Kitchen Chemistry Experiment 2
Cumbustion

How does the weight of a metal change when it is burned?

Materials	Substitutions
aluminum pie pan
balance
uncoated extra-course steel wool	steel wool
bunsen burner	LPG burner
crucible tongs	corn-on-the-cob tongs

Procedure

1. Weigh your empty pan. Record the mass.
2. Weigh the pan with one half of a pad of medium-course steel wool in it. Record its weight.
3. Light the burner and adjust for a hot flame.
4. Pick up the steel wool with the tongs. Hold the steel wool in the flame for several minutes. Rotate the pad with the tongs so all parts are exposed. After all of the pad has a dull gray appearance, turn off the burner. Place the burned steel wool in the the pie pan. Sweep any "popped" mass into the pie pan as well.
5. Obtain the mass of the pan and burned steel wool. Record all of your results in the data table.

Data and Observations

1. Weight of the empty pan __________g
2. Total weight of the pan and unburned steel wool __________g
3. Weight of the unburned steel wool (#2-#1) __________g
4. Weight of the pan and burned steel wool __________g
5. Weight of the burned steel wool (#4-#1) __________g
6. Difference between the weight of the burned and unburned steel wool (#5-#3) __________g

Extensions

What kind of change took place? Why did the mass of the steel wool change as a result of burning? Can you explain the differences in the masses? "Steel" wool is composed of elemental iron (Fe). Write a balanced chemical equation for the burning of steel.

Try burning another metal, like magnesium. You may need to include the weight of the tongs (initial and final) in this experiment as some of the oxide will be left there. When magnesium burns, it gives of a very bright light. Don't look directly at the light! It will hurt your eyes. Write a balanced chemical equation for the burning of magnesium. How are these two equations similar? How can the oxidation of a metal (corrosion) be prevented?

Teacher's Notes

There is a gain in weight or mass when steel wool is burned. The increase in the weight is due to the oxygen that combined with the iron.

The balanced chemical equation for the combustion, or oxidation, of iron is 4 Fe (s) + 3 O2 (g) ---> 2 Fe2O3 (s)

The corrosion of iron is prevented by not allowing the metallic object to be in contact with oxygen. This can be accomplished by painting, coating with oil, galvanizing (painting with zinc) the steel objects. Corrosion weakens the iron because the iron oxide (rust) flakes off and therefore reducing the amount of the steel.

It is best to use coarse to medium coarse steel wool . Fine steel wool will give you the effect of a "sparkler"--popping all over the lab bench and possibly onto paper!!! Move papers and towels away from the burner while heating .

Students must use a sufficient amount of steel wool to notice a change after heating. The difference in mass will be very small . A aluminum pie pan under the burner can be used to collect the mass that has "popped". Since every 0.1 g will count, you will want to eliminate the "popped" mass as a source of error.

An old fashioned flash bar for a camera will demonstrate this concept well. Weigh it before exposure then again afterwards. Why is there a change in mass?

Safety Precautions

Precautions shoud be taken when lighting the burner, of course. Turn the gas on only after the match has been struck. This will prevent an excess amount of gas from building up around the burner. The heated steel wool will be very hot and must be picked up with the tongs only. If the pan becomes hot, it should not be put on the balance until it cools.

Caution students to prepare for some popping of the fine steel filaments. Move paper and flammables away from the burner.