The law of conservation of mass, also known as principle of mass/matter conservation is that the mass of a closed system (in the sense of a completely isolated system) will remain constant over time. The mass of an isolated system cannot be changed as a result of processes acting inside the system. A similar 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.
As opposed to mass conservation, the principle of matter conservation (in the sense of conservation of particles which are agreed to be "matter") may be considered as an approximate physical law, that is true only in the classical sense, without consideration of special relativity and quantum mechanics. Another difficulty with the idea of conservation of "matter," is that "matter" is not a well-defined word scientifically, and when particles which are considered to be "matter" (such as electrons and positrons) are annihilated to make photons (which are often not considered matter) then conservation of matter does not take place, even in isolated systems.
Mass is also not generally conserved in "open" systems (even if only open to heat and work), 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 (isolated) systems, as viewed over time from any single inertial frame, continues to be true in modern physics. The reason for this is that relativistic equations show that even "massless" particles such as photons still add mass and energy to closed systems, allowing mass (though not matter) to be conserved in all processes where energy does not escape the system. In relativity, different observers may disagree as to the particular value of the mass of a given system, but each observer will agree that this value does not change over time, so long as the system is closed.
The historical concept of both matter and mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics. In relativity, the mass-energy equivalence theorem states that mass conservation is equivalent to energy conservation, which is the first law of thermodynamics.
Historical development and importance
An important idea in ancient Greek philosophy is that "Nothing comes from nothing", so that what exists now has always existed, since no new matter can come into existence where there was none before. An explicit statement of this, along with the further principle that nothing can pass away into nothing, is found in Empedocles (ca. 490–430 BCE): "For it is impossible for anything to come to be from what is not, and it cannot be brought about or heard of that what is should be utterly destroyed". A further principle of conservation was stated by Epicurus (341–270 BCE) who, describing the nature of the universe, wrote that "the totality of things was always such as it is now, and always will be". Jain philosophy, which is a non-creationist philosophy and based on teachings of Mahavira (6th century BCE), states that universe and its constituents like matter cannot be destroyed or created. The Jain text Tattvarthasutra (2nd Century) states that a substance is permanent, but its modes are characterised by creation and destruction. A principle of the conservation of matter was also stated by Nasīr al-Dīn al-Tūsī (1201–1274) during the 13th century. He wrote that "A body of matter cannot disappear completely. It only changes its form, condition, composition, color and other properties and turns into a different complex or elementary matter".
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