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Conservation of mass
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The law of conservation of mass/matter, also known as law of mass/matter conservation says that the mass of a closed system will remain constant, regardless of the processes acting inside the system. An equivalent statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
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The law of conservation of mass/matter, also known as law of mass/matter conservation says that the mass of a closed system will remain constant, regardless of the processes acting inside the system. An equivalent statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. This is also the central idea behind the first law of thermodynamics.
The law of "matter" conservation (in the sense of conservation of particles) may be considered as an approximate physical law that holds only in the classical sense, before the advent of special relativity and quantum mechanics. Another difficulty with conservation of "matter" is that "matter" is not a well-defined word in science, and when particles which all consider to be "matter" (such as electrons and positrons) are anihilated to make photons (which are often not considered matter) then conservation of matter does not hold, even in closed systems.
Mass is also not generally conserved in open systems, when various forms of energy are allowed into, or out of, the system (see for example, binding energy). However, the law of mass conservation for closed systems, as viewed over time from any single inertial frame, continues to hold in modern physics. The reason for this is that relativistic equations show that even massless particles such as photons still add mass to closed systems, allowing mass (though not matter) to be conserved in all processes where energy does not escape the system.
The historical concept of both matter and mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics. In modern physics, only mass conservation for closed systems continues to hold exactly.
Historical development and importance
Beginnings of the theory of conservation of mass were stated by Epicurus (341-270 BC). In describing the nature of the universe he wrote: "the sum total of things was always such as it
is now, and such it will ever remain," and that nothing is created from nothing, and nothing that disappears ceases to exist.
An early yet incomplete theory of the conservation of mass was stated by Nasir al-Din al-Tusi (1201-1274) in the 13th century. He wrote that a body of matter is able to change, but is not able to disappear.
The law of conservation of mass was first clearly formulated by Antoine Lavoisier (1743-1794) in 1789, who is often for this reason (see below) referred to as a father of modern chemistry. However, Mikhail Lomonosov (1711-1765) had previously expressed similar ideas in 1748 and proved them in experiments. Others who anticipated the work of Lavoisier include Joseph Black (1728-1799), Henry Cavendish (1731-1810), and Jean Rey (1583-1645).
Historically, the conservation of mass and weight was kept obscure for millennia by the buoyant effect of the Earth's atmosphere on the weight of gases. For example, since a piece of wood weighs less after burning, this seemed to suggest that some of its mass disappears, or is transformed or lost. These effects were not understood until careful experiments in which chemical reactions such as rusting were performed in sealed glass ampules, wherein it was found that the chemical reaction did not change the weight of the sealed container. The vacuum pump also helped to allow the effective weighing of gases using scales.
Once understood, the conservation of mass was of key importance in changing alchemy to modern chemistry.
When chemists realized that substances never disappeared from measurement with the scales (once buoyancy effects were held constant, or had otherwise been accounted for), they could for the first time embark on quantitative studies of the transformations of substances. This in turn led to ideas of chemical elements, as well as the idea that all chemical processes and transformations (including both fire and metabolism) are simple reactions between invariant amounts or weights of these elements.
Generalization
In special relativity, the conservation of mass does not apply if the system is open and energy escapes. However, it does continue to apply to closed systems.
The mass associated with chemical amounts of energy is too small to measure
The change in mass of certain kinds of open systems where no atoms or massive particles are allowed to escape, but other types of energy (such as light or heat) were allowed to enter or escape, went unnoticed in the 19th century, because the mass-change associated with addition or loss of the fractional amounts of heat and light associated with chemical reactions, was very small.
In relativity, the theoretical connection of all energy with mass was made by Albert Einstein in 1905. However, Einstein pointed out that the change in mass of systems for which the chemical amounts of energy were allowed in or out of systems, was predicted by his theory to be so small that it could not be measured with available instruments. Einstein speculated that the energies associated with radioactive phenomena were so large as compared with the mass producing them, that they might be measured as loss of a fraction of mass in systems, once they were removed. This later indeed proved to be possible, and proved a successful test of Einstein's theory.
Criticisms
The conventional statement of the law of conservation of mass - that matter can neither be created nor destroyed - has been subject to wide criticism due to its apparant absurdity in stating that matter cannot be created-as it is not possible to define an entity which cannot be created - and also due to its self-contradition in stating that matter cannot be destroyed either. Critics have suggested that the statement of the law of conservation of mass be modified to reflect the fact that it merely defines the scope of the physical science, rather than purporting to make a universal statement which is obviously contradicted by the existence of matter in the universe.
Mass conservation remains correct if energy is not lost
The conservation of relativistic mass implies the viewpoint of a single observer (or the view from a single inertial frame) since changing inertial frames may result in a change of the total energy (relativistic energy) for systems, and this quantity determines the relativistic mass.
The principle that the mass of a system of particles must be equal to the sum of their rest masses, even though true in classical physics, may be false in special relativity. The reason that rest masses cannot be simply added is that this does not take into account other forms of energy, such as kinetic and potential energy, and massless particles such as photons, all of which may (or may not) affect the mass of systems. For moving massive particles in a system, examining the rest masses of the various particles also amounts to introducing many different inertial observation frames (which is prohibited if total system system energy and momentum are to be conserved), and also when in the rest frame of one particle, this procedure ignores the momenta of other particles, which affect the system mass if the other particles are in motion in this frame.
For the special type of mass called invariant mass, changing the inertial frame of observation for a whole closed system has no effect on the measure of invariant mass of the system, which remains both conserved and invariant even for different observers who view the entire system. Invariant mass is a system combination of energy and momentum, which is invariant for any observer, because in any inertial frame, the energies and momenta of the various particles always add to the same quantity. The invariant mass is the relativistic mass of the system when viewed in the center of momentum frame. It is the minimum mass which a system may exhibit in all possible inertial frames.
The conservation of both relativistic and invariant mass applies even to systems of particles created by pair production, where energy for new particles may come from kinetic energy of other particles, or from a photon as part of a system. Again, neither the relativistic nor the invariant mass of closed systems changes when new particles are created. However, different inertial observers will disagree on the value of this conserved mass, if it is the relativistic mass. However, all observers agree on the value of the conserved mass, if the mass being measured is the invariant mass.
The mass-energy equivalence formula requires closed systems, since if energy is allowed to escape a system, both relativistic mass and invariant mass will escape also.
The formula implies that bound systems have an invariant mass (rest mass for the system) less than the sum of their parts, if the binding energy has been allowed to escape the system after the system has been bound. This may happen by converting system potential energy into some other kind of active energy, such as kinetic energy or photons, which easily escape a bound system. The difference in system masses, called a mass defect, is a measure of the binding energy in bound systems — in other words, the energy needed to break the system apart. The greater the mass defect, the larger the binding energy. The binding energy (which itself has mass) must be released (as light or heat) when the parts combine to form the bound system, and this is the reason the mass of the bound system decreases when the energy leaves the system.. The total invariant mass is actually conserved, when the mass of the binding energy that has escaped, is taken into account.
See also
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