The Law of Conservation of Mass (or Matter) in a chemical reaction can be stated thus:
In a chemical reaction, matter is neither created nor destroyed.
It was discovered by Antoine Laurent Lavoisier (1743-94) [15K GIF] about
1785. However, philosophical speculation and even some quantitative
experimentation preceeded him. In addition, he was certainly not the first to
accept this law as true or to teach it, but he is credited as its discoverer.
Pre-history leading up to Lavoisier
Anaxagoras, circa 450 B.C. said:
"Wrongly do the Greeks suppose that aught begins or ceases to be; for nothing comes into being or is destroyed; but all is an aggregation or secretion of pre-existing things; so that all becoming might more correctly be called becoming mixed, and all corruption, becoming separate."
Circa 1623, Francis Bacon wrote:
"Men should frequently call upon nature to render her account; that is, when they perceive that a body which was before manifest to the sense has escaped and disappeared, they should not admit or liquidate the account before it hs been shown to them where the body has gone to, and into what it has been received."
Joseph Black (1728-1799) made extensive studies of the carbonates of the alkali and alkaline earth metals and is considered the discoverer of carbon dioxide (which he called "fixed air"). In 1752, he wrote the following, which will be explained below:
"A piece of perfect quicklime, made from two drams of chalk, and which weighed one gram and eight grains, was reduced to a very fine powder, and thrown into a filtered mixture of an ounce of a fixed alkaline salt and two ounces of water. After a slight digestion, the powder being well washed and dried, weighed one dram and fifty-eight grains. It was similar in every trial to a fine powder of ordinary chalk, and was therefore saturated with air which must have been furnished by the alkali."
I want you to notice that the quicklime came from two drams of chalk and at the end he produced one dram and 58 grains of chalk. Since one dram = 60 grains, we can see there is a difference of only 2 grains. As best as I can tell, one grain is equal to a modern value of about 0.4 grams. Here in modern terms, are the chemical reactions Black carried out:
He made lime (CaO) from chalk (CaCO3) by heating it:
CaCO3 ---> CaO + CO2
Then, he reacted the lime with an excess of fixed alkali (K2CO3) and got back chalk:
CaO + K2CO3 ---> CaCO3 + K2O
K2O is potassium oxide (in modern terms) and in the water would react to produce KOH, which was called caustic alkali.
Black was interested in showing that the weight change from chalk to lime was only due to the loss of fixed air and he never went beyond that. In fact, right before the above quote is this:
"With respect to the second proposition, . . . ."
That second proposition is as follows:
"If quick-lime be no other than a calcarious earth deprived of its air, and whose attraction for fixed air is stronger than that of alkalis, it follows that, by adding to it a sufficient quantity of alkali saturated with air, the lime will recover the whole of its air, and be entirely restored to its original weight and condition . . . "
I'm not a true historian of chemistry, but I don't think Black missed the "big picture" because he was so focused on his own agenda. The spirit of careful, quantitative measurements in chemistry was, in the mid-1700's, still fairly new. Black was a careful experimenter, but I believe he was too early in the game, so to speak, to recognize the Law of Conservation of Mass. To the modern eye, his work is clear evidence for the Law of Conservation of Mass, but Black just never got to that point.
Henry Cavendish (1731 - 1810) was one of the great chemists of the eighteenth century (or any other century for that matter). Among his many discoveries was the composition of water and the recognition that atmospheric air was a mixture of nitrogen and oxygen in constant proportion. In 1784, he wrote the following:
"In Dr. Priestley's last volume of experiments is related an experiment of Mr. Warltire's, in which it is said that, on firing a mixture of common and inflammable air by electricity in a close[d] copper vessel holding about three pints, a loss of weight was always perceived, on an average about two grains, though the vessel was stopped in such a manner that no air could escape by the explosion . . . . [This experiment], if there was no mistake in it, would be very extraordinary and curious; but it did not succeed with me . . . though the experiment was repeated several times with different proportions of common and inflammable air, I could never perceive a loss of weight of more than one-fifth of a grain, and commonly none at all."
Cavendish adds a footnote one sentence later saying: "Dr. Priestley, I am informed, has since found the experiment not to succeed." remember also that one gran equals about 0.4 gram, so Canvendish, in the above quote, was discussing a weight difference of about 0.08 grams.
Cavendish is famous even today for the careful, meticulous nature of his work, but he also missed credit for announcing the Law of Conservation of Mass. I think it was because he was taken with other things. For example, just two paragraphs after the above is written, Cavendish begings discussing the fact that common air (in modern terms, the atmosphere) consistently has a maximum reduction in volume of about one-fifth after reacting with inflammable air (in modern terms, hydrogen gas).
