MS-PS1-5: Modeling Why Atoms and Mass Are Conserved in Reactions
In plain English: MS-PS1-5 is the NGSS middle school performance expectation that asks students to develop and use a model showing that the total number of atoms does not change in a chemical reaction, so mass is conserved. Students count atoms in drawings and bead models and compare masses measured in open and closed containers. It is usually taught in grade 7 or 8 physical science.
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.
Next Generation Science Standards · Disciplinary Core Idea: Matter and Its Interactions · Official standard
Students build and use models of atoms to explain why mass is conserved in a chemical reaction. An atom is the smallest unit of an element, and a molecule is a group of atoms joined together. In a chemical reaction, the atoms of the starting substances (the reactants) come apart and join in new ways to make new substances (the products). No atoms are made and none are destroyed, so the total mass stays the same. This is the law of conservation of matter. Students count atoms in drawings, rebuild molecules from beads, and test the idea with mass data from open and closed containers.
The NGSS Clarification Statement says: "Emphasis is on law of conservation of matter, and on physical models or drawings, including digital forms, that represent atoms." The Assessment Boundary says: "Assessment does not include the use of atomic masses, balancing symbolic equations, or intermolecular forces." So on this page students never look up how heavy one kind of atom is and never write or balance chemical equations: every reaction is shown as a drawing or a bead model, with the atoms in each molecule given, and every mass comes from a balance. All data sets are invented for teaching and labeled as invented.
Learning Objectives
By the end of this lesson, students will be able to:
Count the atoms of each element before and after a reaction in a drawing or a physical model, and show that the totals match
Use an atom model to explain why the total mass of a closed system does not change during a chemical reaction
Explain apparent mass losses and gains in open containers by tracing atoms that leave as a gas or join from the air
Critique a model of a reaction that adds or loses atoms, and correct it
Prior Knowledge Required
Students should already be comfortable with:
Weight (mass) of matter is conserved when substances are heated, cooled or mixed 5-PS1-2
Matter is made of particles too small to be seen 5-PS1-1
Reading an electronic balance in grams, to two decimal places
Signs that a chemical reaction may have happened, such as a gas forming or a color change
Drop a bath bomb into a clear tub of water (or show a short video of one). It fizzes, shrinks and disappears. Ask students to answer in writing before any discussion:
Warm-Up Prompt
"Where did the bath bomb go? Was any of its matter destroyed? Write your idea and one way you could test it with a balance."
List the ideas on the board without judging them (it dissolved, it turned into bubbles, it was used up, it went into the air). Return to the list in Closure.
Direct Instruction15-20 minutes
The atom model of a reaction. Use Diagram 1. Each circle is an atom, and each group of touching circles is a molecule. To use the model, count the atoms of each element on the reactant side and on the product side. If the counts match for every element, the model conserves atoms. Since every atom keeps its own mass, the total mass of the products equals the total mass of the reactants.
Open and closed systems. A system is the set of things we are studying, and a closed system is one that matter cannot enter or leave, such as a bottle with its cap screwed on. In an open system, a gas can escape into the air or atoms from the air can join in, so the balance reading can change even though no atoms are made or destroyed.
Counting atoms in a drawing
Hydrogen peroxide (the bottle in a first-aid kit) slowly breaks down into water and oxygen gas. Diagram 1 shows 2 hydrogen peroxide molecules, each with 2 hydrogen atoms and 2 oxygen atoms, becoming 2 water molecules (2 hydrogen and 1 oxygen each) and 1 oxygen molecule (2 oxygen atoms).
Result: Before: 2 × 2 = 4 hydrogen atoms and 2 × 2 = 4 oxygen atoms. After: 2 × 2 = 4 hydrogen atoms and 2 × 1 + 2 = 4 oxygen atoms. The same 8 atoms are rearranged, so the mass does not change.
A closed system (invented data)
In a teacher demonstration, a fizzy-drink bottle holds 100 mL of water and a paper funnel of baking powder kept dry. With the cap screwed on, the bottle and its contents read 128.45 g. The teacher tips the bottle, the powder fizzes, and a gas fills the bottle.
Result: After the fizzing stops, the balance still reads 128.45 g. A new gas (carbon dioxide) formed, but every atom stayed inside the bottle, so the mass did not change.
