Question bank — Law of conservation of mass (Lavoisier) — proof, examples

False — the universe's mass is conserved, but a container only shows it when it is closed; an open flask can lose gas and appear lighter.

True — equal atom counts of each element times fixed atomic masses give equal total mass, so balancing is conservation at the atomic level.

False — the missing mass left as ${\text{CO}}_2$ and water vapour; trap those gases in a sealed box and the mass is unchanged.

False — the wax's carbon and hydrogen became ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$ still trapped inside; total sealed mass is identical.

False — nuclei change identity and a tiny mass defect converts to energy via $E=m{c}^2$; here $m$ is mass and $c$ the speed of light. See Mass–Energy Equivalence (E=mc²), which shows why that mass change is measurable only for nuclei, not chemical bonds.

False — it only needs atoms to be conserved (Dalton's postulate: atoms are indestructible); you can verify it by weighing alone, without any formula.

True — iron combines with oxygen from the air to form iron oxide, so the solid pulls mass in from an open surroundings.

False — any gas released or absorbed is left out; you must count entering/escaping gas to recover $M_R=M_P$.

False — dissolving is a physical change; atoms are merely dispersed, none created or destroyed, so mass is unchanged. This rests on Dalton's picture of indestructible atoms (Dalton's Atomic Theory).

The error is calling the system closed; the $4.4\text{ g}$ escaped as ${\text{CO}}_2$ gas, so $5.6+4.4=10\text{ g}$ is still conserved.

The error ignores atom conservation; all reactant atoms end up in the product, so the water mass is fixed at exactly $18\text{ g}$.

The error misses that balancing enforces equal atom counts per element, which is literally conservation of mass. That equal-counts bookkeeping is the whole job of Balancing Chemical Equations.

The error is backwards — mercury does react. In a sealed glass retort he heated liquid mercury in a fixed pocket of air; over days the surface grew a red crust of mercury(II) oxide, $2\text{Hg}+{\text{O}}_2\to 2\text{HgO}$. The solid gained exactly the mass the sealed air's oxygen lost, so the whole apparatus weighed the same before and after. The timeline figure lays this out step by step.

The error is trusting the eye, which tracks only solids/liquids; the lost solid became invisible gas of equal mass.

The error ignores mass drawn in from ${\text{CO}}_2$ and water; the tree's added mass comes from the surroundings, not from nothing.

The error applies a nuclear-scale effect to chemistry; in chemical bonds the energy change is so small the corresponding mass change is far below any balance's sensitivity.

Sealing keeps the system closed so no gas can enter or leave, meaning any mass rearranged internally still shows up on the same balance.

If atoms are neither created nor destroyed and each has a fixed mass, the total $M_R={\sum }_i{N}_i{m}_i$ (atom count $N_i$ times atomic mass $m_i$, summed over elements) cannot change when only bonds rearrange.

Gaseous products escape uncounted, so the container loses mass while the atoms are simply relocated into the surrounding air.

Equal per-element counts guarantee equal total mass, but also forbid one element turning into another — it is conservation at the finer, atomic level, and this is the bedrock that lets Stoichiometry and the Mole predict exact amounts.

Because $M_R=M_P$ is a single equation; if all masses but one are known, algebra pins the last one exactly.

Both stem from fixed atomic masses combining in whole-number ratios, so mass is conserved and distributed in constant proportions (a fixed compound always has the same element-by-element mass split).

The solid genuinely transformed into gas (a real disappearance of that phase), but the matter itself persists, so "destroyed" overclaims.

Yes — with no gas crossing the boundary, an open container behaves like a closed one and the balance reads constant.

No — pressure change does not remove atoms; the same atoms are inside, now as liquid, so the total mass is unchanged.

Not measurably — chemical energy release corresponds to a mass change so minuscule ($E=m{c}^2$) that no laboratory balance can detect it.

No — nuclei split and a real mass defect converts to energy, so this is a nuclear regime where Mass–Energy Equivalence (E=mc²) (mass turning into energy) replaces the strict chemical law.

It falls, because the gases carry their mass out; seal the cell and collect the gases and the total is conserved.

It is trivially satisfied; with no change at all, reactant mass equals product mass because they are literally the same matter.

Unchanged — carbon moves from ${\text{CO}}_2$ into plant tissue and oxygen is released, but all atoms stay inside the sealed boundary.