physical and chemical changes.

• Distinguish between endothermic and exothermic reactions based on energy changes.

Key Vocabulary

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Atom

The basic unit of a chemical element that makes up all matter.

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Chemical Reaction

A process where substances interact to form new, different substances.

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Reactant

A substance that is present at the start 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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Property

A characteristic of a substance, such as color, odor, or density.

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Substance

A particular kind of matter with uniform, definite composition and properties.

Key Vocabulary

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Rearrange

To change the position or the specific order of atoms in a molecule.

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Molecule

A group of two or more atoms that are held together by bonds.

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Conservation of Mass

The principle that mass cannot be created or destroyed in a chemical reaction.

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Exothermic

A chemical reaction that releases energy, usually in the form of heat or light.

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Endothermic

A chemical reaction that absorbs energy from its surroundings to proceed with the reaction.

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Physical vs. Chemical Changes: What's the Difference?

Physical Changes

• ​These changes alter the form of a substance, but not its actual chemical identity.

• ​​Examples include changing its shape or its state of matter, like a solid, liquid, or gas.

• ​The molecules stay the same, like water which is always H₂O as a solid or liquid.

Chemical Changes

• ​These changes create entirely new substances by breaking and forming new chemical bonds.

• ​​The atoms of reactants are rearranged to form the resulting substances called products.

• ​For example, when wood burns, it becomes new substances like ash, smoke, and various gases.

Multiple Choice

What is the key difference between a physical and a chemical change?

• A physical change alters a substance's form, while a chemical change creates a new substance.

• A physical change involves only solids, while a chemical change involves liquids and gases.

• A physical change is always about shape, while a chemical change is about the state of matter.

• A physical change rearranges atoms, while a chemical change keeps molecules the same.

Multiple Choice

Why is melting an ice cube into liquid water considered a physical change?

• Because the water molecules remain H₂O, even though their arrangement changes.

• Because changing state is a simple process that can be easily reversed.

• Because the change only involves temperature and not other factors.

• Because the water changes its shape to fit a new container.

Multiple Choice

When wood burns, it becomes ash, smoke, and gas. Based on this outcome, what conclusion can be drawn about this process?

• It is a chemical change because the wood's atoms were rearranged to form new substances.

• It is a physical change because the wood just changed its form into smaller pieces.

• It is a chemical change because the process released a lot of heat and light.

• It is a physical change because the ash and smoke are still made of the original wood molecules.

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Evidence of a Chemical Reaction

• Scientists look for observable signs to see if a chemical reaction happened.

• An unexpected change in color can mean a new substance has been formed.

• A reaction may produce gas bubbles or form a solid called a precipitate.

• Temperature may change, light may be produced, or a new odor may be detected.

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Multiple Choice

What evidence do scientists look for to determine if a chemical reaction has taken place?

• Observable signs that indicate a new substance has formed.

• The original substances used in the experiment.

• The temperature of the laboratory room.

• The equipment used to mix the substances.

Multiple Choice

Besides a change in color or odor, what is another common sign that a chemical reaction has produced a new substance?

• The formation of gas bubbles or a solid.

• The substances mixing together evenly.

• The color of the substances staying the same.

• The temperature of the mixture decreasing slightly.

Multiple Choice

A student mixes a white powder into a clear liquid. Which of the following observations would best support the conclusion that a chemical reaction occurred?

• The mixture fizzes and the container feels warm.

• The two liquids mix together to form a clear solution.

• The total volume of the liquid decreases slightly.

• The solid dissolves completely in the liquid.

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Atoms Rearrange in Chemical Reactions

• Atoms are not created or destroyed; they just rearrange themselves.

• ​Reactant atoms break their bonds and form new bonds as products.

• Think of it like dismantling a LEGO house to build a car.

• This rearrangement gives products different properties from the reactants.

Multiple Choice

What happens to atoms during a chemical reaction?

• They are rearranged to form new substances.

• They are destroyed and new atoms are created.

• They are mixed together but remain unchanged.

• They are permanently broken down into smaller particles.

Multiple Choice

Why do the products of a chemical reaction have different properties than the substances that started the reaction?

• Because new atoms are created during the reaction.

• Because the total number of atoms has changed.

• Because the atoms have been rearranged into new substances.

• Because the reaction absorbs all of the old properties.

Multiple Choice

Using the idea that atoms are like LEGO bricks that get rearranged, what must be true when comparing the substances at the start (reactants) and end (products) of a reaction?

