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2.4 Chemical Reactions >
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Chapter 2 Matter and Change 2.1 Properties of Matter 2.2 Mixtures 2.3 Elements and Compounds 2.4 Chemical Reactions
2.4 Chemical Reactions >
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In this lesson, you will learn to recognize whether the burning match is a chemical change or physical change.
CHEMISTRY & YOU
What happened to the match?
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2.4 Chemical Reactions >
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Chemical Changes
Chemical Changes
What always happens during a chemical change?
2.4 Chemical Reactions >
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Chemical Changes
Words such as burn, rot, rust, decompose, ferment, explode, and corrode usually signify a chemical change.
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2.4 Chemical Reactions >
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Chemical Changes
Words such as burn, rot, rust, decompose, ferment, explode, and corrode usually signify a chemical change.
• The ability of a substance to undergo a specific chemical change is called a chemical property.
2.4 Chemical Reactions >
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Chemical Changes
Iron is able to combine with oxygen to form rust.
• The ability to rust is a chemical property of iron.
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2.4 Chemical Reactions >
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Chemical Changes
Chemical properties can be used to identify a substance.
• But chemical properties can be observed only when a substance undergoes a chemical change.
2.4 Chemical Reactions >
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Chemical Changes
When charcoal is broken into smaller pieces, the change is a physical change.
• The substances present before the change are the same substances present after the change, although the charcoal pieces are not as large.
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2.4 Chemical Reactions >
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Chemical Changes
When charcoal is broken into smaller pieces, the change is a physical change.
• During a physical change, the composition of the matter never changes.
2.4 Chemical Reactions >
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Chemical Changes
During a chemical change, the composition of matter always changes.
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2.4 Chemical Reactions >
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Chemical Changes
When the charcoal is heated and burned, a chemical change occurs.
• The substances in charcoal react with oxygen in the air to form other substances.
2.4 Chemical Reactions >
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Chemical Changes
A chemical change is also called a chemical reaction.
• One or more substances change into one or more new substances during a chemical reaction.
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2.4 Chemical Reactions >
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Chemical Changes
A chemical change is also called a chemical reaction.
• One or more substances change into one or more new substances during a chemical reaction.
• A substance present at the start of the reaction is a reactant.
• A substance produced in the reaction is a product.
2.4 Chemical Reactions >
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In the burning of charcoal, carbon and oxygen are the main reactants, and carbon dioxide is the main product.
Chemical Changes
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2.4 Chemical Reactions >
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What is always true of a chemical change that is never true of a physical change?
2.4 Chemical Reactions >
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What is always true of a chemical change that is never true of a physical change?
During a chemical change, the composition of the matter always changes. During a physical change, the composition of the matter never changes.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
Recognizing Chemical Changes What are four possible clues that a chemical change has taken place?
2.4 Chemical Reactions >
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Recognizing Chemical Changes
Possible clues to chemical change include a transfer of energy, a change in color, the production of a gas, or the formation of a precipitate.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
Every chemical change involves a transfer of energy.
• For example, energy stored in natural gas is used to cook food.
2.4 Chemical Reactions >
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Recognizing Chemical Changes
Every chemical change involves a transfer of energy.
• For example, energy stored in natural gas is used to cook food.
– When the methane in natural gas chemically combines with oxygen in the air, energy is given off in the form of heat and light.
– Some of this energy is transferred to and absorbed by food that is cooking over a lit gas burner.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
Every chemical change involves a transfer of energy.
• For example, energy stored in natural gas is used to cook food.
– The energy causes chemical changes to take place in the food.
– The food may change color and brown as it cooks, which is another clue that chemical changes are occurring.
2.4 Chemical Reactions >
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Recognizing Chemical Changes
You can observe two other clues to chemical change while cleaning a bathtub.
• The ring of soap scum that can form in a bathtub is an example of a precipitate.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
You can observe two other clues to chemical change while cleaning a bathtub.
• The ring of soap scum that can form in a bathtub is an example of a precipitate.
– A precipitate is a solid that forms and settles out of a liquid mixture.
2.4 Chemical Reactions >
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Recognizing Chemical Changes
You can observe two other clues to chemical change while cleaning a bathtub.
• The ring of soap scum that can form in a bathtub is an example of a precipitate. – Some bathroom cleaners that you can use to
remove soap scum start to bubble when you spray them on the scum.
– The bubbles are produced because a gas is released during the chemical change that is taking place in the cleaner.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place.
