Chemical Changes and Reactions

Chemical Changes and Reactions

Middle School

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Learning Objectives

• Define chemical properties and explain how they differ from chemical changes.

• Recognize the signs that indicate a chemical reaction has taken place.

• Explain how chemical equations represent reactants, products, and the Law of Conservation of Mass.

• Describe the five main types of chemical reactions.

• Understand how energy changes in endothermic and exothermic reactions.

Key Vocabulary

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Chemical Reaction

A process in which one or more substances change to make one or more new substances.

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Chemical Bond

A force that holds two atoms together in a molecule, which can be broken and reformed.

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Reactant

The starting materials in a chemical reaction, written on the left side of a chemical equation.

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Product

The new substances that are formed from a chemical reaction, written on the right side of the equation.

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Precipitate

A solid substance that forms in a liquid solution as a result of a chemical reaction.

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Chemical Equation

A shorthand representation using symbols and formulas to describe a chemical reaction.

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Chemical Properties and Changes

Chemical Property

• Describes the ability of a substance to undergo a specific chemical change.

• It is only observable when the substance is turning into a different substance.

• An example is the ability of iron to rust when combined with oxygen.

Chemical Change

• A process where substances are transformed into new substances with different properties.

• This is also known as a chemical reaction.

• Heating zinc and sulfur to form zinc sulfide is an example.

Multiple Choice

Which statement best defines a chemical property?

• A characteristic of a substance that can be observed without changing it

• A process where a substance's shape or size is altered

• The ability of a substance to undergo a specific chemical change

• The process where a substance changes from a solid to a liquid

Multiple Choice

What is the relationship between a chemical property and a chemical change?

• A chemical property and a chemical change are the same process.

• A chemical property describes a physical change, not a chemical one.

• A chemical property can only be observed while a chemical change is happening.

• A chemical change happens first, which then creates a chemical property.

Multiple Choice

When iron combines with oxygen, it rusts and forms a new substance. Which statement provides the best explanation for this process?

• This is a physical property because the iron is still present within the rust.

• This is a chemical change that demonstrates iron's chemical property of being able to rust.

• This is a physical change because the appearance of the iron is altered.

• This is a chemical change that happens to all metals when they are heated.

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Signs of a Chemical Reaction

• The most important sign is the formation of a new, different substance.

• Look for gas bubbles or a solid forming from a liquid (a precipitate).

• An unexpected color change, or new light, sound, or smells are also clues.

• The temperature getting noticeably hotter or colder shows an energy change during the reaction.

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Multiple Choice

What is the most important sign that a chemical reaction has occurred?

• The substance dissolves in water.

• The temperature stays exactly the same.

• The substance changes its shape.

• A new, different substance is formed.

Multiple Choice

Why are the formation of gas bubbles or a solid precipitate considered signs of a chemical reaction?

• Because they are the only signs that can be easily seen.

• Because they indicate that a new substance has been produced.

• Because they show that the original substance has changed state.

• Because they always cause the temperature to increase.

Multiple Choice

A student mixes two clear liquids in a beaker. The mixture fizzes, the beaker feels warm, and a yellow solid settles at the bottom. What is the best conclusion the student can make?

• One liquid simply dissolved in the other.

• A chemical reaction occurred, producing a new gas and a solid.

• The liquids evaporated quickly due to the heat.

• The liquids only changed their temperature.

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Breaking and Making Chemical Bonds

• Atoms in molecules are held together by a force called a chemical bond.

• For a chemical reaction to happen, bonds in the reactants must break.

• The atoms then rearrange and form new bonds to create new products.

• This rearrangement results in substances with completely new and different properties.

Multiple Choice

What is the primary function of a chemical bond?

• To create energy for a chemical reaction.

• To hold atoms together within a molecule.

• To change the individual properties of an atom.

• To separate atoms from each other within a molecule.

Multiple Choice

What must happen to the atoms of reactants for a chemical reaction to produce a new substance?

• The original bonds must break so the atoms can rearrange and form new bonds.

• New bonds must form in the reactants before the original bonds can break.

• The atoms must be held tightly together without breaking their original bonds.

• The atoms must gain new properties before any bonds can change.

Multiple Choice

What is the direct result of atoms from original substances rearranging to form new chemical bonds?

• The original substances keep their original properties.

• New substances are created that have entirely new properties.

• The atoms themselves are changed into new types of atoms.

