Angela's Wand Chapter 13 · Unit 5: Chemical Reactions · Segment 2
What happens to atoms during a chemical reaction, and why does the mass of a sealed system never change?
Students figure out what happens to atoms when a chemical reaction makes new substances: bonds break and new bonds form, so the same atoms regroup into different molecules and extended structures, but no atom is created, destroyed or changed into another element. Because every atom keeps its mass, the products have the same total mass as the reactants, which a sealed container shows directly and an open one only seems to break when a gas leaves or joins. Students learn to read a chemical equation as an atom inventory, build physical models of three regrouping patterns, and use atom accounting the way chemical engineers do when they redesign a process so more reactant atoms end up in products and fewer in waste.
Estimated class time: presentation about 125 minutes · worksheets about 375 minutes in total · labs 5 sessions. Seven 50-minute periods: one for the phenomenon and the atom-regrouping idea, one for reading equations and the bead lab, one for the three regrouping patterns and the mystery-balance lab setup, one for conservation of matter and the lab readings, two for the engineering strand and the gas-trap design challenge, one for synthesis and mastery. Lab 1 needs a three- to five-day wait between its two sessions, so set it up early in the week.
On the first balance sits a cup of vinegar with a small paper scoop of baking soda beside it: 142.6 g in all. The teacher tips the baking soda in. The mixture fizzes for two minutes, then the reading settles at 141.5 g. On the second balance, a tuft of fine steel wool reads 2.00 g. The teacher carries it to a heat-proof tray, lights it with a lighter (teacher only, behind a shield), lets it sparkle and go dark, cools it and puts every crumb back on the balance: 2.14 g. (The readings are illustrative values, realistic for these amounts.) One reaction seems to lose matter and the other seems to gain it. Where did 1.1 g go, and where did 0.14 g come from?
Notice and wonder prompts
California NGSS, Grade 7 Preferred Integrated Course Model. Each entry says whether this chapter's work assesses the performance expectation, gives practice toward it, or supports it as background; the resource lists below repeat this for every worksheet and lab.
Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
Lab 2's bead models, W4's drawn models and W10's tally and drawing items all require students to show that each element has the same number of atoms before and after a reaction and to use that count to explain the mass readings in Lab 1 and Lab 3. Tasks stay inside the assessment boundary: students count atoms and compare masses from a balance; they are never asked to use atomic masses or to balance symbolic equations as the assessed skill.
Assessment boundary: Assessment does not include the use of atomic masses, balancing symbolic equations, or intermolecular forces.
Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
The lesson's engineering spread shows how two improvements to an ibuprofen process were combined into a better one. In Lab 3 the class tests three gas-trap designs, pools the mass-change, sealing-time and cost data, identifies the strongest feature of each design and builds a combined version; W9 and the slide task repeat the same analysis with simulated process data. This PE is listed for grades 6-8 in the California sequence.
Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
Lab 3 opens with each group writing measurable criteria (mass change, sealing time, cost) and the constraints (materials, amounts, no rigid sealed containers) before any trap is built; the precision of those criteria is what makes the later data comparison possible. Listed for grades 6-8 in the California sequence; it is practiced here, not assessed.
Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
Students must already accept that fizzing, color change and the dark crumbly product of burned steel wool signal new substances before they can ask where the atoms went. The chapter uses those property changes as given evidence and does not re-assess them.
Assessment boundary: Assessment is limited to analysis of the following properties: density, melting point, boiling point, solubility, flammability, and odor.
Atoms in Chemical Reactions: same atoms, new partners, same mass. Original, editable slides with speaker notes: a phenomenon opener, learning targets, the chapter's concepts with diagrams, vocabulary in context, 3 checks for understanding, 2 discussion prompts, an application task, a misconception check, a synthesis and an exit ticket.
Ten different tasks, one per instructional blueprint. Each has a student PDF (low ink, US Letter), an editable DOCX, an HTML preview, and a separate teacher answer key with reasoning.
I can observe two reactions whose balance readings change in opposite directions, record what I notice and wonder, and write a first explanation that says where mass could have come from or gone.
I can sort, connect and apply the words for chemical reactions, including telling a coefficient from a subscript and a scientific law from the model that explains it.
I can use evidence from a passage to explain how sealed-vessel experiments showed that matter is conserved, and to tell the difference between the law and the atomic explanation of it.
