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This chapter deals with the composition, structure and properties of matter. We know that pure substances can be classified into elements and compunds. An element consists of only one type of particle. These particles may be atoms or molecules. Sodium, copper, silver, hydrogen, oxygen etc. are some examples of elements. They all contain atoms of one type. However, the atoms of different elements are different in nature. Some elements such as sodium or copper, contain single atoms held together as their constituent particles whereas in some others, two or more atoms combine to give molecules of the element. Thus, hydrogen, nitrogen and oxygen gases consist of molecules in which two atoms combine to give their respective molecules.

When two or more atoms of different elements combine, the molecule of a compound is obtained. The examples of some compound are water, ammonia, carbon dioxide, sugar etc.

A knowledge of basic chemistry is important for understanding just about any area of biology from the function of cells to the behavior of organisms and the ecological relationships between organisms and their environment.
Indeed, chemists are fond of teasing biologists by claiming that all biology is chemistry. While this isn't quite true, in order to make sense of the structure of cells and organisms a little chemistry goes a long way. In this chapter we would describe how physical proprieties of matter can be quantitatively described using numerical values with suitable units.

Chemistry is mainly concerned with the changes that occur when atoms or molecules interact and undergo transformation from one form to another. The information regarding any chemical process is obtained through experiments where one records systematic data as the process progresses with time. The data to be recorded may involve measurement of physical quantities such as pressure, volume, temperature, concentration, density, time etc. The measurement of any physical quantity involves the recording of a pure number which represents the following ratio.

Obviously, when the experimentally determined pure number is multiplied by the value of unit physical quantity, we get the value of the physical quantity being measured.  For example, we can measure the ratio (a pure number) l2 / l1 of length l2 and l1 by using a specified method to count the number by which l1 must be multiplied to give l2. If l1 is the unit length, then the length l2 is equal to the product of the number and the unit length l1.  In words, we can write

= a pure number
or l2 =(a pure number) l1 
if l1 =1 cm, then
    l2 =(a pure number) (1 cm)

Taking a specific example of the length of a page, say 8.4 cm, implies that the length of the page is 8.4 times longer than the unit of length which is 1 cm.

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