An open system that seems to lose mass (invented data)
The same amounts of water and baking powder are mixed in an open plastic cup that starts at 152.30 g. Diagram 2 shows the readings for 3 minutes.
Result: The final reading is 151.71 g, so 152.30 - 151.71 = 0.59 g left the cup. Those atoms did not disappear: they escaped into the room as carbon dioxide gas.
An open system that seems to gain mass (invented data)
Iron nails with a total mass of 25.00 g sit on a damp paper towel in an open dish for three weeks and turn rusty. The dry nails now read 25.36 g. In the model, for every 4 iron atoms, 3 oxygen molecules from the air (2 oxygen atoms each) join to make 2 units of rust, each with 2 iron atoms and 3 oxygen atoms.
Result: Gain: 25.36 - 25.00 = 0.36 g, the mass of oxygen atoms that came from the air. Atom count: 4 iron and 3 × 2 = 6 oxygen before; 2 × 2 = 4 iron and 2 × 3 = 6 oxygen after.
After each example, ask: "Is this system open or closed? If the reading changed, which atoms crossed the boundary of the system, and in which direction?"
Guided Practice15 minutes
A car's catalytic converter changes harmful carbon monoxide gas into carbon dioxide. Pairs draw the model on mini whiteboards: 2 carbon monoxide molecules (1 carbon atom and 1 oxygen atom each) and 1 oxygen molecule (2 oxygen atoms) become 2 carbon dioxide molecules (1 carbon atom and 2 oxygen atoms each). Then they fill in the tally.
Guided practice tally: carbon monoxide and oxygen make carbon dioxide
Atom
Before (reactants)
After (products)
Carbon (C)
2 × 1 = 2
2 × 1 = 2
Oxygen (O)
2 × 1 + 2 = 4
2 × 2 = 4
All atoms
6
6
Draw the reactants on the left and the products on the right, using one color per element and the same size for every atom of one element.
Tally each element on both sides, as in the table.
In a lab test, 56 g of carbon monoxide reacts completely with 32 g of oxygen. Use the tally to predict the mass of carbon dioxide formed, and explain why you can predict it without weighing the product.
Swap boards with another pair and check their tally. If a count does not match, find the atom that was added or left out.
Independent Practice10-15 minutes
Yeast turns sugar into alcohol and carbon dioxide gas. One sugar molecule (glucose) has 6 carbon, 12 hydrogen and 6 oxygen atoms. An alcohol molecule (ethanol) has 2 carbon, 6 hydrogen and 1 oxygen atom, and a carbon dioxide molecule has 1 carbon and 2 oxygen atoms. Two students drew models of what happens to one sugar molecule:
Two student models of yeast breaking down one sugar molecule
Model
Reactants drawn
Products drawn
Model A
1 sugar molecule
2 alcohol molecules + 2 carbon dioxide molecules
Model B
1 sugar molecule
2 alcohol molecules + 1 carbon dioxide molecule
Each student tallies the atoms for both models, decides which model conserves atoms, and writes two or three sentences explaining the choice with the tally as evidence. Then: "Activity 1 uses this reaction in an open bottle. Use the correct model to predict what the balance will show, and why."
Closure10 minutes
Exit ticket: "A glow stick is a closed plastic tube. Before you bend it, it reads 9.84 g on a balance. You bend it, a reaction inside makes it glow, and you weigh it again. Predict the reading and explain it with atoms." (9.84 g: the atoms rearrange inside the tube, and none get in or out.) Return to the warm-up list about the bath bomb and ask students which ideas the atom model supports and which it rules out.
Differentiation Strategies
For Struggling Students
Give a tally table with the element names already filled in, so students only count and compare
Let students build each molecule from beads before drawing it, and count beads instead of circles
Use the sentence frame: "Before the reaction there are ___ atoms of ___. After, there are ___. So the mass ___ because ___."