• The total number of atoms is reduced during the reaction.

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Chemical Formulas and Equations

Chemical Formulas

• A chemical formula is a shorthand way to represent one molecule of a substance.

• It uses element symbols and small numbers called subscripts, like in H₂O.

• The subscript indicates the number of atoms of that element in the molecule.

Chemical Equations

• A chemical equation uses formulas to show how substances change in a chemical reaction.

• Reactants are on the left of an arrow, and products are on the right.

• Equations must be balanced, meaning atoms are not created or destroyed, only rearranged.

Multiple Choice

What is the primary purpose of a chemical formula like H₂O?

• To describe the physical properties of a molecule.

• To represent one molecule of a substance using symbols.

• To show how atoms are rearranged in a reaction.

• To list all the elements found in a substance.

Multiple Choice

What is the fundamental relationship between reactants and products in a chemical equation?

• Products are used to balance the atoms in the reactants.

• Reactants and products are always identical.

• Reactants are the starting substances that change to become the products.

• Reactants are the substances that result from the change, while products are the starting materials.

Multiple Choice

If a chemical equation is balanced, what must be true about the atoms on both sides of the arrow?

• Some atoms from the reactants are destroyed during the reaction.

• The products will have more atoms than the reactants.

• The number of atoms of each element is the same on both sides.

• New types of atoms are created to form the products.

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Types of Chemical Reactions

Combination Reactions

Decomposition Reactions

Multiple Choice

What happens during a combination reaction?

• A single compound breaks down into simpler substances.

• Two or more substances combine to form a single, more complex compound.

• Two compounds exchange parts to form two new compounds.

• A substance changes its state, like ice melting into water.

Multiple Choice

How is a decomposition reaction related to a combination reaction?

• Both reactions result in the formation of a single, more complex compound.

• They are the same type of reaction.

• A decomposition reaction is the opposite of a combination reaction.

• Both reactions require a single compound to start.

Multiple Choice

A scientist observes a reaction where a single compound, AB, turns into two simpler substances, A and B. Which statement best describes this chemical change?

• It is a combination reaction because it follows the formula A + B -> AB.

• It is a combination reaction because two substances are present at the end.

• It is a decomposition reaction because a single compound breaks down.

• It is a decomposition reaction because two compounds are mixed together.

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Energy in Chemical Reactions

Exothermic Reactions

• ​Releases energy into the surroundings, often in the form of heat or light.

• ​​Because energy is released, the container holding the reaction feels warm.

• ​An explosion is a dramatic example of an exothermic reaction.

Endothermic Reactions

• An endothermic reaction absorbs energy from its surroundings, making the area cooler.

• All chemical reactions involve a transfer of energy between the reaction and its surroundings.

• As a result of absorbing energy, the reaction container will feel cold.

Multiple Choice

Which statement is true for all chemical reactions?

• They always make their surroundings feel cold.

• They all involve a transfer of energy.

• They always release energy as an explosion.

• They only happen when light is present.

Multiple Choice

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

• Exothermic reactions are always explosions, while endothermic reactions are not.

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

• Exothermic reactions feel cold, while endothermic reactions feel warm.

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

Multiple Choice

A student mixes two chemicals in a beaker and observes that the beaker begins to feel warm. What is the best explanation for this?

• The reaction is neither exothermic nor endothermic.

• An explosion is about to happen.

• An endothermic reaction is occurring, absorbing heat from the beaker.

• An exothermic reaction is occurring, releasing heat into the surroundings.

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Common Misconceptions

Misconception	Correction
All color changes are chemical reactions.	Only unexpected color changes may indicate a chemical reaction.
Physical changes like melting or boiling are chemical reactions.	Phase changes alter a substance's state, not its chemical identity (e.g., H2O is still water).
You can change subscripts to balance an equation.	Subscripts define the molecule. Only change coefficients to balance.

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Summary

• A chemical reaction creates new substances with new properties.

• Signs of a reaction include gas, a solid forming, or color and energy changes.

• Reactions happen when chemical bonds break and new bonds are formed.

• Chemical equations are balanced to conserve mass.

• Reactions are classified into types like combination, decomposition, and replacement.

• Energy is either released (exothermic) or absorbed (endothermic).

Poll

On a scale of 1-4, how confident are you about identifying the signs and types of chemical reactions?

• 1

• 2

• 3

• 4