Conservation of Mass in Changes
In 1789, Antoine Lavoisier carefully weighed reactants and products in 20 sealed flasks and found the total mass never changed — proving matter cannot simply vanish when a candle burns.
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● Know
- Mass is conserved in every change — physical and chemical.
- This is called the Law of Conservation of Mass — nothing is created or destroyed.
- Total mass of reactants = total mass of products.
● Understand
- In burning, the 'lost' mass leaves as gases (CO₂, water vapour) and ash.
- In dissolving, the solute is still there even though it's invisible — weigh the solution and you'll see.
- Closed systems show conservation cleanly because gases can't escape.
● Can do
- Calculate missing masses using mass in = mass out.
- Explain why open systems seem to 'lose' mass during burning.
- Design a test that demonstrates conservation of mass.
Weigh a fresh candle, light it, let it burn for exactly 5 minutes, then blow it out and weigh the remaining stump — the stump is lighter, but the room has not gained any solid mass. Where did those grams go? If you could collect all the gas and smoke released, you would find that the total mass equals the original candle. That simple experiment leads to one of the cornerstones of chemistry. The law of conservation of mass states that in a closed system, mass is neither created nor destroyed during a chemical reaction. The total mass of the reactants at the start equals the total mass of the products at the end. This becomes powerful when gases are involved.
A closed system prevents matter from entering or leaving. In an open system, gases can escape or be absorbed from the surroundings, which makes mass seem to change even though the law still holds. The key is to account for every atom — including the ones you cannot see.
Place a candle on a balance inside a sealed glass jar. Light the candle and watch the flame. Although the wax seems to disappear, the total mass of the jar and its contents stays constant. The wax combines with oxygen to produce carbon dioxide and water vapour, both of which remain trapped inside.
At ANSTO, Australia's nuclear science agency, researchers track the mass of every atom in nuclear reactions. While Einstein showed that mass and energy are related, in ordinary chemical reactions the mass converted to energy is so tiny that it is undetectable — mass is conserved to all practical purposes.
The most common error is believing that mass disappears during combustion. It does not. When wood burns, the solid ash weighs less than the original log because carbon dioxide and water vapour escape into the air. If you collected every gas and speck of ash, the total mass would equal the original wood plus the oxygen that reacted with it.
Gases have mass, even though they are invisible and often feel weightless. A balloon filled with helium rises not because helium has no mass, but because it is less dense than air. In chemical reactions, gases must be counted as part of the total mass. When iron rusts, it appears to gain mass because oxygen from the air combines with iron to form iron oxide. The extra mass is not created from nothing — it comes from the oxygen.
Similarly, when a fuel burns, the products include carbon dioxide and water vapour. These gases drift away, making the remaining solid ash seem lighter. The mass is conserved, but it is distributed among invisible products that escape into the atmosphere.
Leave a steel nail outside in damp air for a month. It will develop a reddish-brown coating of rust and weigh slightly more than it did originally. The increase comes from oxygen atoms that have bonded with iron atoms to form iron oxide — a classic demonstration that gases contribute mass.
CSIRO atmospheric scientists measure the mass of carbon dioxide in the air as part of Australia's climate research. By tracking how much CO₂ is produced by bushfires, industry, and respiration, they build models that inform national emissions targets and environmental policy.
Many students think burning makes things lighter because mass is destroyed. This is false. Burning is a chemical reaction that converts solid or liquid fuel into gaseous products. The total mass of fuel plus oxygen equals the total mass of ash, smoke, and gases produced. Only the distribution changes, not the total.
At the particle level, a chemical reaction is simply a rearrangement of atoms. The same atoms that existed in the reactants are still present in the products; they are just bonded in different ways. No atom is created, and no atom is destroyed. This is why mass is conserved: the total number and type of atoms does not change, so their combined mass cannot change either.
A closed system makes this easy to observe because nothing can enter or leave. In an open system, you must be careful to account for all gases. If a reaction produces a gas that escapes, the remaining contents will seem to lose mass. If a gas from the surroundings reacts with a solid, the product will seem to gain mass.
Consider the combustion of methane: one carbon atom and four hydrogen atoms in the methane molecule combine with two oxygen molecules. The products are one carbon dioxide molecule and two water molecules. Count the atoms: one C, four H, and four O on each side. The atoms are rearranged, not created or destroyed.
BlueScope Steel monitors mass balance carefully in its blast furnaces. Every kilogram of iron ore, coke, and air that enters is tracked against every kilogram of steel, slag, and exhaust gas that leaves. Maintaining mass balance is essential for efficiency, cost control, and meeting environmental regulations.
Some students believe that conservation of mass only applies to closed systems and is therefore just a laboratory curiosity. This is not true. The law applies everywhere; open systems only make it harder to observe because matter crosses the boundary. Good experimental design accounts for these exchanges.
In a chemical reaction, the total of the reactants equals the total mass of the products. This is because atoms are but not or destroyed.
Students often struggle with mass conservation in open systems because they forget to count invisible gases. When magnesium burns in air, the product — magnesium oxide — weighs more than the original ribbon. The increase is not new mass; it is oxygen from the air that has bonded with magnesium. If you burn the magnesium inside a sealed container, the total mass before and after is identical.
Another common error is forgetting that the number of atoms stays the same even though the molecules are rearranged. Counting atoms on both sides of a reaction is the best way to convince yourself that mass is conserved.
A student burns magnesium ribbon in a crucible with the lid off. The white ash weighs more than the original metal. The student wrongly concludes that the law of conservation of mass is broken. In fact, the extra mass is oxygen from the air that combined with magnesium to form magnesium oxide.
Researchers at the Australian Synchrotron use mass spectrometry to identify substances by their mass with extraordinary precision. This technology helps verify conservation of mass in complex reactions and is used in fields from pharmaceutical development to environmental monitoring.
The observation that ash weighs less than the original wood is often taken as proof that mass is destroyed. It is not. The missing mass is in the gases. If you burned the wood in a sealed chamber and weighed every product — solid, liquid, and gas — the total would match the starting mass exactly.
Here's a student's conclusion. One line has an error — click it.
- The ash weighs more than the original magnesium.
- The student concludes that the law of conservation of mass is wrong because mass increased.
The law of conservation of mass is not just a classroom rule — it is a universal principle that governs every chemical reaction in the universe. Whether you are burning a match, digesting lunch, or watching iron rust, the total mass of the atoms involved remains constant. What changes is how those atoms are arranged and what substances they form.
Understanding this principle helps scientists design better experiments, calculate yields, and identify missing products. If the mass does not balance, you have either missed a reactant, lost a product, or made a measurement error. The law is a powerful diagnostic tool.
To demonstrate conservation of mass, place dilute acid and a carbonate in a sealed conical flask on a balance. Bubbles form inside the flask as carbon dioxide is produced, but the balance reading stays constant. No gas escapes, so every atom is accounted for and mass is conserved.
Marine biologists at the Great Barrier Reef Marine Park Authority study calcium carbonate mass in coral skeletons. By tracking how corals build and lose mass through calcification and erosion, scientists understand reef health and the impacts of ocean acidification on these vital ecosystems.
Some students believe that invisible products such as carbon dioxide or water vapour have no mass because they cannot be seen or weighed easily. This is false. A litre of carbon dioxide at room temperature has a mass of about 1.8 grams. Gases are matter, and all matter has mass.
At the start of this lesson, you thought about Lavoisier's 1789 discovery that matter can never be created or destroyed — a rule every reaction in the universe obeys.
Now that you've practised balancing equations and used scales to check reactions, revisit your initial reaction to that idea. Were you convinced straight away? What did you calculate or observe in this lesson that made conservation of mass feel real?
1. According to the law of conservation of mass, what happens to mass during a chemical reaction in a closed system?
2. Why does a piece of iron appear to gain mass when it rusts in open air?
3. In which type of system is mass most easily observed to be conserved?
4. When baking soda reacts with vinegar, bubbles form and the mixture fizzes. If the reaction is done in an open beaker, why does the mass seem to drop?
5. Which statement best explains conservation of mass at the particle level?