For Advanced Students
Ask students to predict, before Activity 1, how the open bottle and the closed bottle will differ after the cap of the closed one is removed, and then test it
Have students design a closed system for a reaction that uses oxygen from the air, such as rusting, and explain why the jar must hold enough air
Ask students to explain why an inflated balloon on a bottle can make the balance read slightly less even when no gas escapes (the balloon pushes aside more air)
Assessment Guidance
What to Look For
Check that students count atoms element by element, not molecules: the number of molecules can change in a reaction while the number of atoms cannot. Listen for students who say mass "turned into energy" or "burned away", and ask them to name the gas that carried the atoms. Strong answers say whether the system is open or closed before they explain a balance reading.
02
Classroom Activities
3 Activities
1
Open Bottle, Closed Bottle: Yeast and Sugar
30 minGroups of 3-4
Groups start the same yeast reaction in two bottles. One bottle has its cap screwed on (closed), and the other has only a loose cotton ball in its neck so gas can escape (open). They weigh both every 5 minutes and use the atom model to explain the results.
Procedure
Safety: wear goggles. Use only plastic bottles made for fizzy drinks, which are built to hold gas pressure; never use glass. Use warm tap water no hotter than 40 °C. Never leave a bottle with its cap on overnight, and open it slowly with the cap pointed away from everyone.
In each bottle, put 150 mL of warm water, 1 tablespoon of sugar and 1 packet of dry yeast, then swirl.
Screw the cap onto bottle 1. Push a cotton ball loosely into the neck of bottle 2.
Weigh both bottles right away and every 5 minutes for 25 minutes. Record the masses to 0.01 g.
At the end, slowly open bottle 1, let the gas out, put the cap back on the balance with the bottle and weigh it once more.
Sample Data (invented)
Closed bottle at 0, 5, 10, 15, 20 and 25 minutes: 191.41, 191.41, 191.40, 191.41, 191.41, 191.41 g. Open bottle at the same times: 189.87, 189.83, 189.73, 189.60, 189.46, 189.32 g.
Discussion Questions
By how much did the open bottle's reading change in 25 minutes? Which atoms left, and in what substance?
The closed bottle never moved by more than 0.01 g from its starting reading. Is that difference evidence that atoms were lost? Think about how precise the balance is.
Predict the closed bottle's reading after you let the gas out. Then compare with your measurement.
Use the correct model from Independent Practice to explain why foam and bubbles formed in both bottles.
Modification for Distance Learning
Students graph the sample data, write a claim about each bottle, and explain it with a drawing of the yeast reaction. A family can repeat the open bottle at home with a kitchen scale that reads to 1 g, using a larger bottle and a longer wait.
2
Bead Models: Rearrange, Never Remove
20 minPairs
Pairs build the reactant molecules for a reaction card from pony beads (one color per element) joined with short pieces of pipe cleaner, weigh the whole model, take it apart, rebuild it as the products, and weigh it again.
Reaction Cards
Card A, silver tarnishing: 4 silver atoms, 2 hydrogen sulfide molecules (2 hydrogen atoms and 1 sulfur atom each) and 1 oxygen molecule (2 oxygen atoms) become 2 silver sulfide units (2 silver atoms and 1 sulfur atom each) and 2 water molecules.
Card B, a copper penny darkening: 2 copper atoms and 1 oxygen molecule become 2 copper oxide units (1 copper atom and 1 oxygen atom each).
Card C, photosynthesis in a leaf: 6 carbon dioxide molecules and 6 water molecules become 1 sugar molecule (6 carbon, 12 hydrogen and 6 oxygen atoms) and 6 oxygen molecules.
Procedure
Build every reactant molecule on the card and count the beads of each color.
Weigh all the beads and pipe cleaner pieces together on the balance (sample reading for Card C: 14.62 g).
Take the molecules apart and build every product molecule. Use every bead, and do not add any.
Weigh again (sample reading for Card C after rebuilding: 14.62 g) and record any beads left over or missing.
Discussion Questions
Card C uses more beads than Cards A and B together. Did you need any extra beads to build its products, or have any left over?
Taking the reactant molecules apart and building new ones stands for which part of a real reaction?
Suppose one bead rolled off the table before the second weighing. What would the balance show, and why would that make the model wrong?
Variation: Digital Model
Students drag colored circles on a shared slide to build the reactants, then move the same circles into the products, keeping a tally in a text box. The rule is the same: no new circles, and none deleted.
