Understanding Conservation of Mass in Chemical Reactions

By: Catherine Barnhill



Often students cannot understand from chemical formulas how atoms exist in compounds, and how they are rearranged during reaction. Furthermore, the concept of balancing reactions eludes them when they cannot actually see the individual atoms. To illustrate a reaction for them, They are split into groups and given "bingo" type chips of different colors. They then put them into groups as reactants, representing the compounds that enter the reaction. They will then break the bonds of the reactants and attempt to rebond them as products. In most reactions, they will see that without balancing they do not have enough "chips" to complete the reaction. However, if they add more of one or more of the reactants, they can then complete the reaction because they now have enough "chips". They will then transfer findings to a sheet, written as a balanced formula equation.

Keywords: reaction conservation balancing


Students will observe the meaning of coefficients in chemical reactions, and will understand the concept of the law of conservation of mass as it pertains to chemical reactions.


Students will know and understand that during a reaction bonds are broken and new bonds are formed, but that matter is not created or destroyed.
Students will show understanding of how to balance chemical equations.


Academic Preparation

The lecture will be given on chemical reactions and balancing of chemical reactions prior to performing the activity.


1. Students are broken into groups of 3-5.
2. Students receive a handout with a few different UNBALANCED reactions on it.
3. Students will assign certain colored "chips" to elements.
4. Students will "build" the reactants.
5. Students will "break the bonds" by splitting the compounds apart, and will then attempt to rearrange the "chips" into the products.
6. Students will decide first if the reaction is already balanced, and if not, they will decide how many molecules (moles) of each reactant is needed for a complete reaction.
7. Students will then transfer their findings into coefficients by writing the balanced equation for the reaction.
8. Repeat for all equations on the sheet.
***Extension to this activity would be to give extra reactions for which the products are not provided, expecting students to be able to predict the products.


Teacher will move around the room, listening to comments and answering questions. Teacher will check answers on lab sheet, and will give many more practice problems for balancing without the use of the "chips". They will also be required to take a teacher generated test at the end of the chapter to show understanding of the concepts both by balancing equations, predicitng products, and explaining through extended response why equations must be balanced.

Estimated Lesson Duration

1hr 00min