Midway Middle School

 Chemical Reactions often cause observable changes

 Color  Odor  Release or Absorption of Heat  Release or Absorption of Light  Release of Gas

Examples of Physical Changes *Melting ice *Dissolving salt *Cutting paper

*No new product is formed *Substance just physically appears different

Examples of Chemical Changes *Color change *Heat absorbed or released *Formation of gas

*Elements are made of only one kind of atom *Elements are all listed on the periodic table Examples of elements: Oxygen – O Iron – Fe Sodium - Na

*Compounds are made of two or more elements chemically combined Examples of compounds: Water – H 2 O Baking soda – NaHCO 3 Carbon dioxide – CO 2

NaHCO 3 This is the chemical formula for baking soda. *How many elements are there? 4 Sodium(Na) Hydrogen(H) Carbon(C) Oxygen(O )

NaHCO 3 This is the chemical formula for baking soda. *How many total atoms are there? Sodium(Na) 1 - Hydrogen(H) 1 - Carbon(C) 3 - Oxygen(O )

 Matter can NOT be created Nor can it be destroyed  Chemical Equations MUST balance

 The same number of atoms of each element must be in the products that were in the reactants of a chemical equation. Example: ReactantsProduct

Chemical reactions are represented on paper by chemical equations. Example: 2 hydrogen gas (2H 2 ) can react (burn) with oxygen gas (O 2 ) to form 2 water molecules(H 2 0).

yields subscript

Why do Chemical Equations have to balance?

Is this a balanced equation? Yes atoms in = atoms out

Balance the following equation:

 We seem to be o.k. with our number of (C)carbon atoms in both the reactants and products  But, we have only half the (H)hydrogen in our products as in our reactants.

2

*This has balanced our (C)carbon and (H)hydrogen atoms *But, we now have 4 (O)oxygen atoms in our products, and only have 2 in our reactants.

2

  We have a balanced chemical equation for the reaction of methane with oxygen.