I.    All reactions can be classified into one of two groups: exothermic and endothermic.

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II.    Exothermic Reactions:

   A.    A reaction is exothermic if energy (in the form of heat)) is released into the surroundings.

   B.    Examples:

      1.    Freezing: In order for a liquid to freeze, energy must leave the liquid.

      2.    Getting a steam burn on your skin: When you get a steam burn, gaseous water molecules condense on your skin, and condensation is an exothermic                         process. 

      3.    Condensation: Condensation is the conversion of a gas to a liquid (the opposite of evaporation). Liquid molecules have to receive energy in order to                        become gaseous. As a result, gaseous molecules are more energetic than molecules of liquid water. Therefore, when a gas turns into a liquid, excess                        molecular energy is released into the surroundings.

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III.     Endothermic Reactions: 

   A.    A reaction is endothermic if energy (in the form of heat) goes into something (if energy is absorbed).

   B.    Examples:

      1.    Cooking: Heat is needed for foods to cook.

      2.    Melting: In order to melt ice cubes the ice must absorb heat.

      3.    Evaporation: In order for a liquid to convert to a gas, it must absorb heat. Energy is needed to break apart the intermolecular forces that are keeping the                    liquid molecules together so that they can spread apart and become gaseous. 

      4.    Sweating: When you produce sweat it absorbs your heat, allowing your body to cool.

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IV.    Conclusion:

   A.    Think exothermic if energy is not needed for the process to occur. 

   B.    Think endothermic if energy is needed for the process to occur.