2003 Virginia SOLs
Motivation for Learning
The following demo will get your students thinking about the Law of Conservation of Matter. You will probably need to do this in a prior class period.
Note: In this demo, the iron in the steel wool rapidly oxidizes (rusts) to form iron (III) oxide. The result is a rusty piece of steel wool and a balloon that has been completely sucked into the flask. However, the mass both before and after the reaction is the same. This should provide you with a nice hook to begin a discussion on the Law of Conservation of Matter. You will notice that this exercise also stresses balancing chemical equations. You may want to provide students with the unbalanced version of the chemical equation below and have them practice this skill prior to proceeding to the main portion of the activity.
The Law of Conservation of Matter says that matter can neither be created or destroyed, but can be changed in form. In other words, the total mass of the material(s) before the reaction is the same as the total mass of material(s) after the reaction. Generally, this fact has been confirmed by countless experiments. However, there is another way to understand this concept. You can think in terms of the atoms themselves. Take the synthesis reaction between hydrogen gas and oxygen gas. The product of this reaction is water. At this point it is helpful if your students understand the concept of diatomic molecules i.e. H2, N2, O2, F2, Cl2, Br2, I2. The unbalanced chemical equation for this reaction is
If you count up the number of hydrogen atoms on each side, then you find that each side has 2 hydrogen atoms each. Now count up the oxygen atoms on each side of the chemical equation. The left side of the equation has two oxygen atoms and the right side has only one.
This is obviously not equal. Now let's look at the balanced equation.
If you recount the number of atoms on each side for each substance, then you have
To graphically illustrate:
+ O=O ----->
Therefore, it is the Law of Conservation of Matter that sets the ground rules that must be followed to correctly balance chemical equations.
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Complete the following equations and balance:
1. ___ NaOH + ___ CuSO4 -----> _____________________________________________
2. ___ NH4OH + ___ CuSO4 -----> ____________________________________________
3. ___ NH4OH + ___ Zn(NO3)2 -----> __________________________________________
Click here for a solubility chart
Students with special needs
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(Please answer in complete sentences):
1. What is the insoluble solid that is produced generally called?
2. Use the provided solubility chart to predict the identity of the insoluble products.
3. Why is it important to balance a chemical reaction?
4. Balance the following equations:
___ MnO2 + ___ HCl -----> ___ MnCl2 + ___ H2O + ___Cl2
___ Pb(NO3)2 + ___ K2CrO4 -----> ___PbCrO4 + ___KNO3
___ CO + ___ Fe2O3 -----> ___ Fe + ___ CO2
___ Zn(OH)2 + ___ H3PO4 -----> ___ Zn3(PO4)2 + ___ H2O
Answers to Assessment
1. 2NaOH + CuSO4 -----> Na2SO4 + Cu(OH)2¯
2. 2NH4OH + CuSO4 -----> (NH4)2SO4 + Cu(OH)2¯
3. NH4OH + Zn(NO3)2 -----> NH4NO3 + Zn(OH)2¯
1. The insoluble substance produced is generally called a precipitate.
2. The precipitates were
1. Copper (II) hydroxide
2. Copper (II) hydroxide
3. Zinc hydroxide
3. Balanced chemical equations obey the Law of Conservation of Matter and thus are true representations of what actually occurs in nature.
4. The balanced chemical equations are: