Conservation of Mass
Why atoms are never lost, even when the scale seems to disagree
How this deck works. Every slide is colour-coded: Read for the explanation, Look for diagrams and models, Do for experiments and simulations, and Check for questions. Move with the buttons at the bottom, or the ← → keys on a keyboard.
A log weighs 8 pounds. Burn it and the ash weighs a few ounces. Where did the rest go?
Where you left off · Topic 3
In the collision lab, A₂ and B₂ turned into AB over and over, and the counter at the bottom never moved: 12 A atoms and 12 B atoms, always. Reactions rearrange atoms. They do not make them or destroy them.
That idea has a name, and this topic is about taking it seriously enough to put it on a kitchen scale — including in the cases where the scale seems to disagree with it.
Because the scale really does read less after a log burns. And it really does read more after steel wool rusts. Neither one breaks the rule. Working out why is the whole job.
The law of conservation of mass
In a chemical reaction, the total mass of the products always equals the total mass of the reactants.
Matter is not created and it is not destroyed. Every atom that goes into a reaction comes out of it. They are bonded differently, so the substances are different — but if you could weigh every atom before and after, the two numbers would match exactly.
The person usually credited with nailing this down is Antoine Lavoisier, a French chemist working in the 1770s and 1780s. Others had noticed pieces of it, but Lavoisier did the thing that settled it: he ran his reactions in sealed containers and weighed the whole container, so nothing could sneak in or slip out.
That single choice is why he saw the pattern when others had missed it for centuries. Chemists before him weighed open dishes and kept getting results that made no sense.
Teacher note · common misconception
Students hear "mass is conserved" and picture it as a rule about the objects they can see. It is a rule about atoms. Insist on the phrase "the whole system" every single time — nearly every wrong answer in this topic comes from quietly leaving a gas out of the accounting.
Same atoms, same mass
Test yourself
You react 12 g of substance A with 20 g of substance B inside a sealed flask. What is the total mass of the products?
Why the scale sometimes seems to disagree
Closed system
Nothing can get in or out. A sealed jar, a zip bag pressed shut, a capped bottle.
The scale reads the same before and after. Always.
Open system
Gases can escape into the room, or join in from the air. An open cup, a fireplace, a rusting bumper.
The scale can read less — or more. Both are normal.
When it reads less: a product is a gas and the gas floated away. Burn a log and most of its mass leaves as carbon dioxide and water vapor going up the chimney. Weigh only the ash and of course you get a smaller number — you are weighing part of the products and calling it all of them.
When it reads more: a reactant came from the air and stayed. Steel wool rusting picks up oxygen atoms from the room, and those atoms are now locked into the solid sitting on the scale. Nothing was created; oxygen simply moved from the air onto the pan.
Teacher note · common misconception
"Burning destroys matter" is stubborn because the everyday evidence looks overwhelming. The fix is not to repeat the rule louder — it is to make students name the missing products out loud. Ask "what left the fireplace, and where did it go?" until carbon dioxide and water vapor come out of their mouths, not yours.
Drag the reaction forward and count
Pick a reaction, then move the slider from 0% to 100%. Watch the atoms travel to new partners — and watch the tally at the bottom refuse to move.
What the lab is showing you:
- Every atom on the right at 100% is an atom that was already on the left at 0%. None appeared and none disappeared — they only changed partners.
- The tally along the bottom is an atom inventory. It is the tool you use to check whether a written equation is balanced.
- Try the rusting reaction. Four iron atoms go in and the products are heavier than the iron alone — because six oxygen atoms came out of the air and joined them. That is the whole explanation for the mass gain you will measure.
Test yourself
You put 10 g of baking soda and 40 g of vinegar in an open cup on a scale. It fizzes. Afterward the scale reads 48 g. What happened to the missing 2 g?
Trap the gas and weigh it
Open cup versus sealed bag — the same reaction, two readings.
Materials
- Baking soda
- White vinegar
- 2 zip bags
- A small cup or plastic bottle cap
- A kitchen scale (1 g resolution)
- A teaspoon
Steps
- Put 2 teaspoons of baking soda into the bottom corner of a zip bag.
- Stand the small cup upright inside the bag and pour about 3 tablespoons of vinegar into the cup without letting it touch the baking soda yet.
- Press out as much air as you can, seal the bag completely, and place it on the scale. Write down the mass.
- Without opening the bag, tip the cup over so the vinegar reaches the baking soda. Let it fizz until it stops. The bag will inflate — hold it if you need to, but do not let it burst or open.
- Put the sealed bag back on the scale. Write down the mass.
- Now repeat everything in an open cup instead of a bag, weighing before and after. Compare the two results.
The sealed bag reads the same before and after. The open cup reads lower afterward.
The reaction is identical in both. Baking soda and vinegar produce carbon dioxide gas, water and a dissolved salt. In the bag the carbon dioxide has nowhere to go — it is right there inflating the plastic, still part of what the scale is weighing. Closed system, mass unchanged.
