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Laws of Conservation of Matter, Constant Proportion and Multiple Proportion

Early history of chemistry was marked by incorrect theories about what occurred during chemical reactions. It had long been observed, for instance, that when wood burnt, the resulting ash was very light and fluffy. These types of observations then led to the conclusion that ‘something’, which German chemists, Becher and Stahl called phlogiston, was lost by substances when they are burnt.

Law of Conservation of Mass
It was Anthoine Lavoisier (1743- 1794), a French chemist, who finally laid the phlogiston theory to rest and set chemistry on the proper course again. He demonstrated in his experiments that the combustion process actually occurred by the reaction of the substances with oxygen. He showed through actual measurements, that if a reaction is made to take place in a closed container in order that none of the products of the reaction escapes, the total mass of all substances present after the reaction has occurred is the same as before the reaction began. This is the basis of the law of conservation of mass; which states that matter is neither created nor destroyed in a chemical reaction. This is one of the most important principles in chemistry. After this important discovery, many chemists were inspired to investigate the quantitative (i.e. amount involved) aspect of chemical reactions. These investigations, in turn, led to another important law in chemistry, the law of definite proportions.

Law of Definite Proportions
This law, also known as the law of definite composition, states that in a pure chemical substance the elements are always present in definite proportions by mass. An example of this law is water where the ratio of the mass of hydrogen (2g) and that of oxygen (16g) is always 1:8 irrespective of the source of the water. This means that when 9g of pure water are decomposed we obtain 1g of hydrogen and 8g of oxygen; if 18g of water are broken down, 2.0g of hydrogen and 16.0g of oxygen are produced. In the same reasoning, if 2g of hydrogen are mixed with 8g of oxygen and the mixture is ignited this will produce 9g of water and 1g of hydrogen will remain unreacted. The hydrogen to oxygen ratio of the product still remains 1:8.

Law of Multiple Proportions
This law states that if two elements A and B combine to form more than one chemical compound, then the various masses of one element A, which combine separately with a fixed mass of the other element B are in simple multiple ratio. Example: Carbon forms two compounds with oxygen -CO and CO2. In these compounds, the mass of carbon is fixed while the mass of oxygen varies in the ratio 1:2. This supports the law of multiple proportions.