Students who demonstrate understanding can: 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.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
In a chemical reaction, atoms are not made or destroyed. They are only regrouped. If you count every atom of each element before the reaction and after it, the totals match. Since each atom keeps its own mass, matching atom counts mean matching total mass. That is the law of conservation of matter.
Students show this with physical models or drawings of atoms. When methane burns, one methane molecule and two oxygen molecules become one carbon dioxide molecule and two water molecules. Before: 1 carbon, 4 hydrogen, 4 oxygen. After: 1 carbon, 4 hydrogen, 4 oxygen. Nothing is lost, just rearranged.
The idea also explains puzzling observations. A burning log seems to lose mass and rusting nails seem to gain it, but only because gases are escaping into or coming from the air. In a sealed container, the scale reading stays the same. Students are not asked to balance symbolic equations or use atomic masses at this level.
Ash weighs far less than wood, so students think matter vanished. The missing atoms left as carbon dioxide and water vapor; trapping the gases would show no loss.
Students often ignore gases when weighing. A sealed bottle of fizzing reactants weighs the same before and after, which shows the gas has mass.
Two molecules can become three molecules in a reaction. What must match is the number of each kind of atom, not the number of molecules.
Students sometimes think a reaction can produce an element that was not there to begin with. Every element in the products must already be present in the reactants.
One methane molecule (1 carbon and 4 hydrogen atoms) reacts with two oxygen molecules (2 oxygen atoms each). The products are one carbon dioxide molecule and two water molecules. Show that atoms are conserved.
Answer: Before and after: 1 carbon, 4 hydrogen and 4 oxygen atoms. The atoms are rearranged, not created or destroyed, so mass is conserved.
Hand out colored blocks or paper circles for each element, have students build the reactants, then take them apart and rebuild them as products without adding or removing any pieces. Follow up with a sealed bag of baking soda and vinegar on a balance so students see the reading stay the same.
Items often show particle diagrams with one side incomplete and ask which drawing correctly shows the products, or present mass data from an open container and ask why it decreased.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: B) 6
Atoms are rearranged, not created or destroyed, so the products must contain the same 6 oxygen atoms.
Answer: C) It left as gases such as carbon dioxide and water vapor
Atoms from the wood combined with oxygen and escaped into the air as gases. Matter was not destroyed.
Answer: D) 150 grams
Nothing can enter or leave the sealed bag, so the total mass stays at 150 grams even though a gas forms.
Answer: conservation of mass (also accepted: law of conservation of mass, conservation of matter, law of conservation of matter)
The law of conservation of matter (or mass) says atoms are only rearranged in a reaction, so total mass stays the same in a closed system.
Answer: B) The atoms were regrouped and the number of each kind of atom is unchanged
The number of molecules can change. What stays the same is the number of each type of atom.
Answer: oxygen (also accepted: o)
Rust forms when iron combines with oxygen (and water) from the air, so the oxygen atoms add to the nail's mass.
Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
A full lesson with slides, activities and an exit ticket on conservation of mass in reactions, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-PS1-5 with an answer key, ready in about a minute.
Make a worksheet βTurn conservation of mass in reactions into a quiz students answer online that marks itself, with a class summary for you.
Build a test βNo. The assessment boundary excludes balancing symbolic equations and using atomic masses. Students use physical models or drawings of atoms to show that the counts match.
In an open container, gases can escape or be taken in from the air. Seal the system and the total mass stays constant.