MS-PS1-5: Atoms and the Conservation of Mass
Also called NGSS MS-PS1-5 · Topic: Chemical Reactions · Physical Science
The Official Wording
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.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-PS1-5 Means in Plain English
Middle schoolers use models of atoms, such as drawings or physical pieces, to show that a chemical reaction rearranges atoms into new substances. Every atom at the start is still there at the end, so the total mass of everything involved stays the same.
Students count atoms in models but do not use atomic masses, balance written equations, or study forces between molecules. This builds on 5-PS1-2, where weight stays the same when matter changes, and leads to HS-PS1-7, where students calculate mass in reactions.
MS-PS1-5 I Can Statements
- I can count the atoms of each kind before and after a reaction.
- I can explain why mass stays the same in a closed container.
- I can explain why an open reaction seems to lose mass.
MS-PS1-5 Worked Example
Two fizzing reactions on a digital scale, one open and one sealed, followed by an atom drawing.
Does a fizzing antacid tablet lose mass when it reacts with water?
Set It Up: Two antacid tablets broken into quarters, water, an open cup, an empty 500 mL screw-top soda bottle with a small vial inside, and a digital scale that reads to 0.1 g.
- Weigh an open cup of water with one tablet's pieces beside it on the scale.112.6 g before
- Drop the pieces in, wait for the fizzing to stop, and weigh again.112.1 g after, 0.5 g gone
- Put the other tablet's pieces in the bottle, stand the vial of water upright beside them, cap it and weigh.145.3 g before
- Tip the bottle so the water reaches the pieces, wait, and weigh with the cap still on.145.3 g after
- Draw the bubbles as carbon dioxide, one C and two O in each, and trace the atoms back to the tablet.Gas atoms came from the tablet
The open cup lost 0.5 g because gas escaped into the room, while the sealed bottle kept every atom and its mass did not change.
Why it works: A reaction only rearranges atoms into new substances, so when nothing can leave the container, the total mass before and after is the same.
Common Mistakes with MS-PS1-5
Thinking gas has no mass
“Gas weighs nothing, so the mass really disappeared.”
Try this: Gas is made of atoms with mass. Trap it in a sealed container and the scale shows the mass never left.
Atoms appearing in the model
A drawing with two O atoms before and three after
Try this: Count each kind of atom on both sides of the drawing. The counts must match, because reactions only rearrange atoms.
Thinking new substances mean new atoms
“The reaction made new atoms of carbon dioxide.”
Try this: The atoms are old ones in new groups. Carbon dioxide is a new substance, but its carbon and oxygen came from the starting materials.
Where MS-PS1-5 Fits
Practice MS-PS1-5
Quick Check
- A candle burns and seems to get lighter. Where did the mass go?
- Before a reaction a model has 4 H atoms and 2 O atoms. How many are there after?
- Why must the container be sealed to show mass is conserved?