Chemical Change

Chemical Change

Middle School

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Learning Objectives

• Define and differentiate between physical and chemical changes.

• Analyze density, melting point, and flammability data to spot chemical reactions.

• Describe how reactant atoms rearrange to create new substances with new properties.

• Identify signs of chemical reactions, like color change, gas bubbles, or heat.

Key Vocabulary

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Physical Change

This is a change in the form or appearance of a substance without creating a new one.

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Chemical Change

A chemical change is a process that results in the formation of new chemical substances.

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Reactants

Reactants are the starting materials or substances that undergo a change during a chemical reaction.

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Products

Products are the new substances that are formed as a result of a chemical reaction.

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Flammability

Flammability describes the ability of a particular substance to burn or ignite, causing fire or combustion.

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Precipitate

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

Key Vocabulary

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Density

Density describes the amount of mass packed into a specific, certain, or given amount of volume.

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Melting Point

The melting point is the specific temperature at which a solid substance starts to transform into a liquid.

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Boiling Point

The boiling point is the exact temperature at which a liquid substance begins to turn into a gas.

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Solubility

Solubility refers to the maximum ability of a substance, the solute, to dissolve in another substance, the solvent.

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Odor

Odor is the distinctive or characteristic smell that is given off by a particular chemical substance you encounter.

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Atom

An atom is the smallest and most basic building block of a chemical element, like hydrogen or oxygen.

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Physical vs. Chemical Changes

Physical Change

• ​This change alters a substance's appearance, but does not create a new substance.

• ​​Examples include cutting paper, melting ice, or bending a metal spoon.

• ​Its characteristic properties, like boiling point or density, remain the same.

Chemical Change

• ​This type of change, also known as a chemical reaction, produces new substances.

• ​​The original substances are called reactants, and the new ones are called products.

• ​Examples include burning a piece of wood or an iron nail rusting.

Multiple Choice

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

• A physical change creates a new substance, while a chemical change does not.

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

• A physical change involves reactants and products, while a chemical change does not.

• A chemical change alters a substance's mass, while a physical change alters its density.

Multiple Choice

During a physical change, what happens to a substance's characteristic properties, such as its boiling point?

• The properties change to match the new substance formed.

• The properties, like boiling point, remain the same.

• The properties become reactants in a chemical reaction.

• The properties are lost during the change.

Multiple Choice

When wood is burned, it turns into ash and smoke. Why is this considered a chemical change?

• Because the wood has changed its appearance.

• Because the change involves a solid turning into a gas.

• Because the wood and oxygen are reactants that form new products like ash and smoke.

• Because the process can be easily reversed.

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Characteristic Properties of Substances

• Every pure substance has unique properties used for its identification.

• Key examples include density, melting point, boiling point, and solubility.

• Other properties are flammability (the ability to burn) and odor.

• Comparing properties tells us if a new substance has formed.

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Multiple Choice

What is the primary purpose of using a substance's characteristic properties?

• To change the color of a substance.

• To measure the mass of a substance.

• To help identify a pure substance.

• To determine the volume of a substance.

Multiple Choice

Which of the following is a characteristic property that can be used to identify a substance?

• Temperature

• Volume

• Flammability

• Mass

Multiple Choice

A scientist measures the boiling point of a liquid as 100°C. After a chemical reaction occurs, the boiling point of the new liquid is 115°C. What is the best conclusion the scientist can draw?

• The substance only changed its state from liquid to gas.

• A new substance was formed because a characteristic property changed.

• The amount of the substance decreased during the experiment.

• The two substances did not actually mix together.

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Atoms in Chemical Reactions

• A chemical reaction rearranges atoms; it does not create or destroy them.

• ​Atoms in reactants break their bonds and form new combinations.

• These new combinations of atoms are called the products.

• Products have different properties, such as a new odor or density.

Multiple Choice

What happens to atoms during a chemical reaction?

• They are destroyed completely.

• They are created from nothing.

• They are rearranged into new substances.

• They are converted into pure energy.

Multiple Choice

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

• Reactants are the new substances formed, and products are the starting materials.

• Reactants and products are both destroyed during the reaction.

• Reactants and products are two names for the same substances.

• Reactants are the starting materials, and products are the new substances formed.

Multiple Choice

If a chemical reaction begins with two clear, odorless liquids and results in a yellow solid, what can be concluded?

• The atoms of the reactants were destroyed.

• A new substance with different properties was formed.

• The reaction created entirely new atoms.

• The properties of the reactants and products are identical.

Evidence of a Chemical Reaction

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Change in Color

• A color change can indicate that a new substance has been formed.

• For example, bread turns brown in a toaster due to chemical reactions.

• The heat causes new compounds to form, changing the bread's color.

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Production of a Gas

• Bubbles forming may signal that a new gas is being made.

• This can happen when solids or liquids react with each other.

• Vinegar and baking soda create bubbles of CO₂ gas.

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Formation of a Solid

• A precipitate is a solid that forms from a liquid solution.

• This can happen when two different liquids are mixed together.

• The new solid is evidence a chemical reaction has occurred.

Multiple Choice

What is the most likely reason for a change in color, the appearance of bubbles, or the formation of a solid in a substance?

• A chemical reaction has occurred

• The substance has gotten hotter

• The substance has dissolved in water

• The substance has changed its state

Multiple Choice

How does the formation of bubbles in a liquid, like when vinegar is added to baking soda, serve as evidence of a chemical reaction?

• It shows that a new gas is being produced

• It means the liquid is starting to boil

• It indicates that a solid is dissolving

• It proves the liquid is getting colder

Multiple Choice

If you mix two clear liquids and the mixture becomes cloudy because a solid substance forms and settles at the bottom, what is the best conclusion you can draw?

• A new solid, called a precipitate, has formed from a chemical reaction

• The two liquids were not able to mix together properly

• One of the liquids has evaporated, leaving a solid behind

• The temperature change caused a solid to form

Energy in Chemical Reactions

• Some reactions release energy as heat or light, making surroundings warmer.

• Some reactions absorb energy from surroundings, making them feel colder.

• Energy moves from chemicals to surroundings, or from surroundings into chemicals.

Multiple Choice

What is the primary role of energy in a chemical reaction?

• It is either released or absorbed.

• It is always destroyed.

• It is always created from nothing.

• It remains unchanged within the chemicals.

Multiple Choice

What happens to the surroundings when a chemical reaction releases energy?

• The surroundings become warmer.

• The surroundings become colder.

• The temperature of the surroundings does not change.

• The reaction stops producing light.

Multiple Choice

If a chemical reaction in a beaker makes the beaker feel cold, what is the best explanation for what is happening?

• The reaction is absorbing energy from its surroundings.

• The reaction is releasing energy into its surroundings.

• The chemicals are creating their own energy.

• The temperature has no connection to the energy of the reaction.

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Common Misconceptions

Misconception	Correction
Bubbles always mean a chemical reaction is happening.	Bubbles can also form from physical changes, like opening a soda can.
When a substance disappears by dissolving, it's a chemical change.	Dissolving is a physical change. The substance is still present, just mixed in.
All changes involving heat are chemical changes.	Physical changes like melting ice also require heat. No new substance is formed.

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Summary

• A physical change alters form; a chemical change creates new substances.

• Characteristic properties like density and flammability are used to identify substances.

• In a reaction, atoms rearrange to form new products with new properties.

• Evidence of a reaction includes color change, gas production, or energy changes.

Poll

On a scale of 1-4, how confident are you about analyzing properties to determine if a chemical reaction has occurred?

• 1

• 2

• 3

• 4