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The law of conservation of mass is the first of the laws of chemical combination in NCERT Class 11 Chemistry, and most students learn it in one line. The story behind it is more useful than the line: a chemist who refused to ignore the weight of gases, and in doing so ended a theory that most of his colleagues believed. This post covers what Antoine Lavoisier did, the exact words he used, and how to apply the law in numericals where mass seems to appear or vanish.
Who was Lavoisier?
The Science History Institute biography describes a wealthy Parisian, born in 1743, who took a law degree in 1763 but spent his student years attending science courses. In 1768 he was elected to the Académie des Sciences and joined the Ferme Générale, a private company that collected taxes for the monarchy. From 1775, as inspector of royal gunpowder, he had a well-equipped laboratory at the Paris Arsenal.
He did not work alone. His wife, Marie-Anne Paulze Lavoisier, was his scientific collaborator. She drew the plates for his textbook and trained in chemistry herself.
What was phlogiston theory, and why did weighing break it?
In the 1700s many chemists explained burning with phlogiston. The idea, as the biography summarises it, was that combustible things such as charcoal and metals contained phlogiston, and released it into the air as they burned. A burned metal became a "calx".
The trouble was mass. Metals such as lead and tin got heavier when they burned. If they were losing something, why did they weigh more?
In 1774 Joseph Priestley visited Paris and described how he had made a new "air" by heating the calx of mercury. Lavoisier repeated the experiment and reversed the interpretation. The calx was not taking phlogiston out of the air; heating it released a gas, which he named oxygen. When a metal burns, it combines with oxygen, and that is why its mass goes up. He called the calx an oxide, a word we still use. With Pierre-Simon Laplace he went on to show that water is made of hydrogen and oxygen.
What exactly did Lavoisier say about mass?
He set out the principle in his 1789 textbook, Traité élémentaire de chimie. In Robert Kerr's 1790 English translation, Elements of Chemistry, the passage in the chapter on fermentation reads:
"We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment."
He added that "the whole art of performing chemical experiments" depends on this principle. NCERT Class 11 Chemistry, Unit 1 dates the law to 1789 and notes that it came from "exact measurement of masses of reactants and products".
Worked example: where does the mass go?
Heating mercury oxide (Lavoisier's reaction in reverse). 2HgO → 2Hg + O₂. Molar masses: HgO = 216.59 g/mol, Hg = 200.59 g/mol, O₂ = 32.00 g/mol.
Start with 10.83 g of HgO.
Moles of HgO = 10.83 / 216.59 = 0.0500 mol.
Moles of Hg = 0.0500 mol, so mass of Hg = 0.0500 × 200.59 = 10.03 g.
Moles of O₂ = 0.0500 / 2 = 0.0250 mol, so mass of O₂ = 0.0250 × 32.00 = 0.80 g.
Check: 10.03 + 0.80 = 10.83 g. Mass is conserved.
If you heat it in an open tube, the solid left behind weighs 0.80 g less. The mass has not vanished; it left as gas.
Burning magnesium. 2Mg + O₂ → 2MgO. Burn 2.43 g of Mg (0.100 mol, molar mass 24.31 g/mol). You get 0.100 mol MgO, which is 0.100 × 40.30 = 4.03 g. The extra 1.60 g is oxygen taken from the air, exactly the gain that puzzled phlogiston chemists.
Common mistakes:
Ignoring gases. If a product or reactant is a gas, an open container will seem to gain or lose mass.
Balancing by mass instead of atoms. Balance the equation first; mass conservation then follows.
Rounding molar masses too early. Keep at least two decimal places until the final step.
For more practice with moles and masses, see our guide to common mole concept errors.
What did Lavoisier get wrong?
He was not right about everything. The Science History Institute notes that his list of 33 elements included caloric, a weightless "principle" of heat. He named oxygen from the Greek for "acid former" because he believed oxygen was what made substances acidic. Hydrochloric acid, HCl, has no oxygen at all.
How did his life end?
His wealth came partly from the Ferme Générale, which was hated during the French Revolution. He was arrested in 1793 and guillotined in the spring of 1794, according to the same biography.
What can a student take from Lavoisier's way of working?
Keep a balance sheet. In any reaction, list what goes in and what comes out, gases included. This one habit fixes many stoichiometry errors.
Trust measurement over a popular theory. Phlogiston was widely accepted, and his scales disagreed with it.
Names carry ideas. His new names, such as oxide and hydrogen, built his theory into the language, which is why NCERT still uses them.
When a stoichiometry answer does not balance and you cannot see why, the Fermi Problem Library has a step-by-step solver that explains the reasoning, so you can find the step where the mass went missing. To see how the next generation built on Lavoisier, read our post on John Dalton's atomic theory.
Try this now: 5.00 g of calcium carbonate is heated until it becomes calcium oxide. Work out the mass of carbon dioxide released, and check that the totals match.
