The law of conservation of mass, which is a central theme of chemistry, is one of the first conservation laws to be discovered. It was established in 1789 by French chemist, Antoine Lavoisier, who is regarded as the father of modern chemistry. A gifted public servant, he helped to construct the list of elements, and even gave the name ‘oxygen’ to mankind’s most important gas.
Lavoisier burned substances such as phosphorus and sulfur in air and showed that they combined with the air to make new materials which weighed more than the original substances. This demonstrated that the weight gained by the new materials was lost from the air in which the substances were burned. From his observations, Lavoisier discovered that, although matter may change its form or shape, its mass always remains the same in volume. He established the law of conservation of mass which says that matter cannot be created or destroyed by a chemical change; simply that nothing can lose mass. It can only change by being rearranged. This is because the total quantity of matter and energy available in the universe at any given time is a fixed amount, never more or less. This law applies to ordinary chemical reactions (as opposed to nuclear reactions, in which matter can be changed to energy).
During a chemical change (or reaction), the atoms of one or more substances (called reactants) simply undergo some “rearrangements.” The result of these rearrangements is the formation of new, different substances (or products). However, all of the original atoms are still present somewhere in the system. It is because of this important law that we are able to write balanced chemical equations which make it possible to predict the masses of reactants and products that will be involved in a chemical reaction.
Put another way, the law of conservation of mass allows us to determine the amount of a missing reactant or product that was involved in a chemical reaction. For example, if one was told that 8 grams of hydrogen reacted with some oxygen to make 60 grams of water, one could calculate how much oxygen must have been used by applying the law. One would deduct the amount of hydrogen from the amount of water to get the 52 grams of oxygen used.
However, now that relativity and quantum mechanics have taken center stage this law would need to be modified to allow for the conversion of mass to energy because conversion of one type of matter into another is always accompanied by the conversion of one form of energy into another.
Difficulties in understanding
The law of the conservation of mass has not been an easy one to understand, judging by recent research. Researchers in different countries have shown that students have difficulties in understanding this essential concept. For example, Driver et al. (1984) and Andersson (1984) report the responses of 15-year-old students to a question about a piece of phosphorus placed in some water in a sealed flask which is heated by the sun. Students were told that the phosphorus catches fire producing a white smoke, which dissolves in water, and they were asked to state whether the final mass would be the same, greater, or less than the starting mass. Both research studies reported that about one-third of the sample gave conservation- type answers, suggesting that the mass would not change because —the flask is sealed“. A further 16% thought the mass would decrease; only 6% thought the mass would increase.
So, in practical terms, mass is always there even though you cannot see it, or find it. A candle, for example, will burn and melt, but the mass is there on the dish or in the jar. As another example, a lorry carries some chemical waste from a chemical plant to a waste treatment area. The lorry leaves the plant with 2,500 kilograms of waste but arrives at the treatment facility with only 2,000 kilograms. The initial idea is that 500 kilograms of waste has just vanished. But it hasn’t, and needs to be accounted for. As the law of conservation of mass clearly states that mass cannot be created or destroyed, the waste must be somewhere within the closed system, in this case, earth.
The law of conservation of mass is one of a set of known conservation laws and it determines the conditions in which each quantity is conserved.
Source
http://209.85.229.132/search?q=cache:gt5ypl1JNB0J:www.uoi.gr/cerp/2003_October/pdf/04Ozmen.pdf
http://www.visionlearning.com/library/module_viewer.php?mid=49
