The Conservation of Mass

The concept of conservation of mass is a "great idea" that marked the end of alchemy and the beginning of chemistry as we know it. Antoine Lavosier is usually credited with its "discovery" in 1789, by doing chemical experiments in closed, sealed containers and weighing things before and after chemical or heating processes were done on the containers. But at that time there was a belief in "caloric" a "heat substance" and many wondered whether caloric had weight. People were weighing things before and after chemical processes before 1789, and the results were confusing. In some processes caloric seemed to have positive weight, in others it had negative weight.

So what came first? What motivated the experiments that led to the conservation of mass confirmation? Was it the effort to find the weight of caloric? Was it a conviction that mass ought to be conserved? Was caloric considered to be part of that conservation law?

What, particularly, motivated Lavoisier? His experimental work demolished the phlogiston/caloric theory, which had been accepted for 100 years. Was this his purpose, to confirm or deny that heat (caloric) had weight?

The best account I found on the web about this period in the history of chemistry is Rochelle Forrester's The History of Chemistry: From the Phlogiston Theory to the Periodic Table.

The Phlogiston Theory.

Seventeenth century chemists/alchemists tried to understand processes of burning and combustion: the slow physical changes of metals in air (calcination, which we now call oxidation), and the rapid burning of fuels. An example of calcination is rusting of iron. The reverse process is the conversion of iron ore to metallic iron. Calcination occured "naturally" but conversion of ores to metal could only be accomplished by the action of heating by burning wood or coal.

At this time the four elements of the Greeks had already been redefined. Air was no longer considered an element, and in the late 1600s the three that remained were called "earths":

  • Vitreous earth gave solidity to matter.
  • Fluid earth gave it liquidity.
  • Fatty earth (later called phlogiston) gave it combustibility.

Becher had redefined these into three "essences":

  • Salt or terra lapida, the essence of fixity and inertness.
  • Mercury or terra mercurialis, the essence of fluidity.
  • Sulfur or terra pinguis, the essence of combustibility.

German physician and chemist Gerog Ernst Stahl (1660-1734) in his 1697 Zymotechnia Fundamentals renames terra pinguis "phlogiston" considering it to be a single principle. In his phlogiston theory, burning of sulfur and other substances as well as calcination of metals involve loss of phlogiston. The metal had phlogiston, but the calx did not. But when the calx is reheated with carbon or other substances containing phlogiston, the calx regains phlogiston, restoring the original metal. Wood and charcoal were supposed to be rich in phlogiston. During this time few chemists were doing chemical experiments using careful measurements, so phlogiston theory seemed adequate. The theory correctly described many experiments, but there were troublesome cases, and Stahl began to doubt the phlogiston theory by the early 1700s.

The Phlogiston theory seemed to give a good model for many experiments:

  • It accounted for the similar properties of metals, because they all contained phlogiston.
  • The metals and their calxes were related, they just had different amounts of phlogiston.
  • It accounted for why candles go out when placed in a closed jar. The air becomes saturated with phlogiston.
  • A mouse dies in a closed container, or in a container where a candle has been burnt until it goes out, because the air is saturated with phlogiston.
  • Charcoal leaves very little ash when it burns because it is nearly pure phlogiston.
  • Some metal ashes convert back to metals when heated with charcoal because the charcoal restores the phlogiston to the ash.

But there were some troublesome problems and unaccountable cases.

The most perplexing problem was this: Charcoal, when burned, loses almost all its weight, leaving only a light ash. But metals gained weight when oxidizing, as Robert Boyle had shown some 50 years earlier. So in one case, the charcoal loses weight when it loses phlogiston, but in the metal, it gains weight when losing phlogiston.

Jean Rey, in 1630, found that tin gains weight when it forms a calx, by a large amount (about 25%). So how could it gain weight if if it loses phlogiston? Stahl rationalized this cleverly, by suggesting that the weight increased because air entered the metal to fill the vacuum left after the phlogiston escaped.

Some chemists entertained the notion that there might be two kinds of phlogiston, one with negative weight (levity), one with positive weight (gravity). In a way, we can credit or blame Newton for this levity idea. Chemists, influenced by the great success of Newton's work in mechanics, realized they needed to pay more attention to weight in their theories and experiments. They extended the idea of positive weight, adding the idea of levity, or negative weight. As Isaac Asimov comments: "This was a foolish notion, however, and did not last long."

Chemists were still doing chemical experiments in open containers, so they did not realize, as we do now, that the oxygen in the air was participating in the chemical reactions. So merely "weighing more carefully" was not sufficient to resolve the difficulties.

In 1772 Antoine Lavoisier showed that nonmetals burned in air gained surprisingly large amounts of weight. (Phosphorus, for example, increases its weight by a factor of about 2.3.) Such a large change convinced Lavoisier that phosphorus combines with something in air when it burns. Consistent with this hypothesis is his observation that when phosphorus burns in a small amount of air, the air's volume decreases by about 1/5th. Remember that Lavoisier had already shown that air was not inert, and was a substance to be reckoned with when doing chemistry.

Such problems stimulated more careful measurement, and experiments in which care was taken to weigh everything that might take part in the chemical reactions. One way to do this was to study the chemical reaction in a closed, sealed container. When this was done, all those puzzling mass changes that had been the subject of so much puzzlement for nearly 100 years simply disappeared! The theory that phlogiston was a material substance was dying by the 1780s. But then phlogiston was replaced with "caloric", an invisible essence but without weight. Caloric was not taken to be matter, but it could "be in" matter and could "flow" from hot bodies to cold.

The experiments of Count Rumford, and later experiments of Joule finally disposed of the caloric theory. Now the language shifted again. What had been called "caloric" was now called "heat", but even today students still think of heat as a "substance" and speak of "the heat in a body". That language is a relic of the phlogiston and caloric theories, and can lead to conceptual difficulties in understanding the true nature and mechanism of energy transfers by thermal processes.

We should note that this whole episode showed the importance of stating conservation laws with reference to a "closed system". It's somewhat surprising that that idea, so important to all of physics, was not recognized earlier. Even in the work of Newton, the idea was not explicitly emphasized, and Newton did not formulate any conservation laws, though the concept of mass conservation is implicit in his thinking and in Newton's laws of motion. All of the conservation laws, of mass, energy, momentum and angular momentum reference what happens in a closed system, one in which nothing gets in or out that can alter what happens inside.

Heat and Work compared.

There are two ways to transfer energy from one body to another:

  • A body doing work on another.
  • A body heating another.

How can one know which is happening when two bodies exchange energy? Work is done by macroscopic forces that cause a macroscopic displacement of the body acted upon. Heating is done by forces at the microscopic level, causing only microscopic displacments of the small constitutents of matter (molecules and atoms). Heating can occur without any displacement of the center of mass of the body being heated. Work always displaces the center of mass.

Web resources worth a serious look.

  • David Harrison's web pages index. University of Toronto. Many excellent documents at all college levels.
  • Benjamin Crowell's Light and Matter, another complete free online physics textbook.

As you read other books, and web resources, be alert to the fact that many of them still use the word "heat" to mean "internal thermal energy" of a body. I prefer, as do many other authors, to reserve the word "heat" for the transfer of energy from one body to another.

Go to the next chapter, Colliding balls. Energy and momentum in collisions.
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