• The products will have more atoms than the reactants.

• Some of the original atoms are lost during the rearrangement.

• The total number and type of atoms are the same in both the reactants and the products.

• The new substances created must have the same properties as the original substances.

Conservation of Mass

• Mass is not created or destroyed in chemical reactions.

• Atoms are only rearranged during a chemical reaction.

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

Multiple Choice

What is the principle of conservation of mass?

• Mass is converted into energy.

• Mass is not created or destroyed.

• Mass increases during the reaction.

• Mass decreases during the reaction.

Multiple Choice

According to the law of conservation of mass, what happens to atoms during a chemical reaction?

• They are destroyed by the chemical reaction.

• They are created by the chemical reaction.

• They are rearranged to form the products.

• They are converted into a different type of atom.

Multiple Choice

If 10 grams of reactant A and 15 grams of reactant B are combined in a chemical reaction, which statement accurately describes the outcome?

• The total mass of the products will be 25 grams.

• The total mass of the products will be less than 25 grams.

• The total mass of the products will be more than 25 grams.

• The total mass cannot be determined from the information.

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Energy in Chemical Reactions

Exothermic Reactions

• ​These reactions release energy, often as heat, into the surroundings.

• ​​The products of the reaction contain less chemical energy than the reactants.

• ​The temperature of the surroundings increases, making it feel hot.

Endothermic Reactions

• ​These reactions absorb energy from the surroundings, often as heat.

• ​​The products of the reaction contain more chemical energy than the reactants.

• ​The temperature of the surroundings decreases, which makes it feel cold.

Multiple Choice

What is the primary difference between exothermic and endothermic reactions?

• The type of chemicals involved

• The speed at which they occur

• Whether they release or absorb energy

• The color change they produce

Multiple Choice

What is the relationship between the reactants and products in an endothermic reaction?

• The products contain less chemical energy than the reactants.

• The products contain more chemical energy than the reactants.

• The products and reactants have the same amount of chemical energy.

• The reaction does not involve any chemical energy.

Multiple Choice

If a chemical reaction taking place in a beaker causes the beaker to feel cold, what can you conclude about the reaction?

• It is an exothermic reaction because it is releasing cold.

• It is an endothermic reaction because it is absorbing heat from its surroundings.

• It is an exothermic reaction because the temperature decreased.

• It is an endothermic reaction because the products have less energy.

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Synthetic Materials from Natural Resources

• Humans make synthetic materials by chemically changing natural resources like oil and wood.

• This process rearranges atoms to create new materials with more useful properties.

• A great example is making soap from natural resources like fats and lye.

• The new soap can clean with oil and water, unlike the original ingredients.

Multiple Choice

What is a synthetic material?

• A material made by humans from natural resources.

• A material that is found directly in nature.

• A material that has no useful properties.

• A material that is made without chemical reactions.

Multiple Choice

What is the purpose of using chemical reactions to make synthetic materials?

• To rearrange molecules to create new, useful properties.

• To make exact copies of the original natural resources.

• To destroy the molecules of natural resources completely.

• To combine materials without changing them chemically.

Multiple Choice

The creation of soap from fats and an alkali is used as an example of making a synthetic material. What conclusion does this example support?

• The soap has new abilities that the original fats and oils did not have.

• The soap is a natural resource, just like the fats and oils.

• The chemical reaction makes the original materials less useful.

• Only solids can be used to create synthetic materials.

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Common Misconceptions

Misconception	Correction
New atoms are created during a chemical reaction.	Atoms are rearranged to form new substances, not created or destroyed.
A physical change, like boiling water, is a chemical reaction.	Physical changes only alter a substance's state, not its chemical identity (H2O).
Mass is lost when a reaction produces a gas.	Gas has mass. In a closed system, the total mass remains constant.
All chemical reactions happen quickly and dramatically.	Many reactions are very slow, such as iron rusting or fruit ripening.

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Summary

• In a chemical reaction, reactants are transformed into products with new properties.

• Evidence of a reaction includes changes in color, temperature, or gas formation.

• Atoms are rearranged in reactions, and the total mass is always conserved.

• Exothermic reactions release energy, while endothermic reactions absorb it.

Poll

On a scale of 1-4, how confident are you about explaining what happens to atoms during a chemical reaction?

• 1 - Not confident at all

• 2 - A little confident

• 3 - Mostly confident

• 4 - Very confident