13. The International System of Units.

Measurements are observations that convey specific data. However, observations need to be expressed correctly. Specific units are required which can aptly quantify the observations. Thus, a standard measurement system had to be devised which would be accepted worldwide. The metric system that is followed worldwide is the ‘SI‘ unit system.

SI UNITS

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 centimetre units are more useful. So, the standard unit of density in chemistry is grams per cubic centimetre (g/cm³).

1kg = 1000g and 1 m = 100 cm

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

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Measurements are not always accurate. There are ‘uncertainties’ involved while making measurements. These uncertainties are referred to as ‘errors’. Knowing these errors is crucial for the exact judgement of parameters. In the next post, we shall start studying errors. 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