Law of Conservation of Mass: Statement, Formula and Examples
Chemistry · Some Basic Concepts Of Chemistry · NEET
The Law of Conservation of Mass says that in any physical or chemical change, matter is neither created nor destroyed. So the total mass of the reactants (things you start with) is always equal to the total mass of the products (things you get). Antoine Lavoisier proved this in 1789 by carefully weighing substances before and after reactions. Memory hook: "Atoms only rearrange, they never disappear, so mass stays the same."
In a sealed container, calcium carbonate (100 g) breaks into calcium oxide (56 g) and carbon dioxide gas (44 g). The products together still weigh 100 g, showing that mass is conserved when the escaping gas is also counted.
Your doubts, answered
What is the exact statement of the Law of Conservation of Mass?
In any physical or chemical change, matter can neither be created nor destroyed. This means the total mass stays the same before and after the change. So: total mass of reactants = total mass of products. Antoine Lavoisier gave this law in 1789 after carefully weighing substances in combustion reactions.
If a gas is made during a reaction, is mass still conserved?
Yes. Mass is still conserved, but you must catch the gas too. If you burn something in an open dish, the gas escapes into the air, so the leftover solid weighs less and it looks like mass was lost. But if you do the reaction in a closed, sealed container, the mass of everything (solid + gas) stays exactly the same. This closed-container idea is why NEET questions often say 'closed vessel'.
How is the Law of Conservation of Mass linked to Dalton's atomic theory?
Dalton said atoms are tiny particles that cannot be created or destroyed in a chemical reaction. In a reaction, atoms only break apart and join in new ways. Since the same atoms are present before and after, the total mass cannot change. So Dalton's atomic theory explains the Law of Conservation of Mass. NEET note: Dalton's theory also explains the laws of definite and multiple proportions, but it could NOT explain Gay-Lussac's law of gaseous volumes.
What is the formula I use in numericals?
There is no complicated formula. Just write: mass of reactants = mass of products. For example, in CaCO3 -> CaO + CO2 done in a closed vessel, mass of CaCO3 = mass of CaO + mass of CO2. If you know any two of these masses, you can find the third by simple subtraction.
Give one simple example to prove the law.
Heat 100 g of calcium carbonate (CaCO3) in a sealed container. It breaks into calcium oxide (CaO) and carbon dioxide (CO2). If you collect all the CO2 gas, the CaO plus the CO2 together weigh exactly 100 g. Nothing is lost; the mass just moved into two products instead of one.
⚠️ The NEET trap ✗ Dalton's atomic theory could not explain the Law of Conservation of Mass. ✓ Dalton's atomic theory DOES explain the Law of Conservation of Mass (and definite and multiple proportions). The one law it could NOT explain is Gay-Lussac's Law of Gaseous Volumes. 🧠 Dalton is friends with the 3 'proportion/conservation' laws; he only fails at the GAS VOLUME law. So if the question asks 'which law Dalton could not explain', pick the gas one, not conservation of mass.
Real NEET questions
NEET 2025
Dalton's atomic theory could NOT explain which of the following?
A · Law of multiple proportions
B · Law of gaseous volumes ✓
C · Law of conservation of mass
D · Law of constant proportions
Solution: Dalton's atomic theory successfully explains three laws: the Law of Conservation of Mass, the Law of Constant (definite) Proportions, and the Law of Multiple Proportions, because atoms are neither created nor destroyed, only rearranged. It could NOT explain Gay-Lussac's Law of Combining Gaseous Volumes. That law was later explained by Avogadro's hypothesis: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. So the correct choice is (B).
NEET 2019
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through the Haber process is
A · 10
B · 20
C · 30 ✓
D · 40
Solution: Balanced equation: N2 + 3H2 -> 2NH3. Because atoms are conserved (Law of Conservation of Mass), the equation must be balanced: 2 mol NH3 need 3 mol H2. For 20 mol NH3, moles of H2 = (3/2) x 20 = 30 mol. Answer (C). This shows why we balance equations: the number of atoms of each element, and hence total mass, must be equal on both sides.
Solved Some Basic Concepts Of Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Who proposed the Law of Conservation of Mass and in which year?
Antoine Lavoisier proposed it in 1789, based on careful weighing of reactants and products in combustion reactions.
Does the Law of Conservation of Mass apply to physical changes too?
Yes. It applies to both physical changes (like melting ice or dissolving salt) and chemical changes (like burning or a reaction). In all of them, the total mass stays the same.
Why do we balance chemical equations?
We balance equations so the number of atoms of each element is the same on both sides. This directly follows from the Law of Conservation of Mass, since atoms (and therefore mass) cannot be created or destroyed.
Is the Law of Conservation of Mass ever broken?
For NEET chemistry, treat it as always true. It only appears to break in nuclear reactions, where a tiny bit of mass changes into energy (E = mc^2), but that is not part of the basic chemistry syllabus.
What is the difference between conservation of mass and the law of definite proportions?
Conservation of mass says total mass before = total mass after a reaction. The law of definite proportions says a given compound always contains the same elements in the same fixed mass ratio. They are two different laws of chemical combination.
