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

2. Halloween Colors 2 Physical and Chemical Changes Physical change involves altering the size, shape Physical change involves altering the size, shape or state of matter of a substance: 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: Physical changes are reversible: - water vapor condenses to a liquid - 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. Halloween Colors 3 Physical and Chemical Changes Chemical change rearranges the atoms of a Chemical change rearranges the atoms of a substances to form new substances or two substances to form new substances or two different atoms combine to form one new different atoms combine to form one new substance: substance: - burning of paper - adding vinegar to baking soda gives CO 2 - when iron rusts the gray metal becomes red iron oxide upon exposure to red iron oxide upon exposure to oxygen in the air (4Fe + 3O 2 2Fe 2 O 3 ) oxygen in the air (4Fe + 3O 2 2Fe 2 O 3 ) Chemical changes are rarely reversible Chemical changes are rarely reversible

4. Halloween Colors 4 Clues for Chemical Changes Color changes Color changes Formation of a gas Formation of a gas Increase in temperature (Exothermic) Increase in temperature (Exothermic) Decrease in temperature (Endothermic) Decrease in temperature (Endothermic) Formation of a precipitate from solution Formation of a precipitate from solution Spontaneous combustion Spontaneous combustion Formation of a solid from two different liquids Formation of a solid from two different liquids Formation of a liquid from two different solids Formation of a liquid from two different solids Explosion Explosion The original substances are not restored (irreversible) The original substances are not restored (irreversible)

5. Halloween Colors 5 Demonstration of a Chemical Change 2. A 5% solution of potassium dichromate in water is a clear, orange liquid. in water is a clear, orange liquid. K 2 Cr 2 O 7 Potassium Dichromate 1. A 3% solution of hydrogen peroxide in water is a clear, colorless liquid. in water is a clear, colorless liquid. H2O2H2O2H2O2H2O2 Hydrogen Peroxide

6. Halloween Colors 6 Demonstration of a Chemical Change 2. A 5% solution of potassium dichromate in water is a clear, orange liquid. in water is a clear, orange liquid. When the orange solution is added to the colorless When the orange solution is added to the colorless solution, the color immediately turns black and the solution, the color immediately turns black and the mixture becomes cloudy. mixture becomes cloudy. After a short time, bubbles begin to appear and soon After a short time, bubbles begin to appear and soon the bubbling becomes vigorous while the flask the bubbling becomes vigorous while the flask becomes warm. becomes warm. The mixture becomes clear and slowly turns from The mixture becomes clear and slowly turns from black to reddish brown. After about two hours, the black to reddish brown. After about two hours, the color turns bright yellow. color turns bright yellow.

7. Halloween Colors 7

8. Halloween Colors 8

9. Halloween Colors 9 What happened? A chemical change has taken place: A chemical change has taken place: -color change -formation of a gas -increase in temperature The change involves more than one chemical reaction: The change involves more than one chemical reaction: -several color changes over time -solution first becomes cloudy, but then clears up

10. Halloween Colors 10 The Chemical Changes 1. A very fast chemical reaction: K 2 Cr 2 O 7 + 4H 2 O 2 KOH + 3H 2 O + 2CrO 2 + 3O 2 K 2 Cr 2 O 7 + 4H 2 O 2 KOH + 3H 2 O + 2CrO 2 + 3O 2 Orange Colorless Black Gas Orange Colorless Black Gas (Clear) (Cloudy) (Clear) (Cloudy) 2. A moderately fast chemical reaction: CrO 2 + H 2 O 2 CrO 3 + H 2 O CrO 2 + H 2 O 2 CrO 3 + H 2 O Clear Reddish-Brown Solution Clear Reddish-Brown Solution 3. A slow chemical reaction: 2KOH + CrO 3 K 2 CrO 4 + H 2 O 2KOH + CrO 3 K 2 CrO 4 + H 2 O Clear Yellow Solution Clear Yellow Solution