The law of 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 Lavoisier 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.

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 occurred "naturally" but conversion of ores to metal could only be accomplished by the action of heating by burning wood or coal.

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 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.

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",

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.

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 displacements of the small constituents 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.