1. Clues for a Chemical Change
Halloween Colors
Clues for a Chemical Change

2. 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

3. 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 + 3O Fe2O3 )
Chemical changes are rarely reversible

4. Clues for Chemical Changes
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)

5. Demonstration of a Chemical Change
A 3% solution of hydrogen peroxide
in water is a clear, colorless liquid.
Hydrogen Peroxide
H2O2
A 5% solution of potassium dichromate
in water is a clear, orange liquid.
Potassium Dichromate
K2Cr2O7

6. 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
becomes warm.
The mixture becomes clear and slowly turns from
black to reddish brown. After about two hours, the
color turns bright yellow.
A 5% solution of potassium dichromate
in water is a clear, orange liquid.

9. 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

10. The Chemical Changes A very fast chemical reaction:
K2Cr2O7 + 4H2O KOH + 3H2O + 2CrO2 + 3O2
Orange Colorless Black Gas
(Clear) (Cloudy)
A moderately fast chemical reaction:
CrO2 + H2O CrO3 + H2O
Clear Reddish-Brown Solution
A slow chemical reaction:
2KOH + CrO K2CrO4 + H2O
Clear Yellow Solution