I can read a particle diagram and an equation for a reaction, tally the atoms of each element before and after, and build my own before-and-after model that shows why mass is conserved.
I can graph and calculate from balance readings taken during a reaction to show that mass only leaves or joins a system when matter crosses its boundary, and that a closed system keeps its mass.
I can use mass data, product tests and the atom model to argue, in a Claim-Evidence-Reasoning paragraph, that a burning candle loses mass without any matter being destroyed.
I can find the error in a claim about atoms, equations or mass in a chemical reaction, explain what went wrong, and rewrite the claim so it follows the rule that atoms are only regrouped.
I can work with a group to sort balance-reading evidence from twelve changes into cases where mass left, joined or stayed, explain each case with atoms crossing or not crossing a boundary, and write one rule that fits every card.
I can analyze mass data from three ways of making the same product, check each against criteria and constraints, identify the strongest feature of each design, and combine those features into a new route that sends more reactant atoms into useful products.
I can count atoms in a given equation, sort a reaction by how its atoms regroup, predict and explain balance readings in open and closed systems, and compute the efficiency of a chemical process from mass data.
Three different investigation approaches. Each has a teacher guide (purpose, materials with quantities, estimated cost, prep, safety, procedure, expected observations, debrief, answer key, assessment criteria, accessibility adaptations, low-cost alternative) and a student recording sheet.
When the same reaction runs in an open cup and in a sealed bag, and a damp steel pad sits in air for several days, why does the cup lose mass, the sealed bag keep its mass and the pad gain mass, if matter can never be created or destroyed?
If every atom in the products has to come from the reactants, can a bead model tell a real chemical equation from a fake one, and why does the pile of beads always have the same mass?
Which features of three cheap gas-trap designs should be combined so that a fizzing reaction keeps every gram on the balance, seals fast, and costs the least?
Materials overview: Household chemicals only: white vinegar, baking soda, fine steel wool pads, water. Students never use open flame; burning steel wool is a teacher demonstration behind a shield or a described phenomenon. Balances reading to 0.1 g are needed for Labs 1 and 3 (0.01 g helps with the steel-wool pads). Lab 2 uses pony beads in eight colors (one per element on the cards) and pipe cleaners cut into short bond pieces. Lab 3 adds balloons, empty plastic bottles, zip bags, cups, plastic wrap, rubber bands, tissue paper and a stopwatch. Goggles for every wet activity.
Review status: reviewed · 2026-10-19 · Atom tallies and mass sums re-solved in every file: the vinegar reaction (14 atoms a side), 4Fe + 3O2 -> 2Fe2O3, the four Lab 2 cards and three fake cards (bead kit of 48 is sufficient), methane, peroxide, water formation, Mg/MgO (0.24 g -> 0.40 g), antacid spread (9 a side), NH3, W5, W9 and slide-task mass balances. All keys correct except the items fixed below. Buoyancy, corrected in size everywhere it appears (chapter accuracy note, slides 15-17 and 25, Lab 1, Lab 3, W5, W6, W8 notes): half a teaspoon of baking soda with 30 mL of vinegar makes about 1.1 g of CO2 filling about 0.6 L, so a freely puffing bag or balloon reads about 0.6-0.7 g low, not 'a few tenths'. Lab 1's leak threshold (0.5 g) would have flagged every good seal; Lab 1 q2, q3, q7, q8, debrief, expected observations and teacher notes now use the real size and the vent-and-compare test, and Lab 1 is retitled 'Lost, Kept, Gained'. Lab 3's expected design results, q5 and the teacher notes were corrected, and q6 (fraction of gas escaped) now measures between the perfect-trap mark (-0.7 g) and the open-cup mark (-1.1 g); the old subtraction counted the lift as escaped gas. Closed-system wording made precise: 'reads the same' is kept only for rigid containers (slide 15 compare diagram, slide 17 vocab example, exit ticket item 4, W3 paragraph 7, W10 scaffold, W5 q10); bag examples now speak of mass. Added a ChemEd X reference for the balloon buoyancy effect. W6 is the candle CER as written; chapter.json sequence (Period 4), evidenceOfLearning and the CER practice in threeDimensions now describe it that way, and slide 16 links say W6 applies the open-versus-sealed reasoning to a candle. W6 itself: a 1 L jar holds only about 0.28 g of oxygen, so the flame goes out in roughly 15 s after burning a few hundredths of a gram of wax; the 40 s flame-out and 'visibly shorter candle' were changed to match. Lavoisier/Boyle history in W3 checked: Boyle's 1673 sealed-retort tin experiments and 'fire particles'; Lavoisier's 1774 sealed-vessel calcination of tin (no change while sealed, gain on opening matching the tin's gain); the 1789 Traite; Marie-Anne Paulze Lavoisier's translating, records and drawings. Accurate. Paragraph numbers cited in questions and keys match. Other science fixes: W1 chalk demo made consistent (the 0.88 g loss needs all 2.00 g of CaCO3 to react, so the chalk now dissolves completely; note that many classroom chalks are gypsum); W5 capped bottle is now a teacher demo and is squeezed to flush the dense CO2 before the 300 s reading, and the sealed-flask refill is 0.14 g (air refills the oxygen's volume and is lighter per liter than O2); W7 q7 key said the bottle warmed, but vinegar and baking soda absorb energy; W8 Card 1 now uses two tablets (one tablet gives at most about 0.8 g of CO2, not 1.2 g) and the dough card notes its own lift; W9 keys referred to a '2 percent catalyst loss' that is not on the sheet; slide 13 bicycle example used 2 x 2 = 2 + 2; slide 20 notes now say 77 percent is the textbook figure, not EPA's. Ambiguity fixed: Lab 1 q9 offered overlapping choices ('at least' and 'possibly more'); now less than, exactly, or 1.1 g or more. Consistency: chapter sequence slide ranges were off by one from Period 2 on and Period 7 cited a slide 26 that does not exist; slide 2 parking-lot slide numbers likewise; standards note for MS-PS1-5 named a 'presentation task' that is the ETS1-3 task. presentation estimatedMinutes set to the pacing sum (125). caGrade added to every lab standards entry. Originality: 6-word runs shared with the lesson text rewritten in Lab 2 procedure, slide 6 notes, W2 law definition, W4 coefficient label, the chapter DCI lines and the Lab 3 q7 prompt; quoted textbook errors in the accuracy notes shortened to 'break apart'. Remaining matches are section titles and the term 'law of conservation of matter'. Renderability: W5 and W6 dataset values are now strings so trailing zeros on 0.01 g readings print (162.00, not 162.0). Slide text limits, diagram fields, W3 passage length (700 words, 7 paragraphs) and answer-key coverage all checked. Labs remain untested physically, as their modelLimitations say.
Verified against the student edition of Bring Science Alive! 7th Grade Integrated (Student Edition). Printed page numbers appear in the book; PDF pages are the physical pages of the 524-page scan (printed + 36). The textbook is not reproduced on this site.
| Section | Printed pages | PDF pages | Used for |
|---|---|---|---|
| Introduction: the rusting car and the lesson question | 162, 163 | 198, 199 | Mass-gain hook, lesson preview, the publisher's standards listing (MS-PS1-5, MS-ETS1-3) and vocabulary box. |
| 1. Atoms Regroup in Chemical Reactions: electrolysis, writing equations, hydrogen peroxide | 164, 165 | 200, 201 | Bonds break and form, reactant/product layout of an equation, coefficient meaning, the two break-apart examples used in Lab 2 and the slides. |
| 2. Ways Atoms Regroup in Chemical Reactions: break apart, join, swap | 166, 167 | 202, 203 | The three regrouping patterns, the steel-wool and lead-iodide examples, the regrouping-pattern diagram and Lab 2 sort. |
| 3. Conservation of Matter: vinegar and baking soda, the law and the open-air caveat | 168 | 204 | Law of conservation of matter, scientific law definition, the 14-atom count, the escaping-gas idea behind Lab 1 and Lab 3. |
| Key Science Concept: Equations Model Conservation of Matter | 169 | 205 | Atom tally layout for the counting tables; the 'atoms break apart' wording flagged in accuracyNotes. |
| 4. Engineering Green Medicines: efficiency, catalysts, reusing a by-product, combining solutions | 169, 170, 171 | 205, 206, 207 | Atom accounting in industry, the ibuprofen efficiency comparison, the combine-the-best-features strategy behind Lab 3 and the slide task. |
| Lesson Summary | 171 | 207 | Check that the four strands (regrouping, patterns, conservation, engineering) are all covered by the targets. |
