Physical Science · California Grade 7

MS-PS1-5

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.

Clarification statement: Emphasis is on law of conservation of matter and on physical models or drawings, including digital forms, that represent atoms.

Assessment boundary: Assessment does not include the use of atomic masses, balancing symbolic equations, or intermolecular forces.

Source: California Department of Education, NGSS Preferred Integrated Course Model, Grade 7 (accessed 2026-10-18)

Chapters

Chapter 5: Using Stored Energypractice

Lab 2 and the atom-count slides use beads and tokens to show that the 6 carbon, 12 hydrogen and 18 oxygen atoms entering respiration all come out in the products, with mass unchanged. This builds the practice toward the PE, which is taught fully in Lessons 12-13; students are never asked to balance a symbolic equation.

Chapter 13: Atoms in Chemical Reactionsassesses

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.

Worksheets (12)

Chapter	Worksheet	Role	How it connects
Ch. 5	W4 · From Acorn to Muscle Cell: Reading and Building a Model of Food's Path Diagrams and models	practice	Counting the carbon, hydrogen and oxygen atoms on both sides of the overall cellular respiration summary practices the idea that atoms are conserved in a chemical reaction, which students will study in depth in the chemistry chapters.
Ch. 12	W4 · Reading the Reaction Map: From Clue to Confirmation Diagrams and models	supports	The comparison figure's shared box notes that the same atoms are present before and after and that mass is conserved when the gas is trapped. This previews the conservation idea without asking students to count atoms.
Ch. 13	W1 · Lighter Cup, Heavier Pad: Where Did the Mass Go? Phenomenon exploration	practice	Students meet the puzzle the PE resolves: a balance reading that falls and one that rises during a reaction. Their first explanation starts the model of atoms moving in or out of an open container that they will build and use across the chapter.
Ch. 13	W2 · Same Atoms, New Partners: Putting the Words to Work Vocabulary in action	practice	Reading an equation correctly (coefficient versus subscript, reactants versus products) is the skill students need to count atoms before and after a reaction. The sheet builds that reading skill and the language of conservation; it does not yet ask for a full model.
Ch. 13	W3 · The Sealed Vessel: How Careful Weighing Found a Law Reading with evidence	practice	The passage gives the historical evidence for conservation of mass and the atomic reasoning behind it. Students practice the logic of the PE (same atoms, therefore same mass) with text evidence rather than with their own model.
Ch. 13	W4 · Count Before, Count After: Modeling a Reaction with Atoms Diagrams and models	assesses	Students tally atoms in a given particle diagram and then draw their own before-and-after model for a second reaction, explaining that equal atom counts mean equal mass. Equations are given already balanced, so the task stays inside the assessment boundary: counting, not balancing, and no atomic masses.
Ch. 13	W5 · Open Cup, Capped Bottle: Tracking Mass Through a Reaction Data and graphing	practice	Students analyze mass-versus-time data for the same reaction run open and capped, and mass-gain data for burning steel wool, then explain every reading with atoms crossing or not crossing a boundary. This builds the evidence base for the model the PE asks for; the sheet never uses atomic masses or asks students to balance an equation.
Ch. 13	W6 · The Shrinking Candle: Was Any Matter Destroyed? Claim, evidence, reasoning	assesses	Students must use the idea that atoms regroup but are never created or destroyed to explain why an open candle loses mass while a sealed one does not. Their written argument is evidence toward the PE. The sheet gives no equation to balance and uses no atomic masses; the only numbers are balance readings.
Ch. 13	W7 · Fixing Priya's Notebook: Seven Reaction Claims That Break the Atom Rule Error analysis	practice	Every entry in the notebook misuses the chapter's model in a common way: heat counted as matter, a subscript read as a total, a subscript changed to fix a count, a gas treated as massless, atoms described as breaking, molecules confused with atoms. Diagnosing and repairing these claims strengthens the model the PE asks for. Equations are given already correct; students count, they do not balance.
Ch. 13	W8 · Where Did the Mass Go? A Twelve-Card Evidence Sort Collaborative investigation	practice	Each card is a balance reading from a physical or chemical change in an open or closed system. Groups must use the atom model to explain every reading and defend one rule, which is the reasoning the PE asks students to show with a model. No atomic masses and no balancing are involved.
Ch. 13	W9 · Choose the Greener Route: Combining the Best of Three Process Designs Transfer and design	supports	The mass accounting only works because atoms are conserved: reactant mass in must equal product plus by-product plus waste out. Students verify that sum for each route before judging efficiency, but the atom model itself is background here.
Ch. 13	W10 · Atoms in Chemical Reactions: Mastery Check Retrieval and mastery check	assesses	Items ask students to tally atoms in given equations, draw a before-and-after model for a reaction, predict sealed and open readings, and explain both a mass loss and a mass gain with the same rule. All equations are supplied already balanced and no atomic masses appear, so the quiz stays inside the assessment boundary.

Labs (5)

Chapter	Lab	Role	How it connects
Ch. 5	Lab 2 · Atom Tokens: Taking a Sugar Molecule Apart Without Losing a Piece Physical model	practice	Counting and weighing the beads before and after is the practice this PE calls for, done here with one reaction; the full treatment with several reactions comes in Lesson 13. Students count atoms in a physical model; they do not balance a symbolic equation.
Ch. 12	Lab 3 · When Does the Fizz Stop? A Reaction as a System Collaborative systems modeling	supports	Students see that the mass lost equals the mass of gas that escaped and that it stops growing when a reactant is used up. Conservation of mass and atom counting are Lesson 13's job; here the data simply prepare the ground, so the role is supports.
Ch. 13	Lab 1 · Three Balances: Lost, Kept, Gained. Where Did the Grams Go? Evidence-based mystery	assesses	Students draw before-and-after atom pictures of the sealed bag and use the same atom model to explain why an open cup reads lower and a rusting pad reads higher. The explanation that mass changes only when atoms cross the system boundary is the evidence this PE asks for, and no atomic masses or symbolic balancing are needed.
Ch. 13	Lab 2 · Beads Never Vanish: Build Four Reactions and Catch the Fakes Physical model	assesses	The atom inventory, the before-and-after bead masses and the drawing are a physical model of conserved atoms; the claimed-reaction verdicts show whether students can use the model to decide what a reaction cannot do. Students count atoms and compare bead masses only; atomic masses are never used, and fixing a fake equation with whole units is an optional extension, not the assessed skill.
Ch. 13	Lab 3 · Trap Every Gram: Design a Gas Catcher for a Fair Mass Test Engineering design challenge	assesses	Students explain why a trap that keeps every atom keeps every gram, and why a leaky trap reads lower, by tracking the carbon and oxygen atoms of the gas before and after. No atomic masses or symbolic balancing are involved.