Physical and Chemical Changes

Physical and Chemical Changes

Middle School

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Learning Objectives

• Difference between physical and chemical changes.

• Learn how properties decide if a chemical reaction has happened.

• Explain how atoms rearrange to make new substances in chemical reactions.

• Apply the law of conservation of mass to physical and chemical changes.

Key Vocabulary

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Physical Change

A change in a substance's form or appearance, but not its chemical identity or makeup.

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Chemical Change

A change that results in a new substance with completely new chemical and physical properties.

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Conservation of Mass

The principle that states matter is not created or destroyed during any type of chemical reaction.

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Reactant

A substance present at the very beginning of a chemical reaction to be changed.

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Product

The new substance that is formed at the very end of a chemical reaction.

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Flammability

The ability of a certain substance to burn or ignite, which causes fire or combustion.

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What Are Physical Changes?

• A physical change alters a substance's form, not its chemical identity.

• The substance itself remains the same at a molecular level.

• Examples include an ice cube melting or crushing an aluminum can.

• Dissolving sugar in water is another common physical change.

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Multiple Choice

Which of the following is the best definition of a physical change?

• A change that alters a substance's form but not its chemical identity.

• A change that creates a completely new substance with different properties.

• A change that can only happen when a substance melts or boils.

• A change that is permanent and cannot be reversed.

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Identifying Substances by Properties

Physical Properties

• Properties that can be observed or measured without changing the substance’s chemical identity.

• Examples include unique characteristics like density, melting point, and boiling point for each substance.

• For example, water’s boiling point is a physical property and is always 100°C (212°F).

Chemical Properties

• Properties that describe a substance's ability to undergo a chemical reaction, forming new substances.

• Observing a chemical property, like flammability, fundamentally changes the substance's original composition.

• For instance, the ability of wood to burn is a chemical property called flammability.

Multiple Choice

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

• A physical property can be observed without changing the substance, while a chemical property describes its ability to change.

• A physical property describes a substance's color, while a chemical property describes its mass.

• A physical property relates to flammability, while a chemical property relates to density.

• Physical properties are only for liquids, while chemical properties are for all states of matter.

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What Are Chemical Changes?

• A chemical change transforms substances into entirely new substances.

• ​This process is also known as a chemical reaction.

• Atoms are rearranged to form new products, but never created or destroyed.

• For example, hydrogen and oxygen gases react to form liquid water (H₂O).

Multiple Choice

What happens to the atoms of the reactants during a chemical reaction?

• They are rearranged to form the products.

• They are destroyed and disappear completely.

• They are converted into energy.

• They mix together but remain unchanged.

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Signs of a Chemical Reaction

• A chemical reaction may occur when substances' properties change after they interact.

• Look for changes in odor, density, or melting point as indicators.

• The formation of a gas, a solid, or a color change are clues.

• For example, burning steel wool creates ash, which is a new substance.

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Multiple Choice

Which of the following observations would most likely indicate that a chemical reaction has occurred?

• A change in the substance's odor after interacting with another substance.

• A change in the substance's shape when it is bent.

• A change in the substance's size when it is crushed.

• A change of state from solid to liquid.

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The Law of Conservation of Mass

• Matter is not created or destroyed during any physical or chemical change.

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

• In reactions, atoms are rearranged to form new substances, but atoms are not lost.

• For example, 12g of carbon and 32g of oxygen create 44g of CO₂.

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Multiple Choice

What is the central principle of the law of conservation of mass?

• Matter is not created or destroyed in a reaction.

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

• Atoms can be destroyed during a chemical reaction.

• The mass of a substance always changes over time.

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Energy and Changes in Matter

Absorbing Energy

• ​Some changes in matter absorb thermal energy from their surroundings to occur.

• ​​For example, melting an ice cube is a physical change that absorbs heat.

• ​Thermal energy is the total energy of particles and flows from warmer to cooler objects.

Releasing Energy

• ​Other changes in matter release energy into their surroundings, often as heat and light.

• ​​For instance, burning wood is a chemical change that releases its stored energy.

• ​In all changes, energy is conserved, meaning it is only transferred, not lost.

Multiple Choice

How does thermal energy behave when two objects are at different temperatures?

• It flows from the warmer matter to the cooler matter.

• It flows from the cooler matter to the warmer matter.

• It is created by the cooler matter.

• It is destroyed by the warmer matter.

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Common Misconceptions

Misconception	Correction
Substances 'disappear' after a reaction, like burning wood.	Atoms rearrange to form new substances like smoke and ash; they are not lost.
Dissolving salt in water is a chemical change because the salt vanishes.	This is a physical change. The salt can be recovered by evaporating the water.
Temperature and thermal energy are the same thing.	Temperature is the average kinetic energy; thermal energy is the total kinetic energy.

Multiple Choice

Why is melting an ice cube considered a physical change, while baking a cake is a chemical change?

• Melting only changes the state of water, while baking creates new substances with different properties.

• Melting is reversible, while baking is not.

• Melting involves cooling, while baking involves heating.

• Melting is a slow process, while baking is a fast process.

Multiple Choice

If 10 grams of vinegar and 5 grams of baking soda react in a sealed bag, a gas is produced and the bag inflates. According to the law of conservation of mass, what will be the total mass of the contents of the bag after the reaction?

• 15 grams

• More than 15 grams

• Less than 15 grams

• Exactly 10 grams

Multiple Choice

You mix two room-temperature liquids. The container becomes cold to the touch and a solid substance forms at the bottom. What can you infer from these observations?

• A chemical change occurred that absorbed energy.

• A physical change occurred that released energy.

• A chemical change occurred that released energy.

• Only a physical change of dissolving occurred.

Multiple Choice

You are given a clear, odorless liquid. To determine if it is water, what is the most effective strategy using its physical and chemical properties?

• Measure its boiling point to see if it is 100°C and check if it is non-flammable.

• Mix it with salt to see if the salt dissolves.

• Pour it into a different shaped container to see if it takes the shape.

• Taste it to see if it is tasteless.

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Summary

• Physical changes alter a substance's form but do not create a new substance.

• Chemical changes create new substances with new properties by rearranging atoms.

• Signs of a chemical reaction include changes in odor or formation of a gas.

• In any change, matter is not created or destroyed, only conserved.

Poll

On a scale of 1-4, how confident are you about differentiating between physical and chemical changes?

• 1

• 2

• 3

• 4