2. Law of Conservation of Mass
In a chemical reaction, mass can neither be created or destroyed, it can only change form.
Therefore, the mass of the reactants before the reaction occurs must be exactly equal to the mass of the products following the reaction.

3. For example, if a log were placed in a sealed metal box and set on fire. The products (smoke, ash, gases, etc.) would have the exact same mass as the log before it were burnt.
If the box were sitting on a scale as the log burned, the weight would not change at all.

4. Same number of each type of atom on both sides of the arrow.

5. Notice
The number of atoms of each type of element must be the same on both sides of a balanced equation.
Subscripts must not be changed to balance an equation.
A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction.

6. Tips for Balancing a Chemical Equation
Write down the number of atoms per each element that you have on each side of the equation.
If possible, start with the most complicated compound that contains the same element on the left and right side of the equation.
Change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of the element is the same on each side of the equation. Remember! To balance an equation, you change the coefficients, not the subscripts in the formulas

7. Example

8. Example

9. Example

10. Example

11. NH4OH + FeCl3  Fe(OH)3 + NH4Cl
Example
NH4OH + FeCl3  Fe(OH)3 + NH4Cl

12. You Try

13. You Try
Fe3O4 + H2  Fe + H2O

14. You Try
Na + H2O  H2 + NaOH

15. You Try
CH4 + O2  CO2 + H2O

16. You Try
AgNO3 + Cu  Cu(NO3)2 + Ag

18. Online Practice: