The SI units

Scientists have developed and used several systems for expressing the units of physical quantities. However, all measurements in any system are based on the units of the basic or fundamental physical quantities. The units of the fundamental or basic quantities that are independent of each other are called fundamental units.Keeping in view the importance of the proper units for measurement, there have been attempts over centuries in several developed civilizations to suggest standard units of measurements at international level. In the year 1967, the XIII General Conference on Weights and Measures rationalised the MKSA (Metre, Kilogram, Second, Ampere) system of units and adopted a system based on six basic units. It was called the Systeme Internationale de unites known as SI units in all languages. In 1971, the General Conference added another basic unit to the SI units i.e., mole for the amount of substance.

The fundamental units in different systems are different. The international system of units, known as SI units, are commonly used for all scientific purposes. This system has seven basic units for seven physical quantities, which are given in Table below.

Physical quantity	Unit	Symbol
Length	meter	m
Mass	kilogram	kg
Time	second	s
Temperature	Kelvin	K
Amount of substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	cd

Perhaps you may be confused by mass and amount of substance and also with luminous intensity as given in Table 1.1. The mass of a body is the amount of matter contained in the body, while a mole is the amount of any substance equal to its molecular mass.

1 mole of HCl = 36.46 g 2 moles of HCl = 36.46 x 2 = 72.92 g