1. Chapter 2 Matter and Change . 2.4 Chemical Reactions
2.1 Properties of Matter
2.2 Mixtures
2.3 Elements and Compounds
2.4 Chemical Reactions
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2. What happened to the match?
CHEMISTRY & YOU
What happened to the match?
In this lesson, you will learn to recognize whether the burning match is a chemical change or physical change.
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3. What always happens during a chemical change?
Chemical Changes
Chemical Changes
What always happens during a chemical change?
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4. Chemical Changes
Words such as burn, rot, rust, decompose, ferment, explode, and corrode usually signify a chemical change.
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5. 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.
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6. Chemical properties can be used to identify a substance.
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.
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7. 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.
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8. During a chemical change, the composition of matter always changes.
Chemical Changes
During a chemical change, the composition of matter always changes.
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9. When the charcoal is heated and burned, a chemical change occurs.
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.
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10. A chemical change is also called a chemical reaction.
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.
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11. Chemical Changes
In the burning of charcoal, carbon and oxygen are the main reactants, and carbon dioxide is the main product.
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12. Recognizing Chemical Changes
What are four possible clues that a chemical change has taken place?
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13. 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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14. Every chemical change involves a transfer of energy.
Recognizing Chemical Changes
Every chemical change involves a transfer of energy.
For example, energy stored in natural gas is used to cook food.
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15. 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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16. 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.
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17. Recognizing Chemical Changes
If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place.
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18. Clues to chemical change often have practical applications.
Recognizing Chemical Changes
Clues to chemical change often have practical applications.
Production of a Gas
Bubbles of carbon dioxide form when two antacid tablets are dropped into a glass of water.
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19. Clues to chemical change often have practical applications.
Recognizing Chemical Changes
Clues to chemical change often have practical applications.
Color Change
When a test strip is dipped in a solution, the color change is used to determine the pH of the solution.
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20. Clues to chemical change often have practical applications.
Recognizing Chemical Changes
Clues to chemical change often have practical applications.
Formation of a Precipitate
One step in the production of cheese is a reaction that causes milk to separate into solid curds and liquid whey.
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21. CHEMISTRY & YOU
Are the changes that happen to a burning match chemical or physical changes? How do you know?
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22. Conservation of Mass
Conservation of Mass
How are the mass of the reactants and the mass of the products of a chemical reaction related?
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23. Conservation of Mass
During any chemical reaction, the mass of the products is always equal to the mass of the reactants.
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24. 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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25. Mass also holds constant during physical changes.
Conservation of Mass
Mass also holds constant during physical changes.
When 10 grams of ice melt, 10 grams of liquid water are produced.
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26. 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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27. Conservation of Mass
The law of conservation of mass is more easily observed when a change occurs in a closed container.
When the liquids in the figure at left are mixed, they react. None of the products are gases.
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28. To what changes does the law of conservation of mass apply?
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29. 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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30. Chemistry as the Central Science
BIG IDEA
Chemistry as the Central Science
The different forms of matter may undergo physical or chemical changes.
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