13.SI Units.

As we know ,science is studied all around the world.Thus, it is essential that we have a standard system for measurements and have units for physical quantities which are accepted worldwide.Otherwise, it will be too difficult for people to keep a tab of different conventions and units and they will have to constantly convert quantities from one metric system(a system used for measuring) to other. This need gave birth to a metric system which is accepted and followed worldwide – the ‘SI‘ unit system.

SI stands for ‘système international d’ unitès ‘ (French) or ‘The international system of units‘(English).This is an internationally accepted metric system used to express measurements. The three very basic units of this system are –

M  ⇒ Meter        (Length unit)   [L]
kg ⇒ Kilogram  (Mass Unit)      [M]
s   ⇒  Seconds    (Time unit)       [S]

Thus, this system is also called the MKS System. 

There are 7 basic units which describe various physical quantities in this system.They are as follows –





Kilogram kg
Length Meter m
Time Seconds s
Temperature kelvin K
Amount of substance Mole mol
Electric current Ampere A
Luminance Intensity Candela cd

These are the base units.All other quantity units can be derived from these 7 basic units.

e.g. Area      =   length × breadth ⇒ [L×L] ⇒ m2
        Volume =  length × breadth × height ⇒ [L×L×L] ⇒ m3
        Density = Mass / Volume ⇒ [M] / [L]3 ⇒ kg/m3

Physical Quantity


SI unit

Area Length × Breadth m2
Volume Length×Breadth×Height m3
Density Mass per unit volume g/cm3 , Kg m-3  
Frequency Cycles/sec Hz

(Hz= s-1)

Pressure Force/area Pascal(Pa)

(Newton /metre2 ,Nm-2).

Concentration Molarity – moles/dm3 mol dm-3
Force Mass * acceleration Newton (N)

Kg m s-2

Velocity Distance/time m s-1
Heat Energy Joule(J)


The SI units for mass and length are kg and meter respectively. This is because everyday objects weigh between 1-100kgs and have dimensions which can be easily recorded in meters. However, in chemistry we deal with small masses and dimensions too. Thus, gram and centimeter units are more useful. So, the standard unit of density in chemistry is grams per cubic centimeter(g/cm3).

1kg = 1000g and 1 m = 100 cm

e.g. 30kg/m3 = (30 × 1000 g) / (100)3 cm3 = 30000/1000000 = 0.003g/cm3.

But while making measurements there are always ‘uncertainties’ , and if we don’t know these uncertainties, the measurements we make are meaningless.Thus, the knowledge of these ‘uncertainties’ or ‘errors’ is essential for our study.In the next post, we start talking about these errors in greater detail. Till then ,

Be a perpetual student of life and keep learning ….

Good Day !

References and Further Reading –






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