Выбранный для просмотра документ The Law of Conservation of Mass.doc
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 5th 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 18th 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.
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.
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.
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.
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).
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.
Выбранный для просмотра документ The law of conservation of Mass.ppt
Описание презентации по отдельным слайдам:
Conservation Concept State (v) Matter Contribute To be traced back Flammable Dissertation Phlogiston Generalise Assume Set To alter Сохранение Термин Был введен Вещество Признавать, распознавать Относиться, происходить из Воспламеняющийся Диссертация Флогистон Обобщать Принимать Установить, наметить Изменять
Complete the definitions below with the following words: Released, state, distinguished, flammable, trace, reaction, concept, demonstrate, contribute, dominant To…means to show. A … is the condition of something at a particular time. A … is a process in which a chemical change happens. Somebody …is well regarded in their field. A …is an idea. To be …is to be set free. … matter is matter that can burn. To … means to help somebody or something be successful. To …. Something back means to discover how it developed. Something that is … is very important, successful or powerful.
Read the text and match the questions below with the paragraphs. There is one question you don’t need to use. How was it discovered? Which theory did it replace? Can we create or destroy matter? What is the name of the law? What has it made us consider? What is mass? How long ago had other philosophers thought about it?
Listen to a class discussion about some famous people from the past. Then say which of the statements are true . In ancient times there were no strict separation between philosophy and science. Thales rightly believed water was the basis of all composition in nature. Thales’ importance lies in his use of science rather than mythology. Empedocles’ theory of the four elements included earth and water. Anaxagoras agreed with Empedocles’ theory. Anaxagoras also claimed things kept the same form.
Выбранный для просмотра документ chemical kinetics.ppt
Описание презентации по отдельным слайдам:
Chemical kinetics Rate Occur Incredibly Rot Sufficient Concentration Bounce Surface area Collide Solid Compress Decrease Collision Boundary Hydrochloric acid Obtain Catalyst Химический продукт Скорость Возникать Невероятно, чрезвычайно Гнилой Достаточный Крепость, сгущение (раствора) Колебание напряжения Площадь поверхности Сталкиваться. Твердый Прессовать, сжимать Уменьшение Столкновение Граница Хлористоводородная кислота Получать Катализатор
Rate Consumed Rot Collide Catalyst Bounce Instant Compressed Force Sufficient A. Decay B. Spring back like a ball C. Moment D. As if eaten up E. The speed at which something happens within a particular period of time F. One moving object crashes into another G. Strength and power H. Enough I. Pushed hard together J. A substance that without being affected increases speed of chemical reaction
The rate of a chemical reaction depends on different factors. is very fast for metals. is always the same. 2. In order for a chemical reaction to occur the volume of a gas must increase. solids must be compressed. molecules must collide. 3. Higher concentrations of reacting substances will lead to more chemical reactions. increased pressure. reactive products.
4.The metal zinc is more reactive that magnesium. reacts more slowly than magnetism. is less reactive than copper. 5. Higher temperature can make molecules Travel faster with no increase in energy. Travel at the same speed, but with more energy. Travel faster, and with more energy.
Listen to two students talking about chemical reactions. Then decide if the following statements are true or false. A chemical reaction begins with reactants. Weight is released in a chemical reactions. Salt is a metal. Sodium and chlorine are solids. In a chemical reaction combined substances make new ones.
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