1. Chemical and Physical Changes and Reactions
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 nor destroyed 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)