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This is intuitively obvious for simple mechanical actions:  If you take a coffee maker apart, the pieces weigh as much as the original coffee maker.  For chemical transformations, this is not so obvious.

This is intuitively obvious for simple mechanical actions:  If you take a coffee maker apart, the pieces weigh as much as the original coffee maker.  For chemical transformations, this is not so obvious.

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 18th and 19th 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 are 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.

But when analyzing nuclear reactions, the potential-energy-equivalent mass becomes (barely) measurable, and, when methods of measuring atomic weights became sufficiently accurate in the 1910s and 1920s, it was noticed.

But when analyzing nuclear reactions, the potential-energy-equivalent mass becomes (barely) measurable, and, when methods of measuring atomic weights became sufficiently accurate in the 1910s and 1920s, it was noticed.

For example, a Radium atom has a mass (atomic weight) of 226.0254098 amu (atomic mass units).  It decays by [[alpha decay]] into a Radon atom (222.0175777 amu) and a Helium atom (alpha particle, 4.0026033 amu.)  The discrepancy is .0052288 amu.  This is the potential-energy-equivalent mass of the Radium atom.  That is, .0052288 amu of the Radium atom's mass was this potential-energy-equivalent mass.  It disappeared when the potential energy was converted into kinetic energy.  The various conversion factors among amu, kilograms, Joules, and electron volts, along with the conversion factor of 1.11<math>\times</math>10^-17 given above, yields a potential energy loss of 4.870596 MeV.  This got converted into the kinetic energy of both the alpha particle and the recoiling Radon atom.  Using the laws of conservation of energy and of momentum, one can calculate the ratio of alpha energy to recoil energy; it is the same as the ratio of Radon atom mass to Helium atom mass&mdash;55.5 to 1.  Hence the recoil energy of the Radon atom is expected to be about 0.0868 Mev, and the expected alpha particle kinetic energy is 4.784 MeV.  This is the observed alpha particle energy.

For example, a Radium atom has a mass (atomic weight) of 226.0254098 amu (atomic mass units).  It decays by [[alpha decay]] into a Radon atom (222.0175777 amu) and a Helium atom (alpha particle, 4.0026033 amu.)  The discrepancy is .0052288 amu.  This is the potential-energy-equivalent mass of the Radium atom.  That is, .0052288 amu of the Radium atom's mass was this potential-energy-equivalent mass.  It disappeared when the potential energy was converted into kinetic energy.  The various conversion factors among amu, kilograms, Joules, and electron volts, along with the conversion factor of 1.11<math>\times</math>10⁻¹⁷ given above, yields a potential energy loss of 4.870596 MeV.  This got converted into the kinetic energy of both the alpha particle and the recoiling Radon atom.  Using the laws of conservation of energy and of momentum, one can calculate the ratio of alpha energy to recoil energy; it is the same as the ratio of Radon atom mass to Helium atom mass&mdash;55.5 to 1.  Hence the recoil energy of the Radon atom is expected to be about 0.0868 Mev, and the expected alpha particle kinetic energy is 4.784 MeV.  This is the observed alpha particle energy.

The Law of Mass Conservation is believed by some to be in conflict with religious sensibilities, apparently relating to the initial creation of the universe, or its ultimate fate.  While the exact circumstances and causes of the "big bang" are discussed by cosmologists and philosophers, the Law of Mass Conservation is accepted for everything in between.

The Law of Mass Conservation is believed by some to be in conflict with religious sensibilities, apparently relating to the initial creation of the universe, or its ultimate fate.  While the exact circumstances and causes of the "big bang" are discussed by cosmologists and philosophers, the Law of Mass Conservation is accepted for everything in between.

==See Also==

==See also==

*[[Conservation of Energy]]

*[[Conservation of Energy]]