Mr. Chapman
Science 10
Norquay School
Smarties, Conservation of Mass, and Balancing Equations
We know that in a chemical reaction, a substance becomes
a completely different substance. In other words, the
reactants combine in order to form new products. An
important observation for chemical reactions is that in any
chemical reaction, matter is not created or destroyed.
This important principle is known as the Law of Conservation of Matter:
Matter cannot be __________ or _____________, it can only change _________.
We know that even though they are incredibly small, atoms have mass. Therefore, according to
the law of conservation of mass, atoms cannot be created or destroyed. When a chemical
reaction occurs, the atoms that make up the reactants are simply rearranged to form the
products. Look at the following chemical reaction:
Fe + Cu(NO3)2 → Fe(NO3)2 + Cu
Notice that the atoms that formed the reactants have simply been rearranged to form the
products. Let’s count the number of atoms on the left side of the arrow (reactants) and the
right side of the arrow (products):
Left Side
Fe atoms _____ Cu atoms _____
Nitrogen atoms _____ Oxygen atoms ______
Right Side
Fe atoms _____ Cu atoms _____
Nitrogen atoms _____ Oxygen atoms ______
Mr. Chapman
Science 10
Using
Norquay School
to Investigate the Law of Conservation of Mass
Partner up and grab a bag of Smarties. Use the Smarties to create models of the compounds in
each of the following reactions.
1. For the reaction represented by the skeleton equation below, build the reactants out of
Smarties using the following colors:
Zinc – Red
Hydrogen – Orange
Zn +
HCl

ZnCl2
+
H2
Chlorine – Yellow

Now, rearrange your reactants (Zn and HCl) to form the products shown in the
reaction...
Can’t do it? Circle the reactant you need more of (Zn or HCl) in order to complete the products.
How many more molecules of that reactant do you need to finish the products? _____
How many molecules of this reactant will you now have in total? _____

Now, reform your reactants out of Smarties, but with the new amount of reactant.
Now can you rearrange the reactants in order to form the products? What you just did on
the previous page is called balancing the chemical equation. We can now re-write the previous
skeleton equation as a balanced chemical equation:
Mr. Chapman
Science 10
Norquay School
Zn + 2 HCl

ZnCl2
+
H2
The 2 in front of the HCl molecule in the reaction is called a numerical coefficient. A
numerical coefficient indicates the number of a certain type of molecule that is present in a
chemical reaction. In the balanced chemical equation, we can see that ___ zinc molecule is
combining with ___ hydrogen chloride molecules in order to form ___ zinc chloride molecule
and ___ hydrogen gas molecule. Now, count the number of atoms on each side of the chemical
equation to verify that it is balanced:
Left Side
Zinc atoms _____
Hydrogen atoms _____
Right Side
Zinc atoms _____
Chlorine atoms _____

Hydrogen atoms _____
Chlorine atoms _____
Notice that the 2 in front of the HCl molecule multiplies both the number of hydrogen
atoms and the number of chlorine atoms by 2.
In order to balance a chemical equation, we will sometimes need to manipulate the number of
reactant molecules, product molecules, or both. Work through the next couple of examples:
2. For the reaction represented by the skeleton equation below, build the reactants out of
Smarties using the following colors:
Mr. Chapman
Science 10
Norquay School
Potassium – Brown
K
+
Cl2

KCl
Chlorine – Yellow

Now, rearrange your reactants to form the products shown in the reaction...
Once again, it doesn’t work out! Remember that atoms are never created OR destroyed, and in
this case you have one extra chlorine atom that is not part of the products. Try putting a
numerical coefficient in front of the KCl molecule to make up for that extra chlorine. What
number will you put in front of it? Re-write your new chemical equation below:
K
+
Cl2
 ___ KCl
Once again we find one more problem. Count the number of chlorine atoms on the left
side ( ___ ). Count the number of chlorine atoms on the right side ( ___ ). But now look at the
potassium (K) atoms. There are ___ potassium atoms on the right side and ___ potassium atom
on the left side. You can fix this with a numerical coefficient in front of the potassium atom on
the left side. What number will you put there? Write your new chemical equation below:
___ K
+
Cl2
 ___ KCl
Now build the reactants out of Smarties and rearrange them to form products. Does it work? You have
balanced the chemical equation. Sometimes when we mess with one side, we must fix the other.
Mr. Chapman
Science 10
Norquay School
Is balancing chemical equations starting to make sense? You need to put numerical coefficients
on different molecules so that there are EQUAL numbers of each type of atom on the reactant
and product sides of the equations. Here are some basic rules for balancing chemical equations:
Balancing a Chemical Equation:
1. Look at both sides of the reaction and determine which atoms are unbalanced.
2. Apply numerical coefficients to whichever molecules contain unbalanced atoms in order
to balance them. BALANCE HYDROGEN AND OXYGEN ATOMS LAST (this just makes life a
lot easier usually).
Hints:
-
Treat polyatomic ions (examples: NO3 and OH) as if they are one type of atom... usually this
will help to balance the equation
-
When you change one side of the equation with a numerical coefficient, always recheck the
numbers of atoms on each side... recounting is something you will have to do all the time.
Try one more chemical equation with Smarties, and then eat them when you are finished............
__ FeCl3 + __ NaOH  __ Fe(OH)3 + __NaCl
** Hint: If you are doing this with Smarties, treat the OH as one type of atom. For example,
make every OH a blue Smartie. That should make this equation easier to model.