1. Chemical Change Conservation of Mass

2. A.Anytime the atoms in molecules are rearranged to form new molecules, a chemical reaction has taken place 1.New substances are formed B.The starting molecules are known as reactants and the ending molecules are products 2 H 2 (g) + O 2 (g)  2 H 2 O (l) ReactantsProducts

3. C.Indicators of a Chemical Change 1.Formation of a precipitate – solid that is formed when liquids are mixed 2.Exothermic – heat is released by reaction, feels hot a.Ex. Hand Warmers, Burning 3.Endothermic – heat is absorbed by reaction, feels cold a.Ex. Cold Compress 4.Formation of a gas: Does not include boiling which is a physical change a.Ex. Hydrogen peroxide in a wound 5.Change in color - Must be more than just simple mixing a.For example, mixing yellow and blue food coloring results in green. This is a physical change. b.Ex. Steak turns from red to brown when cooked

4. C.Indicators of a Chemical Change (Continued) 1.Change in Taste – Must be more than just mixing a.Ex. Cake batter before and after cooking 2.Change in Smell – Must be more than just mixing a.Ex. Burning sugar on the stove 3.Creation of Light without heating a.Heating a sample until it glows is a physical change b.Ex. Chemical Light Sticks, Fireflies D.These are only indicators. To be sure a chemical change has taken place one would have to verify that new molecules have been formed.

5. A.Atoms cannot be created or destroyed in a chemical reaction, they are simply rearranged 1.All the atoms in the reactant molecules are still present in the products B.If all the atoms are present, the mass of the products must be the same as the reactants 1.Mass is Conserved in a chemical reaction, meaning that the mass is the same before and after