What is Law of Conservation of Mass?

The mass of substances produced (products) by a chemical reaction is always equal to the mass of the reacting substances (reactants).


If we were to bake a cake, the mass of the cake after it has been placed in the oven is the same as the mass of all the ingredients. But this has to be in a closed system, meaning all the moisture and gases released have to be collected as well.

Why is it useful?

Law of Conservation of Mass is a Law in chemistry showing what you put in (the reactants) is always or has to be the same as the amount of products.

If the reaction equation is NOT balanced, then it must be balanced.

Rules for balancing:

  1. You can only add a number IN FRONT of your compounds
  2. You CANNOT change the numbers WITHIN the compounds such as changing the subscripts. Ie. H2O cannot be changed to H4O

How many frames and sets of wheels would you need to balance 2 bikes?

Screen Shot 2015-08-13 at 4.28.06 PM

Steps in balancing equations:

  1. Split your equations into reactants side and products side
  2. Make a table of ALL the different elements present
  3. Write down the number of atoms for each element

Screen Shot 2015-08-13 at 4.29.47 PM

  1. Even out the number of the reactants and products by finding a common multiple.
  2. Balance and check until you have the equation balanced for ALL elements.


Keep polyatomic compounds together and count compounds instead of individual elements.
Start with the more unique elements first such as transition metals than hydrogen.