Property Changes in Chemical Reactions

Property Changes in Chemical Reactions

Middle School

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Learning Objectives

• Analyze data to determine if a chemical reaction has occurred.

• Describe how atoms are conserved and rearranged during a chemical reaction.

• Explain that a reaction forms new substances with different properties.

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

• Interpret a chemical equation to identify reactants and products.

Key Vocabulary

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Chemical Reaction

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

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Physical Property

A characteristic of a substance that can be observed or measured without changing its identity.

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Chemical Property

A characteristic describing how a substance reacts with other substances to form something new.

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Density

Density is the measure of how much mass is contained in a given unit volume.

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Solubility

Solubility is the ability of a substance to dissolve completely in a liquid solvent.

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Flammability

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

Key Vocabulary

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Melting Point

The melting point is the specific temperature at which a solid substance turns into a liquid.

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Boiling Point

The boiling point is the specific temperature at which a liquid substance turns into a gas.

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Reactant

A reactant is a substance that is present at the beginning of a chemical reaction.

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Product

A product is a new substance that is formed as a result of a chemical reaction.

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Conservation of Mass

The law of conservation of mass states that mass is not created or destroyed during a reaction.

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Chemical Equation

A chemical equation uses symbols and formulas to represent a chemical reaction that has occurred.

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Characteristic Properties of Substances

Physical Properties

• These are traits you can observe or measure without changing the substance’s chemical identity.

• Examples include density, melting point, boiling point, and how well a substance dissolves in another.

• For instance, the boiling point of pure water is always 100°C at sea level.

Chemical Properties

• These describe how a substance behaves when it is part of a chemical reaction.

• A key example is flammability, which is the ability of a substance to burn.

• Another is reactivity, like how iron combines with oxygen in the air to form rust.

Multiple Choice

What is the fundamental difference between physical and chemical properties?

• Physical properties can be observed without changing the substance's identity, while chemical properties describe how it reacts.

• Physical properties describe a substance's color and shape, while chemical properties describe its texture and mass.

• Physical properties relate to how a substance burns, while chemical properties relate to its boiling or melting point.

• Physical properties can only be measured in a lab, while chemical properties can be observed anywhere.

Multiple Choice

Which of the following describes a chemical property of a substance?

• The temperature at which a substance boils.

• The ability of a substance to dissolve in water.

• The temperature at which a substance melts.

• The ability of a substance to rust.

Multiple Choice

A student finds that a white, crystalline substance melts at 150°C. When the student pours a liquid on it, the substance fizzes and produces a gas. Which statement best explains these observations?

• Its melting point is a physical property, and its reaction to the liquid is a chemical property.

• Its solid form is a chemical property, and its melting is a physical property.

• Both melting and fizzing are chemical properties because the substance changed.

• Both melting and fizzing are physical properties because the original substance is gone.

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Evidence of a Chemical Reaction

• A chemical reaction creates a new substance with entirely different properties.

• A change in chemical properties, like a substance becoming flammable, is evidence.

• Physical changes like a new color, odor, or forming a gas occur.

• The reaction might produce energy in the form of heat or light.

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Multiple Choice

What is the primary outcome of a chemical reaction?

• A new substance with different properties is created.

• The original substance simply changes its shape or size.

• The substances mix together without changing chemically.

• Energy is lost, but no new substance is formed.

Multiple Choice

What is the relationship between changes like a new color or odor and a chemical reaction?

• They are the new substances themselves.

• They are physical signs that a new substance may have formed.

• They prove that only a physical change has happened.

• They are forms of energy released during the reaction.

Multiple Choice

When wood burns in a fire, it turns into black ash while producing heat and light. What is the best explanation for these observations?

• A chemical reaction occurred, because a new substance formed and energy was released.

• Only a physical change occurred, because the wood just changed form.

• The wood absorbed heat, which caused it to glow.

• The light is a chemical change, but the ash is a physical change.

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The Law of Conservation of Mass

• The mass of substances before a reaction equals the mass after the reaction.

• French chemist Antoine Lavoisier discovered this important scientific law in the 1700s.

• He experimented in closed systems where matter could not get in or out.

• This means mass is conserved; it is not created or destroyed in reactions.

Multiple Choice

Which statement best describes the Law of Conservation of Mass?

• The total mass of substances remains the same after a chemical reaction.

• Chemical reactions cause mass to be lost.

• New mass is created during a chemical reaction.

• The mass of a substance increases when it is burned.

Multiple Choice

Why was it important that Antoine Lavoisier's experiments were conducted in a closed system?

• To ensure no mass could enter or escape the reaction.

• To make the chemical reaction happen faster.

• To allow new mass to be created during the reaction.

• To change the chemical properties of the substances.

Multiple Choice

A piece of wood with a mass of 25 grams is burned completely in a sealed container. If 7 grams of ash are produced, what is the total mass of the gases released?

• 18 grams

• 7 grams

• 25 grams

• 32 grams

What Happens to Atoms in a Reaction?

• Atoms in reactants regroup to form new substances called products.

• Reactant particles collide with enough energy, breaking their chemical bonds.

• Separated atoms rearrange and form new bonds to create products.

Multiple Choice

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

• They are destroyed and no longer exist.

• They are changed into atoms of a different element.

• They are regrouped to form new substances.

• They are created from nothing.

Multiple Choice

What must happen first for reactants to be able to rearrange and form new products?

• New bonds form in the reactants before they make contact.

• Product particles collide and their chemical bonds are broken.

• Reactant particles collide and their chemical bonds are broken.

• Atoms are separated from products to form reactants.

Multiple Choice

Based on the process of a chemical reaction, what is the most likely outcome if reactant particles collide with very low energy?

• The reactants would form new products more slowly.

• The reactants would remain unchanged because their bonds would not break.

• The reactants would turn into different kinds of atoms.

• The reactants would form new bonds without breaking the old ones.

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Representing Chemical Reactions

• A chemical equation uses formulas to describe what happens in a chemical reaction.

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

• Subscripts show the number of atoms in a molecule, like in H₂O.

• Coefficients are used in front of formulas to balance a chemical equation.

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Multiple Choice

What is the primary purpose of a chemical equation?

• To use formulas to describe a chemical reaction

• To list the atomic mass of each element

• To show the temperature and pressure of a reaction

• To separate mixtures into their original substances

Multiple Choice

How does a chemical equation show which substances are reactants and which are products?

• It places reactants on the left of an arrow and products on the right

• It uses larger letters for reactants and smaller letters for products

• It labels reactants with an 'R' and products with a 'P'

• It lists the reactants first in the equation and the products last

Multiple Choice

If a chemical equation has an unequal number of atoms on the reactant and product sides, what is the correct way to balance it?

• By adding or changing the coefficients in front of the formulas

• By changing the subscripts within the chemical formulas

• By removing the arrow from the middle of the equation

• By writing new reactants or products into the equation

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Common Misconceptions

Misconception	Correction
Any property change is a chemical change.	Physical changes don't create new substances, but chemical changes do.
Mass is lost or gained in a reaction.	In a closed system, mass is conserved; it is never lost or gained.
Atoms are changed into other types of atoms.	Atoms are only rearranged into new combinations, not changed into other atoms.

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Summary

• Every pure substance has characteristic physical and chemical properties that identify it.

• A chemical reaction produces new substances with new characteristic properties.

• The law of conservation of mass states that matter is never created or destroyed.

• Reactions involve bonds breaking in reactants and new bonds forming in products.

• Atoms are conserved during reactions; they just get rearranged.

• Chemical equations use coefficients to show that atoms are conserved.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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