3
Where Did the Mass Go? Data Stations
25 minGroups of 3-4
Three stations each give invented mass data for a real reaction. At each station, groups decide whether the system was open or closed, say which atoms crossed its boundary and in which direction, and draw a before and after model of the system. Groups rotate every 8 minutes.
Station Data
Station 1, a candle (teacher data, not a student activity): a candle reads 24.6 g before it is lit and 21.8 g after burning for one hour. Wax reacts with oxygen from the air to make carbon dioxide and water vapor.
Station 2, a copper test strip on a roof: a strip of copper reads 50.00 g when it is put outside and 50.12 g a year later, covered in a green coating. Copper reacts with oxygen, water and carbon dioxide from the air.
Station 3, an old penny in salty vinegar: a dull copper penny sits in a cup of vinegar with a pinch of salt for 10 minutes and comes out shiny. On a 0.001 g balance, the cup with everything in it reads 152.314 g before and 152.314 g after. The penny alone reads 3.112 g before and 3.108 g after.
Discussion Questions
At which station did the measured system lose mass, at which did it gain mass, and at which did it stay the same?
At Station 1, is the mass of the carbon dioxide and water vapor made larger or smaller than 2.8 g? Explain using the atoms that came from the air.
At Station 3, the penny lost 0.004 g but the cup did not. Where are the atoms that left the penny?
Modification for Distance Learning
Post each station as a slide. Groups record their answers and drawings on a shared document, then vote on which station was the hardest to explain and why.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: An Atom Model of Hydrogen Peroxide Breaking Down
Each circle is an atom and each group of joined circles is a molecule. The reactants and the products are made of the same 4 hydrogen atoms and 4 oxygen atoms, joined in new ways, so the total mass does not change.
Diagram 2: Mass Change in a Closed Bottle and an Open Cup (Invented Data)
Baking powder and water react in a closed fizzy-drink bottle and in an open cup, drawn to scale. The closed bottle stays within 0.01 g of its starting mass (the precision of the balance). The open cup loses 0.59 g in 3 minutes as carbon dioxide gas escapes into the room.
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Homework Assignment
~30 min
MS-PS1-5 Homework: Atoms and Mass in Reactions
Directions: All data are invented for practice. For every model, draw the atoms as circles (one color per element) and make a tally table of each element before and after the reaction. Say whether each system is open or closed.
Part 1: Count Atoms in a Model (Problems 1-2)
High in the atmosphere, ozone changes into ordinary oxygen. A model shows 2 ozone molecules (3 oxygen atoms each) becoming 3 oxygen molecules (2 oxygen atoms each). (a) Draw the model. (b) Tally the oxygen atoms before and after. (c) If 9.6 g of ozone changes completely into oxygen in a closed container, what mass of oxygen forms? Explain how the tally lets you predict it.
A student drew this model of sulfur burning in oxygen: before, 2 sulfur atoms and 1 oxygen molecule (2 oxygen atoms); after, 2 sulfur dioxide molecules (1 sulfur atom and 2 oxygen atoms each). (a) Tally the atoms on both sides. (b) What is wrong with the model? (c) Draw a corrected model.
Part 2: Use Mass Data (Problems 3-4)
A student adds a pinch of yeast to 30 mL of 3% hydrogen peroxide in two containers. Open cup: 120.84 g before, 120.52 g after. Plastic fizzy-drink bottle with its cap on: 53.07 g before, 53.07 g after. (a) Which container is a closed system? (b) What mass left the open cup, and in what gas? (c) The same amount of gas formed in both containers. Predict the bottle's reading after the cap is removed and the gas escapes, with the cap placed back on the balance.
When 12 g of charcoal (carbon) burns completely in a grill, it makes 44 g of carbon dioxide. (a) What mass of oxygen from the air joined the carbon? (b) The grill holds much less ash than the charcoal put in. Use the atom model to explain why that does not break the law of conservation of matter.
Part 3: Explain with the Model (Problems 5-6)
In autumn, a pile of leaves is raked into a corner of a yard. By summer the pile is much smaller. A neighbor says: "The leaves were destroyed." (a) Use the atom model to say where most of the atoms went. (b) Name one gas that could carry the carbon atoms away. (c) Describe a closed system in which you could test that mass is conserved as leaves rot.
