Intuition The big idea in one lineIn an isolated system , a chemical reaction only rearranges atoms — it never creates or destroys them — so the total mass stays constant . Atoms are like LEGO bricks: you rebuild the model, but you never gain or lose bricks.
Definition Law of Conservation of Mass (Antoine Lavoisier, 1789)In any physical or chemical change occurring in a closed/isolated system , the total mass of the reactants equals the total mass of the products .
mass reactants = mass products \text{mass}_{\text{reactants}} = \text{mass}_{\text{products}} mass reactants = mass products created nor destroyed in an ordinary chemical reaction.
Keyword that carries everything: closed/isolated . If gas escapes or air rushes in, the balance is broken for the container , not for the universe.
We derive it from the atomic theory rather than just asserting it.
This is exactly why we balance chemical equations : balancing enforces N i before = N i after N_i^{\text{before}} = N_i^{\text{after}} N i before = N i after is the conservation of mass at the atomic level.
Intuition Balancing = conservationA balanced equation is a bookkeeping statement that every atom is accounted for. If atoms balance, mass balances automatically.
Worked example Example 1 — Simple combination2.00 g 2.00\text{ g} 2.00 g 16.00 g 16.00\text{ g} 16.00 g Why: closed system, so M R = M P M_R = M_P M R = M P m H 2 + m O 2 = m H 2 O m_{\text{H}_2} + m_{\text{O}_2} = m_{\text{H}_2\text{O}} m H 2 + m O 2 = m H 2 O 2.00 + 16.00 = 18.00 g of water 2.00 + 16.00 = 18.00\text{ g of water} 2.00 + 16.00 = 18.00 g of water Why this step? All reactant atoms end up in the product; no escape ⇒ add reactant masses.
Worked example Example 2 — Find a missing mass12.00 g 12.00\text{ g} 12.00 g 44.00 g 44.00\text{ g} 44.00 g CO 2 \text{CO}_2 CO 2 m O 2 = m CO 2 − m C = 44.00 − 12.00 = 32.00 g m_{\text{O}_2} = m_{\text{CO}_2} - m_{\text{C}} = 44.00 - 12.00 = 32.00\text{ g} m O 2 = m CO 2 − m C = 44.00 − 12.00 = 32.00 g Why this step? Rearranging M R = M P M_R = M_P M R = M P
Worked example Example 3 — The "disappearing mass" trap10.0 g 10.0\text{ g} 10.0 g CaCO 3 \text{CaCO}_3 CaCO 3 open dish:
CaCO 3 → CaO + CO 2 ↑ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\uparrow CaCO 3 → CaO + CO 2 ↑ 5.6 g 5.6\text{ g} 5.6 g No. CO 2 \text{CO}_2 CO 2 4.4 g 4.4\text{ g} 4.4 g 5.6 ( CaO ) + 4.4 ( CO 2 ) = 10.0 g = original mass 5.6\ (\text{CaO}) + 4.4\ (\text{CO}_2) = 10.0\text{ g} = \text{original mass} 5.6 ( CaO ) + 4.4 ( CO 2 ) = 10.0 g = original mass Why: the system here is not closed. Count the gas and mass is perfectly conserved.
Worked example Example 4 — Forecast-then-VerifyA candle in a sealed jar burns until it goes out. Forecast: will the jar+contents weigh less?
Verify: No — the wax's carbon and hydrogen become CO 2 \text{CO}_2 CO 2 H 2 O \text{H}_2\text{O} H 2 O unchanged . What "disappeared" is only the visible solid wax , converted to invisible gas + vapour of equal total mass.
Common mistake "The candle/wood lost mass, so mass was destroyed."Why it feels right: the solid visibly shrinks and seems to vanish. Our eyes only track solids/liquids, not gases.
The fix: invisible gases (CO 2 \text{CO}_2 CO 2
Common mistake "Mass is conserved even in nuclear reactions."Why it feels right: it worked for every chemistry reaction you saw.
The fix: in nuclear reactions mass converts to energy via E = m c 2 E=mc^2 E = m c 2 chemistry law — bonds break/form but nuclei stay intact, so no measurable mass change. (In chemistry the energy change is so small the mass change is undetectable.)
Common mistake "You can add reactant masses even when a gas escapes."Why it feels right: the equation M R = M P M_R = M_P M R = M P The fix: the law holds only for a closed system. Always ask: can matter enter or leave? If yes, account for the escaping/entering gas explicitly.
Recall Feynman: explain to a 12-year-oldImagine building a toy castle out of exactly 100 LEGO bricks, then smashing it and building a spaceship. You still have exactly 100 bricks — you didn't lose any, you just rearranged them. Atoms work the same way. When wood burns and seems to "disappear," the atoms actually floated away as invisible smoke and gas. If you trapped all that gas in a sealed box and weighed it, the box would weigh exactly the same as before. Nothing is ever really lost — it just changes clothes.
"Atoms just DANCE, they never LEAVE."
D-A-N-C-E = D alton's A toms N either C reated nor destroyed → E qual mass. And LEAVE reminds you: mass only seems lost when a gas leaves an open system.
#flashcards/chemistry
State the Law of Conservation of Mass In a closed system, total mass of reactants = total mass of products; mass is neither created nor destroyed in a chemical reaction.
Who proposed it and when Antoine Lavoisier, 1789.
Why must the system be closed to observe the law So no gas can enter or escape; otherwise the container's mass changes even though the universe's mass is conserved.
From first principles, why is mass conserved Atoms are only rearranged, not created/destroyed; each element's atom count and atomic mass are unchanged, so total mass (
∑ N i m i \sum N_i m_i ∑ N i m i ) is unchanged.
How does balancing an equation relate to the law Balancing enforces equal atom counts on both sides, which is conservation of mass at the atomic level.
10 g CaCO₃ heated openly leaves 5.6 g solid — where did 4.4 g go It escaped as CO₂ gas; total (5.6 + 4.4) = 10 g still conserved.
2 g H₂ + 16 g O₂ → mass of water 18 g.
When does this law appear to fail In nuclear reactions, where mass converts to energy (
E = m c 2 E=mc^2 E = m c 2 ), giving a measurable mass defect.
What did Lavoisier heat, and in what apparatus Mercury in a sealed retort; formed HgO while sealed air lost oxygen, total mass constant.
Atoms rearranged not created
Fixed atomic mass per element
Law of Conservation of Mass
massReactants equals massProducts
Atom counts conserved per element
Balance chemical equations
Sealed vessel keeps gas in
Intuition Hinglish mein samjhoDekho, Law of Conservation of Mass ka matlab bilkul simple hai: kisi bhi chemical reaction mein atoms sirf rearrange hote hain, na to naye atom bante hain, na koi atom gayab hota hai. Isliye agar system closed hai (yaani koi gas na andar aaye na bahar jaaye), to reactants ka total mass = products ka total mass. Lavoisier ne 1789 mein mercury ko ek sealed bartan mein garam karke ye prove kiya — pura apparatus weigh kiya before aur after, mass same nikla.
Sabse important cheez: LEGO wala idea. Jaise 100 bricks se castle todkar spaceship banao, bricks 100 hi rehte hain — waise hi atoms conserve rehte hain. Isiliye hum equations balance karte hain: balancing ka matlab hi hai dono side atoms barabar, aur agar atoms barabar to mass automatically conserve.
Common galti: log candle ya lakdi jalte dekh kar sochte hain "mass destroy ho gaya". Nahi bhai — wo mass invisible gas (CO2 _2 2 3 _3 3 2 _2 2 nuclear reactions mein thoda mass energy ban jaata hai (E = m c 2 E=mc^2 E = m c 2