In the open cup the same carbon dioxide drifts off into the kitchen. The atoms still exist; they are simply no longer on the scale. That is the entire difference between the two readings, and it is why Lavoisier sealed his flasks.
If your sealed bag drops by a gram or two, check for a leak at the seal — that is a real result telling you your system was not as closed as you thought.
Teacher note
Press the air out properly. A bag that starts puffy can push against the scale platform once inflated and give a false low reading that looks like lost mass. Also warn students not to overfill: too much of either reactant and the bag can pop.
A balanced equation is conservation of mass, written down
If atoms cannot be created or destroyed, then a correctly written equation must show the same number of each kind of atom on both sides of the arrow. An equation that does is called balanced.
Look at this one, which is not balanced:
As written, an oxygen atom vanished. That is impossible, so the equation is wrong. Fix it by adjusting coefficients — the full-size numbers in front:
The one rule you must not break:
You may change coefficients as much as you like. You may never change a subscript. Writing H₂O₂ to fix the oxygen count would balance the numbers and destroy the chemistry — H₂O₂ is hydrogen peroxide, a completely different substance from water.
Teacher note · common misconception
When a student edits a subscript to balance an equation, do not just mark it wrong — ask them to name the substance they have just written. Turning water into hydrogen peroxide with one keystroke makes the rule memorable in a way that "coefficients only" never does.
How to check an equation in ten seconds
Make a list of every element. Count it on the left. Count it on the right. Compare.
Check the fixed version yourself: 4 Fe + 3 O₂ → 2 Fe₂O₃. Iron: 4 on the left; 2 × 2 = 4 on the right. Oxygen: 3 × 2 = 6 on the left; 2 × 3 = 6 on the right. Balanced.
Test yourself
Which equation is balanced?
Balance an equation with your hands
Build the reactants, then rebuild them into the products.
Materials
- Two colors of dried beans, cereal or beads
- A third color (about 6 pieces)
- Paper and a marker
- A large plate or tray
Steps
- Assign colors: black beans = carbon, white beans = hydrogen, red beads = oxygen. Write the key on your paper.
- Draw a big arrow across the tray. Left side is reactants, right side is products.
- On the left, build one methane molecule: 1 carbon touching 4 hydrogen. Next to it build two oxygen molecules: 2 pairs of oxygen.
- Count everything on the left and write it down: carbon ___, hydrogen ___, oxygen ___.
- Now move — do not add, do not remove — every piece across the arrow and rebuild them into carbon dioxide and water molecules. Use all of them.
- Count the right side and write it down. Then write the equation you just built.
You get exactly one CO₂ and exactly two H₂O, with nothing left over.
You started with 1 carbon, 4 hydrogen and 4 oxygen. Carbon dioxide takes the carbon and 2 oxygen. That leaves 4 hydrogen and 2 oxygen, which is precisely two water molecules — not one, and not three. The equation you built is CH₄ + 2 O₂ → CO₂ + 2 H₂O.
The leftover test is the point. If you finish with spare pieces or find yourself needing one more bean, your equation is not balanced. Try it again with 3 oxygen molecules instead of 2 and watch it fail — you will have two red beads with nowhere to go.
Teacher note
This is the highest-value ten minutes in the whole unit for students who struggle with balancing on paper. The rule "you may not add or remove pieces, only rearrange them" is coefficients-only stated physically. Students who have done it with beans stop editing subscripts.
What does the scale read afterward?
Assume you weigh the whole setup shown, and only that. Ask yourself first: is this system open or closed?
Test yourself
A steel wool pad is left damp in an open dish on a scale for a week. It rusts, and the scale reads more than it did on day one. Has the law of conservation of mass been broken?
Eight words you now own
Tap any card to flip it. Tap again to flip it back.
Where you already use this
A gallon of gas, twenty pounds of exhaust
A gallon of gasoline weighs about 6 pounds, but burning it puts roughly 20 pounds of carbon dioxide out the tailpipe. The extra mass is oxygen your engine pulled in from the air.
Ashes in a fire pit
A pile of firewood becomes a handful of ash. The missing mass went up as carbon dioxide, water vapor and smoke — it is in the sky above your backyard, not gone.
Losing weight
Most of the mass a person loses leaves the body as carbon dioxide, breathed out. The atoms have to go somewhere, and the lungs are the exit.
A rusted-out truck frame
Rust flakes off and the frame looks like it is losing material, but the rust itself is heavier than the iron it replaced. Oxygen from the air is now part of it.
Bread in the oven
A loaf comes out lighter than the dough that went in. Water vapor and carbon dioxide left through the crust while it baked.
A backyard compost pile
A full bin shrinks to a fraction of its size. Microbes convert the material and most of the mass departs as carbon dioxide and water.
Test yourself
Which setup would let you prove conservation of mass most convincingly with a kitchen scale?
Topic 4 quiz — 8 questions
One try per question. You get the reasoning right after you answer.
Up next · Topic 5
Exothermic and Endothermic Reactions. Mass is always conserved — you have proved that. Energy is a different story: your steel wool got warm, and an instant cold pack gets freezing. In the last topic you find out where reaction energy comes from and where it goes.