In any chemical reaction, the total mass of the products always equals the total mass of the reactants — mass is neither created nor destroyed. This is the law of conservation of mass: atoms are rearranged into new substances, but every atom present at the start is still present at the end, just in a different arrangement.
What is the law of conservation of mass?
The law of conservation of mass states that the total mass of substances before a chemical reaction equals the total mass of substances after. No atoms are gained, no atoms are lost — they are simply rearranged.
This principle was formalised by the French chemist Antoine Lavoisier in 1789, following careful quantitative experiments. Before his work, the phlogiston theory incorrectly explained burning — Lavoisier's precise mass measurements showed that oxygen from the air combines with burning materials, and that all the mass is accounted for if gases are included.
Why must mass be conserved?
At the particle level, the reason is simple: atoms cannot be created or destroyed in a chemical reaction. A balanced chemical equation shows exactly this — the same atoms appear on both sides, arranged differently:
Worked example — Iron reacting with sulfur:
Fe + S → FeS
| Substance | Formula | Mass |
|---|---|---|
| Iron (reactant) | Fe | 56 g |
| Sulfur (reactant) | S | 32 g |
| Iron sulfide (product) | FeS | 88 g |
Total mass of reactants = 56 + 32 = 88 g Total mass of products = 88 g ✓
The 56 iron atoms and 32 sulfur atoms from the reactants are all present in the product — they have simply bonded together in a different arrangement.
Why do some reactions appear to gain or lose mass?
In a closed container, mass is always perfectly conserved. But when reactions are carried out in open containers, the apparent mass measured on a balance can change — not because mass is actually lost or gained, but because gases enter or leave the system:
Apparent mass decrease — a gas is produced and escapes: When calcium carbonate (marble chips) is added to hydrochloric acid, carbon dioxide gas is produced and bubbles off into the air:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
The CO₂ escapes from the open flask, so the mass reading on the balance falls. Mass is still conserved in total — the missing mass is in the CO₂ now in the surrounding air — but the balance only measures what is in the flask.
Apparent mass increase — a gas is absorbed from the air: When magnesium burns in air, it reacts with oxygen:
2Mg + O₂ → 2MgO
The solid product (magnesium oxide) is heavier than the original magnesium ribbon because oxygen atoms from the air have been incorporated into it. If you weigh the product, the mass is greater than the starting magnesium — but total mass is conserved because the oxygen mass has been transferred from the air into the solid.
How can you use conservation of mass to find a missing mass?
If you know all masses except one, you can calculate the unknown:
Worked example: 10 g of magnesium is burned in excess oxygen. The product, magnesium oxide, has a mass of 16.7 g. What mass of oxygen was used?
Mass of oxygen used = mass of product − mass of magnesium = 16.7 − 10.0 = 6.7 g
This works because mass of reactants = mass of products: Magnesium + Oxygen → Magnesium oxide 10 g + 6.7 g = 16.7 g ✓
Another example: 100 g of limestone (calcium carbonate) is heated strongly. The limestone decomposes:
CaCO₃ → CaO + CO₂
If 56 g of calcium oxide (quicklime) is produced, what mass of CO₂ was released? Mass of CO₂ = 100 − 56 = 44 g (this matches Mr(CO₂) = 44, confirming the calculation).
How does conservation of mass relate to balancing equations?
A balanced chemical equation is the mathematical expression of the law of conservation of mass. The coefficients in front of each formula ensure the same number of each type of atom appears on both sides.
| Equation | Left side atoms | Right side atoms |
|---|---|---|
| 2H₂ + O₂ → 2H₂O | 4 H, 2 O | 4 H, 2 O ✓ |
| Fe + S → FeS | 1 Fe, 1 S | 1 Fe, 1 S ✓ |
| 2Mg + O₂ → 2MgO | 2 Mg, 2 O | 2 Mg, 2 O ✓ |
An unbalanced equation would imply that atoms had been created or destroyed — which is physically impossible. Balancing is not a mathematical trick; it reflects the fundamental reality that atoms are conserved.
Frequently asked questions
What does the law of conservation of mass state?
The law of conservation of mass states that in any chemical reaction, the total mass of the products always equals the total mass of the reactants. Atoms are not created or destroyed — they are rearranged. The law was formalised by Antoine Lavoisier in 1789 and is a cornerstone of chemistry because it means all chemical equations can and must be balanced: the same atoms appear on both sides.
Why does the mass appear to decrease when marble chips react with acid?
When marble chips (calcium carbonate) react with hydrochloric acid, one of the products is carbon dioxide gas. If the reaction is carried out in an open container on a balance, the CO₂ bubbles out into the surrounding air and is no longer on the balance. The balance reading falls — not because mass has been destroyed, but because mass (as CO₂) has left the container. If all the CO₂ were collected and weighed, the total mass would equal the starting mass of the acid and marble chips.
Why does magnesium gain mass when it burns?
When magnesium burns, it reacts with oxygen from the air: 2Mg + O₂ → 2MgO. The product, magnesium oxide, contains both magnesium and oxygen atoms, so it is heavier than the original magnesium. Mass has not been created — oxygen atoms from the air have been incorporated into the solid product, and the total mass of magnesium plus oxygen used still equals the mass of magnesium oxide produced. This is a classic example of why it is important to include all substances (including gases) when checking conservation of mass.
How do you use conservation of mass to solve exam questions?
The key principle is: total mass of reactants = total mass of products. If a question gives you the masses of all but one substance, rearrange this equation to find the unknown. Identify which substance's mass is missing, sum the masses of everything else on that side, and subtract from the total on the other side. Be alert to whether the missing substance is a gas that has escaped (product side) or a gas absorbed from the air (reactant side).
For Socratic KS3 chemistry with Professor Curie — reasoning from particle picture to conservation law before writing any equation — visit aitutors.me.