In a class test, a classmate's closed fizzy-drink bottle read 0.02 g less at the end than at the start, even though its cap was on. (a) Give two possible explanations that do not involve atoms being destroyed. (b) Describe a quick test for each explanation.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Atom Models and Tallies
Drawings show the right atoms in each molecule, and the tally compares every element before and after
Tally is complete but one count is wrong, or drawings are missing
Open and Closed Systems
Correctly identifies the system as open or closed and names the atoms or gas that crossed its boundary
Identifies the system but does not explain what crossed the boundary
Mass Reasoning
Uses measured masses correctly and links every mass change to atoms that left or joined
Correct arithmetic but the explanation does not use atoms
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data are invented for practice. Choose one answer for each multiple-choice question; your score updates as you answer, and Reset quiz clears everything so you or your students can try again. Short-answer questions show a model answer.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
What happens to the atoms of the reactants during a chemical reaction?
Answer: B
In a reaction the atoms separate and join in new ways, but the same atoms are there before and after. Choice A breaks the law of conservation of matter. Choice C is a common error: energy is released or taken in, but the atoms remain. Choice D is wrong because atoms do not change size.
Question 2 of 20 · Multiple Choice
A model shows the reactants of a reaction: 2 molecules that each have 1 carbon atom and 2 oxygen atoms, and 4 molecules that each have 2 hydrogen atoms. How many atoms are in the reactants in all?
Answer: D
The first molecules give 2 × 3 = 6 atoms and the second give 4 × 2 = 8 atoms, so 6 + 8 = 14. Choice A counts the molecules instead of the atoms. Choice B counts 2 + 8, one atom for each of the first two molecules. Choice C counts 4 + 8, leaving out one atom from each of the first molecules.
Question 3 of 20 · Multiple Choice
A reaction happens in a closed container. The reactants have a total mass of 30.0 g. What is the total mass of the products?
Answer: A
In a closed system no atoms get in or out, and the same atoms make up the products, so their mass is 30.0 g. Choice B would need atoms to escape. Choice C would need atoms to enter from outside.
Question 4 of 20 · Multiple Choice
Crushed eggshell is left in vinegar in an open cup. It fizzes, and after a day the cup weighs 0.8 g less. What is the best explanation?
Answer: C
The fizzing makes carbon dioxide gas, which escapes from the open cup, so its atoms are no longer weighed. Choice A breaks the law of conservation of matter. Choice D is wrong because each atom keeps its own mass.
Question 5 of 20 · Multiple Choice
A silver necklace left in the air turns dark and gains a tiny amount of mass. Why does its mass go up?
Answer: A
The dark coating is a new substance made when silver atoms join with sulfur atoms from a gas in the air (and, in some reactions, oxygen), so the necklace now includes atoms that came from outside it. Choice B is wrong because atoms keep their mass. Choice D is wrong because atoms are never created.
Question 6 of 20 · Multiple Choice
A model shows 1 phosphorus molecule (4 phosphorus atoms) and 6 chlorine molecules (2 chlorine atoms each) reacting. The only product molecules each have 1 phosphorus atom and 3 chlorine atoms. How many product molecules form?
Answer: C
There are 4 phosphorus atoms and 6 × 2 = 12 chlorine atoms. Four product molecules use 4 phosphorus atoms and 4 × 3 = 12 chlorine atoms, so every atom is used exactly once. Choice A keeps the 4 phosphorus atoms together in one molecule. Choice B copies the number of chlorine molecules. Choice D counts the chlorine atoms instead of the molecules.
Question 7 of 20 · Multiple Choice
A student's drawing of a reaction shows 6 oxygen atoms in the reactants and 4 oxygen atoms in the products, and no oxygen anywhere else. What does that tell you?
Answer: B
Atoms are not destroyed in a reaction, so the products must also contain 6 oxygen atoms. The drawing needs 2 more oxygen atoms, either in a product or as leftover reactant. Choice A treats the faulty model as true. Choice C is wrong because reactions do not change one element into another.
Question 8 of 20 · Multiple Choice
In a closed jar, 4.8 g of magnesium reacts completely with 3.2 g of oxygen, and magnesium oxide is the only product. What mass of magnesium oxide forms?
Answer: D
The products have the same atoms as the reactants, so the mass is 4.8 + 3.2 = 8.0 g. Choice B subtracts the masses, choice C multiplies them, and choice A counts only the magnesium.
