ChemistryConservation of Mass: Atoms Are Only Rearranged
Atlas, in goggles and gloves, places a sealed clear flask of baking soda and vinegar fizzing together on a digital balance in a tidy lab.
- Explain that a chemical reaction rearranges atoms into new substances.
- State that atoms are never created or destroyed during a reaction.
- Predict the total mass of products from the total mass of reactants in a sealed system.
- Account for hidden gas to correct the misconception that mass is lost in open reactions.
- Distinguish between a sealed and an open reaction system and explain why sealing matters for measuring mass.
Key terms
- Chemical reaction
- A process in which atoms break old bonds and form new ones to build new substances.
- Conservation of mass
- The rule that total mass stays the same before and after a reaction.
- Sealed system
- A closed container that prevents any matter, including gas, from escaping.
- Open system
- A container that allows gas to escape into the surrounding air.
Atoms Are Only Rearranged
A chemical reaction never makes atoms appear or vanish; it simply breaks the connections between atoms and rebuilds them into new arrangements. Picture the same set of building blocks taken apart and reassembled into a new shape. Because every atom present at the start is still present at the end, the total mass cannot change. This is why conservation of mass holds for every chemical reaction, from rusting iron to burning fuel to baking soda fizzing with vinegar.
Why Sealing the Container Matters
Many reactions produce a gas, and gas is easy to overlook because it floats away invisibly. In an open cup, carbon dioxide from a fizzing tablet escapes into the room, so the leftover liquid weighs less even though no atoms were destroyed. Sealing the container traps that gas inside, keeping every atom on the balance. That is why a careful chemist seals the flask first: it lets the scale prove that mass is conserved instead of seeming to disappear.
Worked examples
Combine 12 g of baking soda with 18 g of vinegar in a sealed flask. What is the product mass?
- Add the reactant masses: 12 g + 18 g = 30 g.
- Atoms are only rearranged in the reaction, so none are created or destroyed.
- Because the flask is sealed, no gas escapes, so the total mass is unchanged.
Answer: 30 g, because mass is conserved in the sealed flask.
An open cup of fizzing tablet starts at 50 g and ends at 47 g. Where did 3 g go?
- Mass cannot be destroyed, so the missing 3 g must have left as something.
- The reaction produced carbon dioxide gas, which is invisible.
- In the open cup that gas escaped into the air, carrying 3 g with it.
Answer: The missing 3 g left the cup as carbon dioxide gas; it was not destroyed.
Activity
Predict the total mass after each sealed reaction, then check whether atoms were created, destroyed, or only rearranged.
Practice
Predict the product mass when 25 g and 40 g of reactants combine inside a sealed jar.
Explain why a burning candle in open air seems to lose mass as it burns.
Common mistakes to avoid
- Forming a new substance adds mass.No new atoms appear when a substance forms, so the total mass stays exactly the same.
- Mass is destroyed when a reaction loses weight.The lost mass usually escaped as gas into the air; atoms are never destroyed in a chemical reaction.
Check your understanding
You react 20 grams of substance A with 30 grams of substance B in a sealed jar. What is the total mass of the products?
A fizzy tablet reacts in an open cup, and afterward the cup weighs less. What best explains this?
During a chemical reaction, what actually happens to the atoms?
Why does Atlas seal the flask before measuring mass during a gas-producing reaction?
Recap
In any chemical reaction atoms are only rearranged, never created or destroyed, so total mass is conserved. Reactions can seem to lose mass when a gas escapes, which is why sealing the container lets the balance prove that mass stays the same.
Reflect
Where do you look first when a reaction appears to lose mass?