2.4 Chemical Reactions >
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Recognizing Chemical Changes
If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place. • The clue may be the result of a physical
change.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place. • The clue may be the result of a physical
change. – For example, energy is always transferred when
matter changes from one state to another.
– Bubbles form when you boil water or open a carbonated drink.
2.4 Chemical Reactions >
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Recognizing Chemical Changes
If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place. • The only way to be sure that a chemical
change has occurred is to test the composition of a sample before and after the change.
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2.4 Chemical Reactions >
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Recognizing Chemical Changes
Clues to chemical change often have practical applications.
• Bubbles of carbon dioxide form when two antacid tablets are dropped into a glass of water.
Production of a Gas
2.4 Chemical Reactions >
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Recognizing Chemical Changes
Clues to chemical change often have practical applications.
• One step in the production of cheese is a reaction that causes milk to separate into solid curds and liquid whey.
Formation of a Precipitate
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2.4 Chemical Reactions >
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CHEMISTRY & YOU
Are the changes that happen to a burning match chemical or physical changes? How do you know?
2.4 Chemical Reactions >
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CHEMISTRY & YOU
Are the changes that happen to a burning match chemical or physical changes? How do you know?
A burning match is undergoing a chemical change. There is a transfer of heat, a production of light, and a change of color. The composition of the materials also changes during the burning process.
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2.4 Chemical Reactions >
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How can you determine for sure that a chemical change has taken place?
2.4 Chemical Reactions >
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How can you determine for sure that a chemical change has taken place?
The only way to determine for sure if a chemical change has taken place is to test the composition of a sample before and after the change.
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2.4 Chemical Reactions >
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Conservation of Mass
Conservation of Mass How are the mass of the reactants and the mass of the products of a chemical reaction related?
2.4 Chemical Reactions >
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Conservation of Mass
When wood burns, substances in the wood combine with oxygen from the air. • As the wood burns, a sizable amount of
matter is reduced to a small pile of ashes.
• The reaction seems to involve a reduction in the amount of matter. But appearances can be deceiving.
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2.4 Chemical Reactions >
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Conservation of Mass
During any chemical reaction, the mass of the products is always equal to the mass of the reactants.
2.4 Chemical Reactions >
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Conservation of Mass
Two of the products of burning wood—carbon dioxide gas and water vapor—are released into the air. • When the mass of these cases is
considered, the amount of matter is unchanged.
• Careful measurements show that the total mass of the reactants equals the total mass of the products.
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2.4 Chemical Reactions >
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Conservation of Mass
Mass also holds constant during physical changes.
• When 10 grams of ice melt, 10 grams of liquid water are produced.
2.4 Chemical Reactions >
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Conservation of Mass
The scientific law that reflects these observations is the law of conservation of mass.
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2.4 Chemical Reactions >
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Conservation of Mass
The scientific law that reflects these observations is the law of conservation of mass. • The law of conservation of mass states
that in any physical change or chemical reaction, mass is conserved.
2.4 Chemical Reactions >
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Conservation of Mass
The scientific law that reflects these observations is the law of conservation of mass. • The law of conservation of mass states
that in any physical change or chemical reaction, mass is conserved.
• Mass is neither created nor destroyed.
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To what changes does the law of conservation of mass apply?
2.4 Chemical Reactions >
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To what changes does the law of conservation of mass apply?
The law of conservation of mass applies to chemical and physical changes.
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2.4 Chemical Reactions >
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Key Concepts
During a chemical change, the composition of matter always changes.
Four possible clues to chemical change include a transfer of energy, a change in color, the production of a gas, or the formation of a precipitate.
During any chemical reaction, the mass of the products is always equal to the mass of the reactants.
2.4 Chemical Reactions >
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• chemical property: the ability of a substance to undergo a specific chemical change
• chemical reaction: a change in which one or more reactants change into one or more products; characterized by the breaking of bonds in reactants and the formation of bonds in products
• reactant: a substance present at the start of a reaction
Glossary Terms
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• product: a substance produced in a chemical reaction
• precipitate: a solid that forms and settles out of a liquid mixture
• law of conservation of mass: in any physical change or chemical reaction, mass is conserved; mass can be neither created nor destroyed
Glossary Terms
2.4 Chemical Reactions >
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BIG IDEA
Chemistry as the Central Science The different forms of matter may undergo physical or chemical changes.
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END OF 2.4