Презентация на тему Закон сохранения массы

The Law of Conservation of Mass

The Law of Conservation of Mass is one of the most important concepts in chemistry. The law states that matter can neither be created nor destroyed. This means that in any chemical reaction, the mass of the reacting substances at the start of the reaction will be the same as the mass of the products at the end of the reaction. Matter can change its form in a reaction, for example from a liquid state to a gas, but the mass will remain the same.

The Law of Conservation of Mass is also known as the Lomonosov-Lavoisier Law because, as we saw in unit 6, both of these scientists contributed to its development. Lomonosov first described the law in a letter to a friend and then published his ideas in a dissertation dated 1760. Lavoisier reached the same conclusions much later, in 1789, and was the first to formulate the law in clear scientific terms. For this reason the law takes its name from both these brilliant men.

The idea of conservation of mass, however, can be traced back as far as ancient Greece. In the 5^th century BC, Anaxagoras, a philosopher and scientist, said that nothing comes into existence or is destroyed and that everything is a mixture of pre-existing things. Over the course of history, many other distinguished scientists also expressed their views on the conservation of mass.

The dominant theory in the 18^th century was the phlogiston theory. According to this theory, all flammable materials (that is, materials that can burn) contain a substance called phlogiston, which is released during the burning process. That means that when flammable materials burn, the new substance, without phlogiston, should weigh less than the original substance. But this theory was wrong. Experiments showed that some metals actually increased in weight when they burnt. Lomonosov's experiment in 1756 demonstrated that the increase in weight was due to air. Many years later, Lavoisier proved that oxygen was required for combustion (burning); without it, the mass of burnt matter remained the same.

The Law of Conservation of Mass was not discovered in the usual scientific way. Lavoisier did not reach his conclusions by generalising from a large number of similar cases because, at that time, there was not enough scientific information for him to do so. Instead, Lavoisier assumed that his theory was true and then he set about proving it. His belief was justified because he did indeed prove the Law of Conservation of Mass.

The fact that the total amount of matter in сhemical reactions is always conserved and never disappears even though the matter may be in an altered form, is not only important for science, but also for other fields of human knowledge, particularly philosophy. It has led us to think about the nature of existence, and where we truly come from.

Antoine Lavoisier

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Lavoisier discovered oxygen and its role in combustion and respiration (breathing); he disproved the phlogiston theory which was popular at the time; he drew up a list of 33 elements or substances that could not be broken down further and formed the basis of the modern-day list of elements. Added to that, he proposed the Law of Conservation of Mass.

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His father was a lawyer, and in line with his family's wishes, Lavoisier completed a law degree, but his main interest was in science. In 1764, at the age of 21, he published his first paper on chemistry, and in 1768 when he was just 25 years old he was made a member of the French Academy of Sciences, one of the most important scientific institutions in the world.

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Lavoisier, too, was fascinated by combustion and disagreed with the phlogiston theory, which he set out to disprove. He did this by carefully weighing the reacting materials and the products that were made in a chemical reaction. This was a very important step in the development of chemistry, and is now known as quantitative chemistry, that is, chemistry that involves accurate measuring. In order to accurately measure changes in mass that happened during his experiments, Lavoisier developed a balance that could weigh to g. Measurement was important because Lavoisier strongly believed that matter was conserved through any reaction and this belief led to the development of the Law of Conservation of Mass.

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Through this, he discovered that it did not support the phlogiston theory because after burning, the mass of the material was greater than it had been at the start. If the elements had really contained phlogiston and lost it during the reaction, they should have weighed less, not more. Further experiments were required to find out what was happening in these reactions, and Lavoisier discovered that air was absorbed as these elements burnt. He realised that something (later identified as oxygen) was taken in during combustion rather than being given out (the phlogiston theory).

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One of these was that respiration was caused by chemical reactions with oxygen in the air. By carefully composing and decomposing water, he discovered that it is made up of oxygen and hydrogen. He gave names to elements which reflected their functions. For example, he came up with the name oxygen because it means acid-former, and that is what oxygen does. This system of chemical nomenclature is still largely in use today.