Today, we know that the atmosphere is about 79% nitrogen and almost 21% oxygen, with small amounts of other gases (carbon dioxide, water, argon, etc.). In 1784, this was a very, very important discovery.
However, notice how he says "extraordinary and curious" in the above quote. He must have had some awareness of what we now call the Law of Conservation of Mass, but he never announced it as a proven, scientific principle.
The work of Lavoisier
Lavoisier wrote in 1785:
"Nothing is created, either in the operations of art or in those of nature, and it may be considered as a general principle that in every operation there exists an equal quantity of matter before and after the operation; that the quality and quantity of the constituents is the same, and that what happens is only changes, modifications. It is on this principle that is founded all the art of performing chemical experiments; in all such must be assumed a true equality or equation between constituents of the substances examined, and those resulting from their analysis."
At this point, he was well into his scientific career. It turns out he had assumed the validity of the law and then assembled ample verification of it before making a formal announcement. However, there is an important point related to Lavoisier and the law. As one historian in 1914 wrote:
"What Lavoisier did, was to assume this permanency of weight to apply to the substances with which chemists dealt, and to be independent of the effect of heat, till then supposed by many to be ponderable." Ponderable means to have weight.
In 1890, another historian wrote:
"Lavoisier established a radical different between on the one hand ponderable matter, . . . matter of which the balance proved the invariability before, during, and after combustion; and on the other hand, the igneous fluid, of which the introduction from an outside source, or the withdrawal during combustion,, neither increased nor diminished the weight of substances; contrary to what the partisans of phlogiston has thought."
Lavoisier was able to establish that heat played no role in adding or decreasing weight, as had been claimed by the phlogiston theory. This is not the place to discuss phlogiston, except to say it was a chemical theory that had lasted about 100 years and was decisively destroyed by the work of Lavoisier. (Lavoisier's prime scientific rival, Joseph Priestley of England, accepted the phlogiston theory.)
Lavoisier was able to assemble a number of experiments, all done in closed vessels, in which the weight remained constant, within experimental error. This included tin or lead being reacted with oxygen as well as the analysis of mercury calx (HgO). Over the years of his work, Lavoisier had several large burning lenses (which focused the sun's rays), constructed and these were instrumental in reaching the high temperatures need to cause the chemical reactions to take place. (Lavoisier was also able to burn a diamond with a large lens and show that only CO2 was produced.)
Your teacher will never require you to know the history involved, but will probably test this statement:
In a chemical reaction, matter is neither created nor destroyed.
It is the Law of Conservation of Mass. Antoine Laurent Lavoisier is its discoverer.
Final comments on the science involved
The manner in which the Law of Conservation of Mass was discovered did not follow the usual "scientific" way that is taught to students. Lavoisier DID NOT arrive at the law by induction, that is generalizing from a large number of specific cases. There was simply not enough data for him to do this.
What Lavoisier did was to ASSUME the validity of the law during the course of his work and then let the verification come from the fact that deductions from the law always - within experimental error - showed the deduction to be correct. Another way to say it is to say that, again within experimental error, the results of a complete analysis of a substance ALWAYS add up to 100% of the starting material.
What is interesting is the issue of experimental error. Suppose an experiment is performed in which mass is lost or gained. This IS NOT taken as evidence of the failure of the law, but as a failure of the experiment. At least at the beginning, a person like Lavoisier must have had a very strong, almost unscientific, belief that he was right, no matter what the data showed or didn't show.
This happened in 1905 with Einstein and the special theory of relativity. The very first scientific article which dealt with relativity after Einstein announced it was by a man named Walter Kaufmann and it CONCLUSIVELY refuted Einstein, showing him to be incorrect. Einstein was undeterred by this, stuck to his guns and was shown to be correct, to the point where everybody knows who Einstein was and hardly anybody remember Kaufmann.
This belief in the correctness of your conclusion also guided Robery Milikian in 1913, when he determined the charge on the electron to a very high degree of accuracy. His laboratory notebooks are littered with comments like "bad value" or "something wrong, don't use." How did he KNOW a given experimental run produced a poor value? He must have had some idea of where he wanted to go before going there and this affected his selection of data.
If you have a desire to go into a scientific field for your career, I urge you to learn about the history of your chosen area. There are many lessons to learn from those who went before us about how science is done.
Very, very last comment
There actually is a better law called the Law of Conservation of Mass-Energy. Conservation of mass was amended due to the discovery of E = mc2 by Einstein. We also know that 100 kJ = about 10¯9 gram and in these modern times, that is very near to the detection limit of some of the better mass spectroscopy instruments in the world. I have heard that this tiny mass loss (actually conversion of mass to energy) in a chemical reaction has been detected, but I do not have a journal reference for this.
You may wish to research how the validity of the Law of Conservation of Mass was called into question in the early part of the 1900's and how the idea of the neutrino is related to this.