1. End Show © Copyright Pearson Prentice Hall Slide 1 of 19 Chemical Reactions Iron is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel. Over time, objects made of iron will rust if they are left exposed to air. You will learn to recognize chemical changes and to distinguish them from physical changes. What always happens during a chemical change? 2.4

2. End Show Slide 2 of 19 © Copyright Pearson Prentice Hall Chemical Reactions > 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

3. End Show Slide 3 of 19 © Copyright Pearson Prentice Hall Chemical Reactions > Chemical Changes A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change. 2.4

4. End Show Slide 4 of 19 © Copyright Pearson Prentice Hall Chemical Reactions > Chemical Changes A magnet separates iron from sulfur. This is an example of a physical change. 2.4

5. End Show © Copyright Pearson Prentice Hall Chemical Reactions > Slide 5 of 19 Recognizing Chemical Changes 2.4 Recognizing Chemical Changes What are four possible clues that a chemical change has taken place?

6. End Show © Copyright Pearson Prentice Hall Slide 6 of 19 Chemical Reactions > Recognizing Chemical Changes Possible clues to chemical change include: a transfer of energy a change in color the production of a gas the formation of a precipitate. 2.4

7. End Show Slide 7 of 19 © Copyright Pearson Prentice Hall Chemical Reactions > Chemical Changes A precipitate is a solid that forms and settles out of a liquid mixture. Clues to chemical changes have practical applications. 2.4

8. End Show © Copyright Pearson Prentice Hall Chemical Reactions > Slide 8 of 19 Conservation of Mass 2.4 Conservation of Mass How are the mass of the reactants and the mass of the products of a chemical reaction related?

9. End Show © Copyright Pearson Prentice Hall Slide 9 of 19 Chemical Reactions > Conservation of Mass 2.4 During any chemical reaction, the mass of the products is always equal to the mass of the reactants.

10. End Show Slide 10 of 19 © Copyright Pearson Prentice Hall Chemical Reactions > Conservation of Mass The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved. The conservation of mass is easily observed when a change occurs in a closed container. 2.4

11. End Show © Copyright Pearson Prentice Hall Slide 11 of 19 Section Quiz -or- Continue to: Launch: Assess students’ understanding of the concepts in Section 2.4 Section Quiz. 2.4.

12. © Copyright Pearson Prentice Hall Slide 12 of 19 End Show 2.4 Section Quiz. 1.Which of the following is a chemical reaction? a.melting of lead b.dissolving sugar in water c.rusting of iron d.crushing of stone

13. © Copyright Pearson Prentice Hall Slide 13 of 19 End Show 2.Which of the following is NOT a possible clue that a chemical change is taking place? a.a change of state b.a change in color c.production of a gas d.formation of a precipitate 2.4 Section Quiz.

14. © Copyright Pearson Prentice Hall Slide 14 of 19 End Show 3.During any chemical change, the mass of the products is a.always equal to the mass of the reactants. b.always greater than the mass of the reactants. c.always less than the mass of the reactants. d.sometimes different than the mass of the reactants. 2.4 Section Quiz.