Clues for a Chemical Change Halloween Colors Clues for a Chemical Change

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

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

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)

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

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.

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

The Chemical Changes A very fast chemical reaction: K2Cr2O7 + 4H2O2 KOH + 3H2O + 2CrO2 + 3O2 Orange Colorless Black Gas (Clear) (Cloudy) A moderately fast chemical reaction: CrO2 + H2O2 CrO3 + H2O Clear Reddish-Brown Solution A slow chemical reaction: 2KOH + CrO3 K2CrO4 + H2O Clear Yellow Solution