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Your NCERT chemistry book says the law of conservation of mass was put forth by Antoine Lavoisier in 1789. That is the standard account, and our post on Lavoisier and the end of phlogiston tells it. Russian historians of chemistry point to Mikhail Lomonosov, who wrote down the same idea decades earlier and tested it with a sealed flask. This post covers what he wrote, what his experiment showed, and a worked example of the trap that caught Robert Boyle.
Who was Mikhail Lomonosov?
Lomonosov lived from 1711 to 1765. The chemistry faculty of Moscow State University, the university he planned, publishes a summary of his life and work based on the 1947 biography by chemist and historian Boris Menshutkin, issued by the USSR Academy of Sciences. It calls him the first Russian academician in chemistry.
According to that source, Lomonosov repeatedly urged the courtier Ivan Shuvalov to found a university in Moscow and wrote detailed plans for it. The university opened on 12 January 1755, and its first professors were his students.
What did he say about matter in 1748?
The faculty's chapter on Lomonosov and the conservation law says he first stated the idea clearly in a letter to the mathematician Leonhard Euler on 5 July 1748. He used the same words in 1758 in a dissertation for the Academy, and in a 1760 essay on the solidity and fluidity of bodies. In paraphrase: every change in nature is such that whatever is taken from one body is added to another, so if matter decreases in one place it increases somewhere else. He added that the same rule governs motion: a body that moves another loses as much of its own motion as it gives.
Menshutkin notes that philosophers had long assumed matter could not be created or destroyed. What was new was treating it as the foundation of chemistry, which needed experiments with a balance.
What was wrong with Boyle's experiment?
The same chapter describes the problem. When tin or lead is heated in air it turns into a powdery calx (we would say oxide), and the calx weighs more than the metal. In 1673 Robert Boyle sealed lead in a glass retort, heated it, opened it, and weighed it. He noticed air rushing in with a whistle when he broke the seal, and found a gain in weight. He explained the gain by "fire particles" passing through the glass and joining the metal.
What did Lomonosov's 1756 experiment show?
Lomonosov repeated the test in 1756, but weighed the vessel before opening it. His own note, as quoted by Menshutkin, says he worked with tightly sealed glass vessels to find whether metals gain weight from heat alone. In paraphrase, he concluded that Boyle's opinion was false, because without outside air getting in, the weight of the burnt metal stays the same.
That result points to two things: the gain comes from air combining with the metal, and the phlogiston story, in which something leaves the metal, does not fit a sealed vessel whose weight does not change. The source stresses that he reported this to the Academy's conference but did not publish it, so it stayed unknown. Lavoisier repeated Boyle's test in 1773 and went further, showing that the metal's gain equalled the loss from the air.
That review, The Law of Conservation of Matter: Lomonosov and Lavoisier by David E. Lewis, traces the law from ancient ideas to Lavoisier and addresses the 1960s arguments over Lomonosov's role. What is not in dispute is the experiment's logic, which you can check yourself.
Worked example: why opening the flask fools you
These are sample numbers for a classroom example. A sealed glass flask containing air and 1.000 g of tin weighs 85.000 g in total. It is heated until 0.200 g of the tin has turned into tin(IV) oxide, Sn + O₂ → SnO₂. Take Sn = 118.7 g/mol and O₂ = 32.0 g/mol.
Moles of tin reacted: 0.200 / 118.7 = 0.001685 mol.
Oxygen used: the same number of moles of O₂, so 0.001685 × 32.0 = 0.0539 g.
Inside the sealed flask: the solid gains 0.0539 g and the air loses 0.0539 g. The flask still weighs 85.000 g. This is Lomonosov's result.
Now open it: air rushes in to replace the oxygen used up. The same number of moles enters, and air averages about 29 g/mol, so about 0.001685 × 29 ≈ 0.049 g of air comes in. The open flask weighs about 85.049 g. This is Boyle's "gain".
The gain is real, but it is air from outside, not something created by fire.
Common mistakes: treating an open system as closed, and saying mass "increased" when a substance has only combined with something you did not weigh.
If you keep losing marks on mass and mole calculations, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. The NCERT Class 11 chapter Some Basic Concepts of Chemistry sets out the law itself.
What can a student take from Lomonosov's way of working?
Weigh the whole system. The right boundary turned a puzzle into a law.
Test a famous result. He repeated Boyle's experiment instead of accepting it.
Publish what you find. Work that stays in a drawer cannot change a field.
Next step: redo the worked example with 0.500 g of tin reacting, and predict the reading after the flask is opened.
