atomA particle that reactions shuffle but never destroy. A reaction ends with exactly the atoms it began with, which is why the total mass does not change.
balanced equationAn equation whose two sides hold the same atoms. It puts the law of conservation of mass on paper. In 2H2 + O2 -> 2H2O each side has four hydrogen atoms and two oxygen atoms.
chemical equationA reaction told in symbols instead of in words. The arrow points toward the products. The first chemical equation known was drawn by Jean Beguin in 1615.
chemical reactionA change that turns substances into different ones. Bonds between atoms break and new bonds form. Burning and rusting are chemical reactions.
closed systemA system sealed so that no matter gets in or out. A stoppered flask is a closed system. No substance can enter or leave it during a reaction.
conservation of massThe law that mass is never gained or lost in a change. Stating it was a crucial step from alchemy to modern chemistry. It is the reason a chemical equation has to balance.
Show all 36 terms (30 more)
The law and its key terms, continued
massThe amount of stuff in a thing, the same anywhere. It is given in grams or kilograms. An object's mass does not change if it is taken to the Moon or out into space.
matterWhat all things are made of; it has mass and bulk. Every object you can touch is matter, made up of atoms. Light, which has no mass, is not matter.
open systemA system that trades matter with what is around it. Magnesium heated in an open crucible takes in oxygen from the air, so the solid that forms has more mass than the metal had.
productA substance that comes out of a reaction, newly made. A product usually has properties different from those of the substances it came from.
reactantA starting material that a reaction consumes. A catalyst is not a reactant, because the reaction does not use it up.
weightThe force with which a planet pulls on an object. Weight is measured in newtons. On the Moon an object weighs one sixth of what it does on Earth, though its mass has not changed.
Measuring, laws and equations 12
atomic theoryThe scientific account of matter as tiny particles. John Dalton set out his version in the early 1800s. In it atoms cannot be created or destroyed, and a reaction only combines, separates or rearranges them.
balanceA laboratory tool for finding an object's mass. The traditional kind hangs two pans at equal distances from a pivot and compares an unknown mass with known ones.
coefficientThe 2 in 2H2O: it counts whole units of a formula. An equation is balanced by choosing suitable coefficients.
isolated systemA system that swaps neither matter nor energy. A closed system can still gain or lose heat. An isolated system cannot.
kilogramThe unit once set by a metal cylinder near Paris. Its symbol is kg, and it is the base unit of mass. Since 2019 it has been defined by constants of nature.
law of definite proportionsThe rule that a compound's makeup by mass is fixed. In any sample of pure water, about eight ninths of the mass is oxygen and the rest is hydrogen. Joseph Proust stated the law in 1797.
rearrangementThe regrouping of atoms into new combinations. When a log burns, the atoms of the wood and of the oxygen it uses end up in carbon dioxide, water vapor and ash.
scientific lawA rule of nature drawn from repeated experiments. A law describes or predicts what happens. It can still be restricted or extended by later observations.
subscriptThe low-set figure that says how many of an atom. In C6H12O6, the formula of glucose, the subscripts show 6 carbon, 12 hydrogen and 6 oxygen atoms.
systemThe part of the world picked out to be studied. In a mass experiment the system might be a flask and everything in it. All that lies outside is called the surroundings.
tareThe mass of the empty container, to be subtracted. On a laboratory balance the tare is the mass of the flask. Subtract it from the total and what is left is the mass of the contents.
total massThe masses of everything in a system, added together. If 10 grams of sugar are stirred into 200 grams of water, the solution has a total mass of 210 grams.
