Conservation of mass law

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The law of conservation of mass (sometimes also called Lomonossow - Lavoisier law) is a law of conservation in chemistry . It says that in chemical reactions the total mass of the substances involved does not change significantly.

In the case of physical processes, this sentence only applies to closed systems , i.e. only to systems that neither absorb energy nor release it into the environment.


Usually the first explicit formulation is attributed to Antoine Lavoisier, who also checked the conservation of mass in numerous experiments with scales. He formulated the principle explicitly in his famous Traitè Élementaire de Chimie of 1789, but used it much earlier. Preservation of the mass was a principle of nature that was widely accepted long before Lavoisier and which goes back to antiquity and was used as a matter of course by Joseph Black , Henry Cavendish or Michail Lomonossow . It was formulated even earlier by Jean Rey in 1630. Partington writes in his history of chemistry that it goes back to antiquity and cites formulations by Edme Mariotte (1678) and Jean Pierre Chardenon (1764). Johan Baptista van Helmont (1580–1644) was also a supporter of mass conservation. Based on his “ willow tree experiment ”, he concluded that the growth of the willow tree came from water alone and that water was therefore the universal element of all natural things. Herbert M. Howe had also noted that the experiment had already been proposed by Nikolaus von Kues in his work Idiota de staticis experimentis (1450) and by the author of the Pseudo-Clementine Recognitiones (1504).

Lomonosov was also an early proponent of mass conservation. Soviet scientists later argued that he had formulated the Law of Conservation of Masses before Lavoisier (for example in the New Commentaries of the Petersburg Academy, published in 1750), that Lavoisier had taken it over from Lomonosov and that Lomonosov had also underpinned it experimentally (around 1756). All of these claims were rejected by Philip Pomper. Lomonosov criticized Robert Boyle's experiment in calcining metals (the term calcination then simply referred to dry heating in air). Boyle incorrectly stated that he had observed an increase in the weight of the metal on calcination in closed vessels and attributed the fire particles to the metal, which Lomonosov rejected. Instead, Lomonosov used a strange explanation for the calcination of metals in closed vessels: he believed that the same masses could have different weights if their surfaces were different (more surface area for the gravitational fluid). According to Lomonossow, this was the case with incineration in closed vessels, since the calcination would loosen the connection between the particles and increase the surface area. He also advocated this in a letter to Leonhard Euler in 1748. Lomonossow, however, correctly attributed the weight gain with normal calcination in the presence of air to the uptake of constituents of the air.

Together with the “ law of constant proportions ” ( Proust , 1797) and the “ law of multiple proportions ” ( Dalton 1808), the law of conservation of mass turned the then still young science of chemistry into a quantitative science. The three laws were theoretically supported by Dalton's atomic hypothesis.

The conservation of mass was experimentally confirmed at the beginning of the 20th century by Hans Landolt and Roland von Eötvös , each with a high level of measurement accuracy. On the other hand, Albert Einstein's special theory of relativity soon contradicted the strict validity of the theorem, since this theory results in the equivalence of mass and energy . This equivalence has now been confirmed many times by experiment. Therefore, the law of conservation of mass can not be applied in nuclear and high energy physics .

Content and meaning

In a modern formulation the sentence reads: In a chemical reaction in a closed system, the sum of the masses of the starting materials is equal to the sum of the masses of the products . This applies to such a good approximation that the theorem remains valid for purposes of chemistry.

An application example from everyday life: If you want to know how much carbon dioxide a car emits per kilometer driven, you just need to know the type of fuel and the fuel consumption. One liter of diesel contains around 700 grams of carbon, which combines with 1880 grams of oxygen to form 2580 grams of carbon dioxide. If the car has a consumption of 6 liters of diesel per 100 km, it inevitably follows that it emits 155 g CO 2 / km.

Mass defect

In fact, in combustion, the mass of the reaction products is a negligibly small fraction smaller than the mass of the reactants before the reaction, because the energy released in exothermic chemical reactions is binding energy released . According to the mass-energy equivalence, this energy has a mass. The total mass of the substances involved does not remain the same, but rather decreases due to the mass defect , although this effect is very small. When burning 1000 grams of carbon with 2664 grams of oxygen ( calorific value 32.8  M J ), 0.364 µ g theoretically “disappear” due to the mass defect , i.e. H. 9.95 · 10 −11 (99.5 trillionths) of mass.

In certain physical particle reactions, namely the electron-positron pair annihilation , the original mass even "disappears" completely and two photons are created with a mass of zero.

A mass defect also occurs with every exothermic nuclear reaction and with every radioactive decay : the sum of the masses of the resulting particles is smaller than the sum of the initial masses. Since the rest energy can be read from the mass , the atomic nuclei or particles that are created have less rest energy than the initial nuclei or particles. The total energy is retained, but not the mass. In particular, the energy released from the mass defect by nuclear fission and nuclear fusion reactions can be used technically after conversion into heat .


  • William R. Newman, Lawrence M. Principe: Alchemy Tried in the Fire: Starkey, Boyle and the Fate of Helmontian Chemistry. Chicago University Press, Chicago / London 2002, ISBN 0-226-57711-2 .

Individual evidence

  1. Lavoisier, Traité, Volume 1, p. 141 (in the 1801 edition). ...; et l'on peut poser en principes que dans toute opération, il ya une égale quantité de matière avant et apres l'operation; que la qualité et la quantité des principes et la meme, et qu'il n'y a peu des changements, des modifications .
  2. ^ Robert Whitaker, A historical note on the conservation of mass, Vol. 52, 1975, pp. 658-659
  3. Partington, History of Chemistry, Volume 3, Macmillan 1962, p. 377
  4. ^ WR Newman, LM Principe: Alchemy Tried in the Fire: Starkey, Boyle and the Fate of Helmontian Chemistry . 2002, p. 68 ff .
  5. ^ Johan Baptista van Helmont: Ortus Medicinae, id est initia physicae inaudita. Progressus medicinae novus, in morborum ultionem ad vitam longam . Ed .: Franciscus Mercurius van Helmont . Elzevir, Amsterdam 1648, p. 108-109 .
  6. ^ WR Newman, LM Principe: Alchemy Tried in the Fire: Starkey, Boyle and the Fate of Helmontian Chemistry . 2002, p. 13 .
  7. HE Hoff: NICOLAUS OF CUSA, VAN HELMONT, AND BOYLE: THE FIRST EXPERIMENT OF THE RENAISSANCE IN QUANTITATIVE BIOLOGY AND MEDICINE. In: Journal of the history of medicine and allied sciences. Volume 19, April 1964, pp. 99-117, doi : 10.1093 / jhmas / xix.2.99 , PMID 14143941 .
  8. ^ HM Howe: A Root of van Helmont's Tree. In: Isis . Volume 96, Number 4, 1965, pp. 408-419.
  9. Pomper, Lomonosov and the discovery of the law of the conservation of matter and in chemical transformations, Ambix, Volume 10, 1962, pp. 119-127
  10. A similar description can be found in Partington, A History of Chemistry, Macmillan 1962, Volume 3, pp. 203f
  11. ^ Holleman-Wiberg: Textbook of inorganic chemistry. 57-70 Edition, de Gruyter, 1964, pp. 11-12.