Science Leadership Scenario - The Story

You are heading into Mr. Criswell's eleventh grade classroom to observe a Chemistry lesson he is teaching. The lesson you are observing is part of a series of lessons to help students develop a model of combustion as a rapid version of the process of oxidation and recognize its relationship to slower versions of that process - e.g. rusting.

Eventually, students will use their model of combustion to explain how various common fuels burn including being able to represent combustion processes in the form of simple word equations.

Students have been considering the difference between physical and chemical changes, beginning with the idea that chemical changes produce a new substance while physical changes do not.

After the discussion of physical and chemical change, students were asked to set up the following investigation:

weigh a piece of steel wool
rinse it with water
push the steel wool into the bottom of a test tube
invert the test tube in a beaker containing water
adjust the water levels inside and outside the test tube until they are equal.

At the beginning of today's class students spend a few minutes of looking for and noting changes in this system. The most obvious changes are that the steel wool undergoes a color change (as a result of rusting) and that the water level inside the test tube goes up (as a result of removal of oxygen from the air).

The students are encouraged to propose hypotheses concerning the cause of these changes and then to design simple variations to test these hypotheses. Many students speculate that rusting is somehow involved in the process; however, Mr. Criswell never confirms this.

After the students have had some time to work in their lab groups around the investigation of rusting, Mr. Criswell introduces one of the fundamental questions facing chemistry in the 18th century: What happens to mass during a chemical change?

In pairs, students are given three investigations in which they produce a chemical change and are asked to compare the masses before and after this change has occurred. At the beginning of today's class students spend a few minutes of looking for and noting changes in this system. The most obvious changes are that the steel wool undergoes a color change (as a result of rusting) and that the water level inside the test tube goes up (as a result of removal of oxygen from the air).

In pairs, students are given three investigations in which they produce a chemical change and are asked to compare the masses before and after this change has occurred.

The first reaction involves the burning of steel wool; the second results in the formation of a precipitate, as well as the subsequent production of a gas; and the third leads to the release of gas. All three of the reactions are designed to produce data that initially seems to contradict Lavoisier's Law of Conservation of Mass because they do not take into account a gas as either one of the reactants (in the 1st case) or a product (in the 2nd and 3rd cases).

After the students share their results with each other, Lavoisier's Law is presented. Mr. Criswell notes that this law has been generally accepted (with some caveats) since Lavoisier introduced it in the late 1700's.

Mr. Criswell: "So can we try and summarize what happened to the masses in all three of the investigations?"

Kevin: "Well, sometimes mass went up, sometimes it went down and sometimes it stayed the same, so it's not the same every time for every reaction."

Beakers Mr. Criswell: "But, we have the Law of Conservation of Mass from Lavoisier that says that should not happen, so what is going on with our data?"

Emily: "Well, in ours we could see a gas bubbling off, so maybe the mass went down for us because we lost some with the gas."

Mr. Criswell: "OK, so that is a possible explanation for why the mass went down, but what would make it go up? Group that did the burning steel wool, yours went up, right?

Rosh: "Yeah, but we are not sure why exactly."

The class data does not support the conclusion that mass is conserved; what can account for this discrepancy?

Students are then presented the challenge of re-designing their experiments to take these gases into account. While this is a simple proposition in the case of the reaction involving a precipitate and the one involving the production of a gas, the burning of the steel wool is problematic, particularly because the result obtained - that the mass actually shows an increase - is counterintuitive.

The lesson ends for the day at this point with the understanding that tomorrow students will be asked to redesign the conservation of mass experiments.

Excellence in Science Institute

Science Leadership Scenario - The Story