1. The Law of Conservation of Mass:
The total mass of the reactants must equal the total mass of the products
Total mass of reactants = Total mass of products

2. This makes sense because atoms cannot be created or destroyed
Atoms present at the beginning of the reaction must still be present after the reaction has taken place
How??
Atoms rearrange themselves, bonding to new atoms to make a different product
Fruit salad game example,
Square dancing

3. So when you write a chemical equation…
The number of atoms of each type must be the same on each side
Equations must be balanced!
This can be done in 4 easy steps
reactants
products

4. Step 1: Write the word equation
products
Methane + oxygen  carbon dioxide + water
reactants

5. Step 2: Write the skeleton equation
Replace each name with its formula
Methane + oxygen  carbon dioxide + water
CH4 + O2  CO2 + H2O

6. Step 3: Count atoms
Count the number of atoms of each type in reactants (left side) and products (right side)
CH4 + O2  CO2 + H2O
Type of Atom
Reactants
Products
Balanced? C 1  H 4 2  O
(2+1) = 3

7. Step 4: Balancing
Multiply each compound by the appropriate coefficients to balance the number of atoms
Start with the atom that is in the least number of compounds (less disruption)
Usually leave oxygen and hydrogen for last (very common atoms)
Trial and error (be patient )

8. Step 4: Balancing CH4 + O2  CO2 + 2H2O Type of Atom Reactants
Products
Balanced? C 1  H 4 O 2
(2+2) = 4 
Remember when you put a coefficient in front of a compound, ALL atoms are multiplied!!

9. Step 4: Balance CH4 + 2O2  CO2 + 2H2O Type of Atom Reactants Products
Balanced? C 1  H 4 O

10. Human Balancing Act Sodium + Chlorine Gas  Table Salt
Na (s) + Cl2 (g)  NaCl (s)
2 Na (s) + Cl2 (g)  2 NaCl (s)