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An examination for selection of candidates for appointment as Special Class Apprentices in Mechanical Department of Indian Railways will be held by the Union Public Service Commission.

Monday, 4 July 2011


Chemistry is basically experimental science. In it we study physical and chemical properties of substance and measure it upto possibility. The results of measurement can reported in two steps,
(a) Arithmetic number, (b) Unit of measurement.
Every experimental measurement vary slightly from one another and involves some error or uncertainty depending upon the skill of person making the measurements and measuring instrument. The closeness of the set of values obtained from identical measurement called precision and a related term, refers to the closeness of a single measurement to its true value called accuracy.
Significant figures
 1. All non zero digits are significant. 
 2. All zeros occurring between two non zero digits are significant.
 3. In a number less than one, all zeros to the right of decimal point and to the left of a non zero digit are not significant.
 4.All zeros on the right of the last non zero digit in the decimal part are significant
 5.All zeros on the right of the non zero digit are not significant. 
 6.All zeros on the right of the last non zero digit become significant, when they come from a measurement
 System of units : A complete set of units, both fundamental and derived for all kinds of physical quantities is called system of units. The common systems are given below
(1) CGS system : This system is also called Gaussian system of units. In this length, mass and time have been chosen as the fundamental quantities and corresponding fundamental units are centimetre (cm), gram (g) and second (s) respectively.
(2) MKS system : This system is also called Giorgi system. In this system also length, mass and time have been taken as fundamental quantities, and the corresponding fundamental units are metre, kilogram and second.
(3) FPS system : In this system foot, pound and second are used respectively for measurements of length, mass and time. In this system force is a derived quantity with unit poundal.
(4) S. I. system : It is known as International system of units, and is extended system of units applied to whole physics. There are seven fundamental quantities in this system. These quantities and their units are given in the following table


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