3.1. Units of Measurement http://www.ck12.org
There can be different units that are used to measure the same physical quantity. For example, temperature can be
expressed in degrees Fahrenheit (°F), degrees Celsius (°C), or degrees Kelvin (K). No one set of units is more correct
than the other. However, as we begin measuring, calculating, and sharing measurements, we will want a standard
set of values that we and others can use. An international governing body has developed a metric system of units of
measurement for scientists called theSystème International (SI). Some of these units are listed inTable3.1.
TABLE3.1: SI Base Units
Physical Quantity Name of SI Unit Abbreviation
mass kilogram kg
length meter m
time second s or sec
temperature Kelvin K
amount of substance mole mol
electric current ampere A
luminous intensity candela cdBase Units vs. Derived Units
With thebase unitslisted in theTable3.1, we can describe many physical details of a given chemical substance.
Base units are measurements that have their own independent scale and cannot be expressed in terms of other base
units. All other measurement quantities, such as volume, force, and energy, can be derived from these seven base
units. For instance, volume is calculated by multiplying together three different lengths (height, width, and depth).
We call these combinations of base unitsderived units. Some examples of derived units are listed inTable3.2.
TABLE3.2: SI Derived Units
Physical Quantity Name of SI Unit Abbreviation
area square meter m^2
volume cubic meter m^3
speed, velocity meter per second m/s
acceleration meter per second squared m/s^2
force Newton (mass×acceleration) N (kg m/s^2 )
mass density kilogram per cubic meter kg/m^3
energy joule (force×distance) J (kg m^2 /s^2 )Magnitude and Scale
When we think about the physical quantities that are measured in chemistry, we must also consider the concepts
of magnitude and scale. The following video introduces these concepts: http://www.youtube.com/watch?v=0fKBh
vDjuy0 (9:01).