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REVIEW: Signs of a Chemical Change  Change in temperature  Production of a gas  Change in color  Production of light or sound  Formation of a solid (precipitate) These are “CLUES” or “DIRECT EVIDENCE” that a chemical reaction has occurred!

 A process that produces a chemical change is called a chemical reaction.  There are two types of substances in a chemical reaction, the reactants and the products.  Reactants are the substances that exist before the reaction begins.  Products are the substances that form as a result of the reaction.

ReactantsProducts Baking Soda + Vinegar→ Gas + White Solid Charcoal + Oxygen→ Ash + Gas + Heat Iron + Oxygen + Water→ Rust Silver + Hydrogen sulfide→ Black Tarnish + Gas Copper Sulfate + Aluminum→ Copper + Aluminum Oxide Sliced Apple + Oxygen→ Apple turns brown

 Chemists try to find out which reactants are used and which products are formed in chemical reactions.  A chemical equation tells chemist at a glance the reactants, products, and proportions of each substance present. CH 3 COOOH+ NaHCO 3 →CH 3 COONa +H2OH2OCO 2 Acetic AcidSodium Hydrogen carbonate Sodium AcetateWaterCarbon Dioxide VinegarBaking Soda

 According to the Law of Conservation of Mass, the mass of the products must be equal to the mass of the reactants in the chemical equation.  This principle was first stated by the French chemist Antoine Lavoisier ( ), who is considered the first modern chemist.  He showed that chemical equations are like math equations, where the right and left sides of the equation are equal. CH 3 COO OH + NaHCO 3 →CH 3 COONa + H2OH2OCO 2

 Often, energy is either released or absorbed during a chemical reaction.  Endothermic reactions absorb thermal energy and the reaction feels cold.  Exothermic reactions release thermal energy and the reaction feels hot.

 1) Synthesis reaction – Two or more substances combine to make a new substance.  Example – Sodium and Chlorine bond to form NaCl or table salt.

 2) Decomposition reaction – One substance breaks down in two or more substances.  Example – Breaking up Lithium Bromide.

 3) Replacement reaction – One element replaces another element to make a new substance.  Example – Potassium trades with Lithium. Y?  Each has one valence electron.