Changes

This law was a development of research in the 18^th and 19^th centuries, notably [[Antoine Lavoisier]]'s experimental discovery that, when all substances were properly accounted for—specifically gases, during the process of combustion—the total weight of a closed system is not changed by a chemical reaction.  A great deal of careful research went into this law—gases are not easy to weigh.  Note that ''volume'' is not preserved in chemical reactions (or in taking apart a coffee maker).  But ''mass'' was found to be preserved.

This law was a development of research in the 18^th and 19^th centuries, notably [[Antoine Lavoisier]]'s experimental discovery that, when all substances were properly accounted for—specifically gases, during the process of combustion—the total weight of a closed system is not changed by a chemical reaction.  A great deal of careful research went into this law—gases are not easy to weigh.  Note that ''volume'' is not preserved in chemical reactions (or in taking apart a coffee maker).  But ''mass'' was found to be preserved.

As an example of this in the field of chemistry, the combination of 22.99 grams of [[Sodium]] with 35.45 grams of [[Chlorine]] (those quantities one [[mole]] each, or [[Avogadro's number]] of atoms) yields 58.44 grams of Sodium Chloride.

As an example of this in the field of chemistry, the combination of 22.99 grams of [[Sodium]] with 35.45 grams of [[Chlorine]] (those quantities are one [[mole]] each, or [[Avogadro's number]] of atoms) yields 58.44 grams of Sodium Chloride.

Over time, the understanding of this principle has developed.  But in the early twentieth century it became clear that radioactivity seemed to violate both the Law of Mass Conservation and the classic "Law of Conservation of Elements".  It was found that small changes in mass occur when elements undergo radioactive disintegration.  The effect was only observed for nuclear transformations.  For a while this led to the belief that the law does not apply to nuclear reactions, and holds only for other phenomena.  This may still be taught in some elementary-schools.

Over time, the understanding of this principle has developed.  But in the early twentieth century it became clear that radioactivity seemed to violate both the Law of Mass Conservation and the classic "Law of Conservation of Elements".  It was found that small changes in mass occur when elements undergo radioactive disintegration.  The effect was only observed for nuclear transformations.  For a while this led to the belief that the law does not apply to nuclear reactions, and holds only for other phenomena.  This may still be taught in some elementary-schools.