Changes that test the law 12
combustionA fuel's rapid, heat-giving joining with oxygen. The ash from a fire has less mass than the fuel had. The gases produced, which drift away, hold the rest, and the oxygen that was used as well.
dissolvingThe scattering of a solid's particles in a liquid. The dissolved solid cannot be seen, but all of its particles are still there.
gasThe state of air and steam, with no set shape. A gas has mass even when it cannot be seen. If 100 grams of water are boiled away, the steam has a mass of exactly 100 grams.
mass-energy equivalenceThe rule E = mc2: a tiny mass equals vast energy. The formula is Albert Einstein's, and the c in it stands for the speed of light.
meltingA solid's turning to liquid as it is heated. When 500 grams of ice cubes melt in a bowl, the bowl holds 500 grams of water.
nuclear reactionA change that alters the nuclei of atoms themselves. Chemical reactions leave nuclei alone. A nuclear reaction can release so much energy that the change in mass is large enough to measure.
phlogistonAn imagined element once thought to escape in burning. The idea was dropped before the end of the 1700s. It could not explain why some metals gain mass when they burn.
physical changeA change that leaves each substance what it was. A physical change can usually be undone: salt dissolved in water comes back when the water evaporates.
rustingIron's slow gain of oxygen, which turns it flaky. Rust is an iron oxide: the iron has combined with oxygen, with water or damp air helping the reaction along.
state of matterSolid, liquid, gas or plasma: a way matter can exist. Solids, liquids and gases are the familiar states, and plasma is a fourth. A change of state does not change the mass.
thermal decompositionThe breaking down of a substance by heat. Heated copper carbonate breaks down into solid copper oxide and carbon dioxide gas. The solid left has less mass because the gas has escaped.
volumeThe room a sample takes up; freezing can change it. Volume is not conserved the way mass is. A bottle of water that freezes holds ice that takes up more room, yet its mass is the same.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Conservation of mass, Matter, Mass, Weight, Mass versus weight, Closed system, Open system (systems theory), Thermodynamic system, Isolated system, Scientific law, Weighing scale, Kilogram, Tare weight, Atom, Chemical reaction, Reagent, Product (chemistry), Chemical equation, Subscript and superscript, History of atomic theory, John Dalton, Law of definite proportions, Physical change, State of matter, Melting, Solvation, Gas, Combustion, Rust, Thermal decomposition, Phlogiston theory, Nuclear reaction, Mass-energy equivalence, Antoine Lavoisier and Mikhail Lomonosov. The law, mass and weight, closed and open systems, the examples of freezing, melting, dissolving, boiling and burning and the worked masses were checked a second time against BBC Bitesize (https://www.bbc.co.uk/bitesize/articles/zxh7jsg, Conservation of mass; https://www.bbc.co.uk/bitesize/guides/z7wp2v4/revision/1, Law of conservation of mass; https://www.bbc.co.uk/bitesize/guides/z7wp2v4/revision/2, Reactions in closed systems); the kilogram against the National Institute of Standards and Technology (https://www.nist.gov/si-redefinition/kilogram, Kilogram: Introduction; https://www.nist.gov/pml/owm/si-units-mass, SI Units - Mass). The fact after each meaning is from the same pages. Licence: own work
How it works
This conservation of mass vocabulary worksheet lists matter, mass, weight, closed system, open system, atom, chemical reaction, reactant, product, chemical equation and two more terms beside their meanings in another order. An answer key prints as the last page.
Read the full guide
Measuring, laws and equations
Set opens on The law and its key terms. The second set has system, isolated system, scientific law, balance, kilogram, tare, total mass, rearrangement, coefficient, subscript, atomic theory and law of definite proportions.
Changes that test the law
The third set has physical change, state of matter, volume, melting, dissolving, gas, combustion, rusting, thermal decomposition, phlogiston, nuclear reaction and mass-energy equivalence.
Mole, molar mass and limiting reactant are on the stoichiometry vocabulary worksheets.
Questions and answers
Who discovered the law of conservation of mass?
Antoine Lavoisier is usually given the credit: his precise measurements in the late 1700s confirmed it. Mikhail Lomonosov had outlined the principle in 1756, and Jean Rey had shown evidence for it as early as 1630.
What is an example of the law of conservation of mass?
When 24 grams of carbon burn in 64 grams of oxygen, 88 grams of carbon dioxide form: 24 plus 64 is 88. Every atom that went in is still there, in a new arrangement.
Does the law of conservation of mass apply to physical changes?
Yes. Water that freezes, ice that melts and sugar that dissolves all keep their mass. A bottle holding 300 grams of water still holds 300 grams when the water has turned to ice.