8th grade Science

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 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)