Question 9 of 20 · Multiple Choice
In a closed jar, 10 g of substance X and 6 g of substance Y react. Afterward, 2 g of X is left over unreacted, and there is one new product. What is the mass of the new product?
Answer: A
All 16 g is still in the jar, and 2 g of it is leftover X, so the product has 16 - 2 = 14 g. Choice B forgets the leftover. Choice C adds the leftover instead of subtracting it. Choice D subtracts the 2 g twice.
Question 10 of 20 · Multiple Choice
In a bead model of a reaction, why does the total mass of the beads stay the same?
Answer: C
Every bead stands for an atom, and the same beads are rearranged into the products, so the total mass cannot change. This matches a real reaction, where the same atoms are rearranged. Choices A and D describe changes that do not happen to the beads.
Question 11 of 20 · Multiple Choice
A rigid plastic jar holds lemon juice and a spoonful of baking soda kept apart, with its screw lid on tight. It reads 85.40 g. The jar is tipped so they mix and fizz. What does the balance read when the fizzing stops, with the lid still on?
Answer: B
The jar is a closed system, so the gas that formed stays inside and every atom is still weighed: 85.40 g. Choice A assumes the gas escaped. Choice C assumes the gas added mass from outside, but no atoms entered the jar.
Question 12 of 20 · Multiple Choice
Which of these is a closed system?
Answer: D
A jar with a tight lid keeps matter from getting in or out. The pot, the campfire and the cup are all open to the air, so gases can escape from them or join them.
Question 13 of 20 · Multiple Choice
A 1 kg pile of dry wood burns completely and leaves about 0.01 kg of ash. Where did most of the atoms of the wood go?
Answer: C
Burning joins atoms of the wood with oxygen from the air to make gases, mostly carbon dioxide and water vapor, which rise into the air. Choice A breaks the law of conservation of matter. Choice B confuses the energy released with the matter. Choice D does not explain the large loss: the ash is almost all that stays behind.
Question 14 of 20 · Multiple Choice
The reactants in a model have 8 hydrogen atoms and 4 oxygen atoms, and the only product is water, with 2 hydrogen atoms and 1 oxygen atom per molecule. How many water molecules form?
Answer: A
Four water molecules use 4 × 2 = 8 hydrogen atoms and 4 × 1 = 4 oxygen atoms, which matches the reactants exactly. Choice B would use only 4 hydrogen and 2 oxygen atoms. Choice C copies the number of hydrogen atoms, and choice D adds all the atoms.
Question 15 of 20 · Short Answer
A model shows 2 carbon atoms and 1 oxygen molecule (2 oxygen atoms) reacting to form 2 carbon monoxide molecules (1 carbon atom and 1 oxygen atom each). Make a tally and explain whether the model conserves atoms.
Before: 2 carbon and 2 oxygen atoms. After: 2 × 1 = 2 carbon and 2 × 1 = 2 oxygen atoms. The counts match for every element, so the model conserves atoms, and the products have the same mass as the reactants.
Question 16 of 20 · Short Answer
A closed plastic container (120.0 g) holds 50.0 g of water and a 4.0 g fizzy vitamin tablet. (a) What does the balance read before the tablet dissolves? (b) After it fizzes with the lid on? (c) The lid is then opened and 0.6 g of gas escapes. What is the new reading?
(a) 120.0 + 50.0 + 4.0 = 174.0 g. (b) Still 174.0 g, because the gas stays in the closed container. (c) 174.0 - 0.6 = 173.4 g: the atoms in the escaped gas are still in the room, just not on the balance.
Question 17 of 20 · Short Answer
Two identical reactions make a gas. In a plastic bottle with its cap on, the reading stays at 210.36 g. In an open cup, the reading drops by 0.42 g. A student says: "Gases weigh nothing." Use both results and the atom model to respond.
The open cup lost 0.42 g when its gas escaped, so the gas carried atoms that have mass: gases do have mass. In the closed bottle the same gas stayed inside, so its atoms were still weighed and the reading did not change. The student's claim does not fit either result.
