Halloween Colors
Clues for a Chemical Change
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Physical and Chemical Changes
 Physical change involves altering the size, shape
or state of matter of a substance:
- boiling of water
- melting of ice
- dissolving salt in water
- shaping a piece of clay
 Physical changes are reversible:
- water vapor condenses to a liquid
- water freezes to form ice
- salt becomes solid when water evaporates
- a piece of clay can be reshaped
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Physical and Chemical Changes
 Chemical change rearranges the atoms of a
substances to form new substances or two
different atoms combine to form one new
substance:
- burning of paper
- adding vinegar to baking soda gives CO2
- when iron rusts the gray metal becomes
red iron oxide upon exposure to
oxygen in the air (4Fe + 3O2
2Fe2O3 )
 Chemical changes are rarely reversible
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Clues for Chemical Changes
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Color changes
Formation of a gas
Increase in temperature (Exothermic)
Decrease in temperature (Endothermic)
Formation of a precipitate from solution
Spontaneous combustion
Formation of a solid from two different liquids
Formation of a liquid from two different solids
Explosion
The original substances are not restored (irreversible)
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Demonstration of a Chemical Change
1. A 3% solution of hydrogen peroxide
in water is a clear, colorless liquid.
Hydrogen Peroxide
H2O2
2. A 5% solution of potassium dichromate
in water is a clear, orange liquid.
Potassium Dichromate
K2Cr2O7
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Demonstration of a Chemical Change
 When the orange solution is added to the colorless
solution, the color immediately turns black and the
mixture becomes cloudy.
 After a short time, bubbles begin to appear and soon
the bubbling becomes vigorous while the flask
2. Awarm.
5% solution of potassium dichromate
becomes
in water is a clear, orange liquid.
 The mixture becomes clear and slowly turns from
black to reddish brown. After about two hours, the
color turns bright yellow.
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What happened?
 A chemical change has taken place:
- color change
- formation of a gas
- increase in temperature
 The change involves more than one chemical reaction:
- several color changes over time
- solution first becomes cloudy, but then clears up
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The Chemical Changes
1. A very fast chemical reaction:
K2Cr2O7 + 4H2O2
Orange Colorless
(Clear)
KOH + 3H2O + 2CrO2 + 3O2
Black Gas
(Cloudy)
2. A moderately fast chemical reaction:
CrO2 + H2O2
CrO3 + H2O
Clear Reddish-Brown Solution
3. A slow chemical reaction:
2KOH + CrO3
K2CrO4 + H2O
Clear Yellow Solution
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