SI units and dimensions measurement recommended by the International Union of Pure and Applied Chemistry (IUPAC). In the International unit (SI) system study the seven base physical units and dimensions for measurements, conversion, and analysis. Therefore, we use the base SI unit and dimension data for measurement and conversion of force (newton or dyne), energy (joule or erg), work, heat, etc. But the SI unit of luminous intensity or candela not needed in chemistry study but it included only for the shake of completeness. In physical chemistry, we commonly deal with the CGS and SI units and dimensions data of pressure, volume, mass, temperature, electric current, etc.

Every physical property has two components, namely, the numerical value and base units. Physical property = numerical value × unit. Thus the quantity 5 Joule means numerical value = 5, and unit = 5 Joule.

International SI Units and Dimensions System

The International unit (SI) system of units and dimensions used for the measurements and study of most of the physical quantities. CGS units broadly use for the measurement of wavenumber. In order to consistency in scientific recording, the international unit system recommended the use of these CGS units.

Length, mass, time, electric current, thermodynamics temperature, amount of substance, and luminous intensity are seven base physical quantities and meter, kilogram, ampere, kelvin, mole, and candela are the unit of these quantities.

SI Units and Dimensions of Measurement

A meter is a length of the path traveled by light in vacuum during a time interval of 1/299792458 second.

The weight of the platinum-iridium cylinder kept at the International Bureau of Weights and Measures in a suburb of Paris, France called one kilogram.

Second is the duration of 9192631770 periods of the electromagnetic radiation corresponding to the transition between two hyperfine levels of the cesium-133 atom in the ground state.

Ampere is the constant current flowing if maintained in two parallel conductors of infinite length, negligible cross-section, and placed one meter apart in a vacuum. Thus the forces produced between these conductors = 2 × 10^{-7} newton per meter length.

The fraction of 1/273.16 of the thermodynamic temperature of the triple point of water.

The amount of substance of a system which contains as many elementary particles entities as there are atoms in 0.012 kilograms of carbon-12.

The candela is the unit of luminous intensity, in a given direction of a source that emits monochromatic radiation of frequency 540 × 10^{12} hertz. But the radiant intensity in that direction of 1/683 watt per steradian.

Unit Conversion Table in Chemistry

In the below table we use multiple and submultiple prefixes for unit conversion by the roman symbol.

CGS and SI Units and Dimensions of Force

Newton second law of motion,
force = mass × acceleration
∴ Force = (mass × length)/(time)^{2}
Because velocity = length/time

Therefore, the CGS unit of force = gm × (cm/sec^{2})
= gm cm sec^{-2} or simply dyne
SI unit of force = Kg × (m/sec^{2})
= Kg m sec^{-2} or simply newton

Therefore, the above base unit measurement data provides the dimension of force = [M L T^{ -2}].

Force Unit Conversion

1 Newton = (1 kg × 1 m)/(1 sec)^{2}
But 1 kg = 10^{3} gm and 1 m = 10^{2} cm
1 Newton = (10^{3} gm × 10^{2} m)/(1 sec)^{2}
=10^{5} gm cm sec-2
∴ 1 Newton = 10^{5} dyne

Calculation of SI-Unit and Dimension of Work, Heat or Energy

From the measurement of Work, W = F × S
∴ CGS unit of work = gm cm sec^{-2} × cm
= gm cm^{2} sec^{-2} or simply erg
Measurement of SI unit of work = kg m sec^{-2} × m
= kg m^{2} sec^{-2} or simply joule

From the above measurement of work
= dimension of force × dimension of length
∴ The dimension of work = [M L T^{-2}] × [L]
= [M L^{2} T^{-2}]

The ability to doing work is termed as energy. Therefore, measurements of SI units and dimensions of energy, heat, and work are the same. CGS and SI units of heat or energy = erg and Joule respectively.