Materials & Prep
Prepare a balance, two sealable containers, a small metal object or set of coins for the opening, a piece of steel wool, a rusted piece of steel wool prepared beforehand, and teacher-recorded data on burning wood and rusting steel wool. Write the data table in the Direct instruction section on the board. If demonstrating burning, confirm the demonstration and ventilation procedures required by your school; this lesson can be taught without a live flame.
Opening
Display a sealed container holding a small metal object. Weigh it, have students predict what will happen to the mass if the object is shaken from one side to the other inside the sealed container, then shake and reweigh it. Use class observations to establish that the balance reading stays the same because the same matter remains inside the container.
Ask“What did the balance include?” Accept answers such as “the container and everything inside it.” Draw a boundary around the container. Explain that today the important question is whether the balance includes all the matter involved in a change. Tell students that the same reasoning will explain two observations that seem contradictory: burned wood appears to lose mass, while steel wool appears to gain mass.
Direct instruction
Display
Open burning of wood: wood before, 25.0 g; ash collected after, 7.0 g. Open rusting of steel wool: dry steel wool before, 3.0 g; rusted steel wool after, 3.8 g. Sealed rusting system: sealed container, air, and steel wool before, 103.0 g; same sealed system after rusting, 103.0 g.
Ask students to silently predict which reading is trustworthy and commit to an explanation before discussion. Surface the misconception directly: the balance is not broken when steel wool gains mass. The open steel wool has combined with oxygen from the air, so the measured steel wool now includes matter that was previously outside the system. Likewise, the 18.0 g missing from the collected wood and ash did not vanish. It left the open system mainly as carbon dioxide and water vapor, along with gases and particles not collected.
ModelDefine the system as the matter included in the measurement. For the open wood measurement, the system is only the collected solid, so 25.0 g of wood cannot be compared fairly with 7.0 g of ash alone. For the sealed rusting measurement, the system is the container, air, and steel wool. Before: 103.0 g. After: 103.0 g. Matter changed form and location, but the total mass stayed constant.
Emphasize the rule: In a chemical reaction, atoms are rearranged into new substances. If every reactant and product, including gases, remains inside the system, total mass before equals total mass after.
Guided practice
Have students work in pairs with the data and complete a three-column organizer: “What was measured,” “Matter that may have entered or left,” and “Claim supported by the data.”
Worked exampleFor open rusting, students identify 3.0 g of steel wool before and 3.8 g of rusted steel wool after. The difference is 0.8 g. Model the reasoning aloud: “I do not conclude that the balance failed. I first ask what could cross the system boundary. Oxygen from the air entered the steel wool. Therefore the larger reading is evidence that the product includes oxygen.”
Turn and talkStudents explain why the open burning measurement and the sealed rusting measurement seem different even though both support conservation of mass. Require the sentence frame: “The mass appears to change because the system includes/excludes . The evidence is .”
Check for understandingPresent this quick case: A closed container and its contents measure 146.2 g before a reaction and 146.2 g afterward. A student says, “No reaction happened because the mass did not change.” Students show agree or disagree and justify with one observation. Expected response: disagree. A reaction can produce new substances while total mass stays the same.
ScaffoldProvide a diagram with a bold system boundary and arrows labeled “matter enters” and “matter leaves.” Students may point to or draw the oxygen arrow for rusting and arrows for gases leaving during open burning before writing their explanation.
Independent explanation
Students write a claim-evidence-reasoning response to the prompt: “Explain why mass is conserved when wood burns or steel wool rusts, even though an open balance may show a change.” Require one example of burning and one example of rusting. Students must include the system boundary, identify matter moving across it, and use at least two values from the data.
SupportGive students the frame: “Total mass is conserved because . In the burning example, leaves or is not measured, so . In the rusting example, enters from the air, causing . The sealed-system data show .”
Circulate and ask, “What matter is included in your before measurement and after measurement?” Redirect “the balance is broken” by asking students to compare the open steel wool with the sealed-system readings.
ExtensionStudents calculate the missing mass in the wood example, 25.0 g minus 7.0 g equals 18.0 g, then explain why 18.0 g is not necessarily all carbon dioxide. A strong response identifies that water vapor and other gaseous products may also be present, so the data show uncollected matter rather than the identity of every product.
Closing
Exit ticket“A student weighs 4.0 g of steel wool, leaves it exposed to air until it rusts, and measures 5.1 g. Explain why the reading increased. Then state what would happen to the mass reading for a sealed container holding the steel wool, air, and container before and after rusting. Use the words system boundary and oxygen.”
Collect responses and look for three elements: oxygen entered the open steel wool system, the open measurements did not include the same matter before and after, and the sealed system would have the same total mass before and after. Ask students to revise one sentence if any of these elements is missing.