University of Virginia
Physics Department

Conservation of Matter and Balancing Chemical Equations

A Physical Science Activity

Adapted from an activity called "Law of Conservation of Matter" from North Carolina State University's Science House learning outreach program - Countertop Chemistry. This lab activity contains some difficult concepts. You may find it necessary to tailor its content based on the level of your students.

2003 Virginia SOLs

 

Objectives

Students will

     

Motivation for Learning

Discrepant Event

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.

Materials

 

Procedure

  1. Tear off an egg sized piece of steel wool. Be careful not to ball it up too tightly.
  2. Place the steel wool into the 250 mL beaker and add white vinegar until the entire piece of steel wool is immersed. Soak for 4-7 minutes.
  3. Remove the steel wool from the vinegar and wring out any excess vinegar.
  4. Place the steel wool into the 250 mL flask and cover the opening of the flask with a balloon.
  5. Mass the entire steel wool-balloon-flask system and record.
  6. Allow this system to sit for 30-45 minutes.
  7. Later, observe the results and again take the mass of the steel wool-balloon-flask system and record.

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.

4 Fe (s) + 3 O2 (g) --------> 2 Fe2O3 (s)

 

 Background Information

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

H2 (g) + O2 (g) -----> H2O (l)

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.

2 atoms H + 2 atoms O -----> (equals) 2 atoms H + 1 atom O

This is obviously not equal. Now let's look at the balanced equation.

2H2 (g) + O2 (g) -----> 2H2O (l)

If you recount the number of atoms on each side for each substance, then you have

4 atoms H + 2 atoms O -----> (equals) 4 atoms of H and 2 atoms oxygen. Here both sides are equal.

To graphically illustrate:

                                 H-H                                           H-O-H
                                               +      O=O      ----->
                                 H-H                                           H-O-H

 

Therefore, it is the Law of Conservation of Matter that sets the ground rules that must be followed to correctly balance chemical equations.

 

Student Activity

To print out the Student Copy only, click here.

Solution Preparation

  1. If your school does not have sodium hydroxide, then it can be obtained from Drano or Red Devil Lye. If you use Drano, the solution does not need to be very concentrated but you would want to filter the aluminum filings that are mixed in with the pellets of NaOH. Lye is CAUSTIC so wear gloves and wash all surfaces anyone might touch. Also be careful with sodium hydroxide pellets. They will absorb water from the air to create a concentrated drop that will burn through most fabrics.
  2. If your school does not have copper (II) sulfate, then it can be purchased at a good hardware store as an algaecide (Bluestone) or root eater. Solutions can be prepared directly and since this is a precipitate reaction, the concentrations are qualitative and approximate.
  3. Ammonium hydroxide is nothing more than household ammonia. Use it straight out of the bottle from the grocery store.
  4. The only chemical that may be difficult for you to find is the zinc nitrate. Probably the easiest thing to do would be to borrow some from the high school that your school feeds into. You will need 20 to 30 g of it.
  5. Again the concentrations are not critical. This lab was tested with all solutions at 0.1 M concentration except the ammonia. Below are the recipes for making a liter of 0.1 M solution for each of the solutions in this lab except ammonium hydroxide.
22 g CuSO4 into 1 liter
4 g of NaOH into 1 liter
24 g of Zn(NO3)2 x 3H2O into 1 liter or 30 g of Zn(NO3)2 x 6H2O into 1 liter

 

Safety Precautions

  1. As mentioned in the solutions preparations section, sodium hydroxide is CAUSTIC and should be handled carefully. Students may need to wear gloves. The base will feel slippery on the skin (just like soap) and should be washed immediately.
  2. Copper solutions can cause eye infections, so students should wash their hands after handling these substances, too.

 

Materials

 

Procedure

  1. Measure 60 mL of NaOH solution in a graduated cylinder and then pour into a small (3 oz) clean plastic cup.
  2. Rinse the graduated cylinder completely before making the next measurement.
  3. Measure 60 mL of CuSO4 solution in the graduated cylinder and then pour it into a clean 5 oz cup.
  4. Carefully place the two solutions on the balance, making sure the solutions do not mix. Mass the solutions and the cups together and record the combined mass.
  5. Pour the NaOH into the 5 oz cup with the CuSO4 solution. Allow the solutions to mix. Record your observations.
  6. Mass both cups and the mixture again. Record the new mass. By how much did the mass change?
  7. Repeat the process in steps 1-4 above for the combinations listed in the data section below. Do not allow the solutions to mix before taking the initial mass.

     

Data Sheet

To print out the Data Sheet only, click here.

 

Reaction

Mass (g) Before

Mass (g) After

Observations

NaOH and CuSO4

 

 

 

 

 

 

NH4OH and CuSO4

 

 

 

 

 

 

NH4OH and Zn(NO3)2

 

 

 

 

 

 

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

Click here for information on laboratories with students with special needs.

 

Assessment

To print out the Assessment Sheet only, click here.

(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.

1. ______________________________

2. ______________________________

3. ______________________________

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

Equations:

1. 2NaOH + CuSO4 -----> Na2SO4 + Cu(OH)2¯

2. 2NH4OH + CuSO4 -----> (NH4)2SO4 + Cu(OH)2¯

3. NH4OH + Zn(NO3)2 -----> NH4NO3 + Zn(OH)2¯

 

Questions:

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:

MnO2 + 4HCl -----> MnCl2 + 2H2O + Cl2
Pb(NO3)2 + K2CrO4 -----> PbCrO4 + 2KNO3
3CO + Fe2O3 -----> 2Fe + 3CO2
3Zn(OH)2 + 2H3PO4 -----> Zn3(PO4)2 + 6H2O