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8 th grade Science

8 th grade Science. Chemical and Physical Changes and Reactions. Chemical Changes. Chemical reactions are taking place all around you and even within you Chemical reaction-a change in which one or more substances are converted into new substances (due to rearrangement)

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8 th grade Science

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  1. 8th grade Science Chemical and Physical Changes and Reactions

  2. Chemical Changes • Chemical reactions are taking place all around you and even within you • Chemical reaction-a change in which one or more substances are converted into new substances (due to rearrangement) • Reactants-substances that react • Products-new substances produced

  3. Chemical Changes • Chemical changes occur when a material changes from one substance to another • Several factors can indicate a chemical change has occurred: • 1. Color change • 2. Temperature change • 3. Production of a gas • 4. Formation of a solid

  4. Physical changes • Physical changes: those changes that occur in nature involving a change in STATE only (phase changes) • A substance may change states of matter as a result of a physical change, but will not become a new substance • Ex: freezing, melting, evaporating, tearing, etc.

  5. Conservation of Mass • Lavoisier-famous scientist who discovered that masses of reactants and products were the same in an experiment • SO, he concluded: • Law of Conservation of Mass-the starting mass of reactants equals the final mass of the products OR • Matter is neither created nordestroyed during chemical reactions

  6. A metal will replace any less active metal • Notice: copper, silver and gold-least active metals • SO: that is why these elements often occur as deposits of the relatively pure element.

  7. All chemical reactions release or absorb energy • This energy can take many forms, such as heat, light, sound and electricity • Chemical bonds are the source of this energy

  8. When chemical reactions take place, some chemical bonds in the reactants must be broken • Breaking these bonds takes energy • Bond formation releases energy

  9. Exothermic Reaction- when the energy given off in a reaction is primarily in the form of heat • *Exothermic reactions provide most of the power used in homes and industries

  10. Endothermic reaction- when the energy needed is in the form of heat • (Endothermic can also refer to phase/physical changes as well)

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