Question 18 of 20 · Short Answer
A student models zinc reacting with oxygen. Before: 2 zinc atoms and 2 oxygen molecules (2 oxygen atoms each). After: 2 zinc oxide units (1 zinc atom and 1 oxygen atom each). Find the problem with the model and give one way to fix it.
Before there are 2 zinc and 4 oxygen atoms; after there are 2 zinc and only 2 oxygen atoms, so 2 oxygen atoms vanished. Fix: draw 1 oxygen molecule left over on the product side (not every reactant has to react), or start with only 1 oxygen molecule.
Question 19 of 20 · Short Answer
27 g of aluminum foil reacts with oxygen until it becomes 51 g of aluminum oxide. What mass of oxygen joined the aluminum? Explain where it came from.
51 - 27 = 24 g of oxygen. The oxygen atoms came from the air around the foil; they joined the aluminum atoms, so the solid got heavier while the total number of atoms did not change.
Question 20 of 20 · Short Answer
A student uses a drag-and-drop computer model of a reaction and ends up with one extra hydrogen atom in the products that was not in the reactants. What should the student conclude, and what should they do next?
The model does not show a real reaction, because atoms cannot be created. The student should recount each element on both sides, find where the extra hydrogen atom came from, and rebuild the products using only the atoms in the reactants.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does MS-PS1-5 mean?
MS-PS1-5 asks middle school students to build and use a model showing that atoms are rearranged, not created or destroyed, in a chemical reaction, so the total mass stays the same. The code means Middle School, Physical Science, core idea 1 (Matter and Its Interactions), performance expectation 5.
What grade is MS-PS1-5 taught in?
It is a grades 6-8 standard, and schools often place it in grade 7 or 8, in a chemistry or matter unit. It fits after students have seen signs of chemical reactions and know that matter is made of particles.
Does MS-PS1-5 require balancing chemical equations?
No. The Assessment Boundary says assessment does not include balancing symbolic equations or using atomic masses. Students show conservation with drawings, physical models such as beads, or digital models, and they count atoms in models that are already given. Writing and balancing equations comes later, in high school chemistry.
What is the assessment boundary for MS-PS1-5?
Assessment does not include the use of atomic masses, balancing symbolic equations, or intermolecular forces. In practice, students count atoms and compare masses measured on a balance, but they do not look up how heavy each kind of atom is or explain the forces between molecules.
Why do some reactions seem to lose or gain mass?
Because the container is open. If a reaction makes a gas, the gas can escape and the balance reads less. If a reaction uses a gas from the air, such as oxygen, atoms join the solid and the balance reads more. In a closed container, the reading stays the same within the precision of the balance.
What is the difference between an open and a closed system?
A closed system lets no matter in or out, for example a bottle with its cap on. An open system, such as a cup or a campfire, lets gases escape or enter. Conservation of matter holds in both, but only a closed system shows it directly on a balance.
What models can students use for MS-PS1-5?
The Clarification Statement names physical models and drawings, including digital forms. Beads, snap cubes, paper cutouts, colored circles on paper and drag-and-drop slides all work, as long as students keep the same pieces on both sides and keep a tally of each element.
What mistakes do students often make about conservation of mass?
Many students think matter is destroyed when something burns or dissolves, or that gases have no mass. Others count molecules instead of atoms, and are confused when the number of molecules changes. Ask them to tally atoms element by element and to say whether the system is open or closed.
Are the labs for MS-PS1-5 safe for middle school?
Yes, with household materials and goggles. Yeast and sugar in warm water make gas slowly, and plastic bottles made for fizzy drinks are built to hold that pressure. Never use glass for a reaction that makes gas, open a bottle with its cap on slowly, and keep any burning (a candle or a campfire) as teacher data, not a student activity.
How does MS-PS1-5 connect to high school chemistry?
In high school, HS-PS1-7 asks students to use mathematical representations to show that atoms, and therefore mass, are conserved. That is where they write and balance chemical equations and use atomic masses, the two things left out of MS-PS1-5 on purpose.
07
Related Standards
4 standards
These standards connect to MS-PS1-5: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
5-PS1-2Prerequisite
Measure and graph quantities to show total weight is conserved when heating, cooling or mixing
Lesson coming soon
Alongside
MS-PS1-2Parallel
Analyze data on properties of substances before and after they interact to spot a reaction
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