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 –

Quantity	Unit	Abbreviation
Mass	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	Formula	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 –

1.http://chemwiki.ucdavis.edu/Core/Analytical_Chemistry/Quantifying_Nature/Units_of_Measure/SI_Units

2.http://www.si-units-explained.info/Prefixes/#.V3XuMMdrVBU

3.http://www.si-units-explained.info/Prefixes/#.V3XuMMdrVBU

4.https://en.wikipedia.org/wiki/International_System_of_Units