Pls answer tom exam

The law of constant proportion is also known as the law of definite proportion. This law, which was introduced by Proust, stated that ‘in a compound, the elements are always present in definite proportions by mass’. Thus, according to this law, the elements always combine to form chemical compounds in a definite proportion by mass. In other words the relative ratio in which the elements compounds with each other to form compounds is fixed and definite and does not vary.

For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen respectively combine together in the ratio of 1:8 by mass to form water. The ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen, or the ratio of masses of hydrogen to oxygen will always be 1:8.


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An atom is defined as the basic unit of element which possesses the properties of the element. By basic, we mean that it is the building block of matter . It is the smallest particle of an element that can take part in a chemical reaction. Every atom consists of protons, neutrons and electrons. For example Carbon will consist of atoms which will show the properties of carbon only and not any other element.

A molecule on the other hand is a group of two or more atoms which combine together to form a single species, whose properties are different from that of the combining elements. It is the smallest unit of a compound. For example, 2 atoms of Hydrogen and one atom of oxygen combine to form one molecule of water.

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