**SI Base Units and SI Prefixes**

In 1960 the General Conference on Weights and Measures adopted the **International
System of units **(or **SI, **after the French *le Système International d'
Unitès)*, which is *a particular choice of metric units.* This system has nine **SI
base units,** *the SI units from which all others can be derived.*

One of the advantages of any metric system is that it is a decimal system. In SI, a
larger or smaller unit for a physical quantity is indicated by an SI prefix**,**
which is *a prefix used in the International System to indicate a power of ten. *For
example, the base unit of length in SI is the meter, and 10^{-2} meter is called a
*centi*meter. Thus 2.80 centimeters equal 2.80 x 10^{-2} meter.** **The
base units used in chemistry are: length, mass, time, and temperature.

Length

The **meter (m) **is *the SI base unit of length.* By combining it with one of
the SI prefixes, you can get a unit of appropriate size for any length measurement.

Mass

The **kilogram (kg) **is *the SI base unit of mass.* This is a somewhat unusual
base unit in that it contains a prefix. To form other SI mass units, prefixes are added to
the word gram (g) to give an appropriate unit.

Time

The **second (s) **is *the SI base unit of time.* Using prefixes such as *milli-,
micro-, nano-, *we can create units appropriate for measuring rapid events. If we are
measuring times for slower events, we use *minutes *and* hours* which are not SI
units.

Temperature

Three temperature scales are currently in use. Their units are ^{o}F (degrees
Fahrenheit), ^{o}C (degrees Celsius), and K (kelvin). The Fahrenheit scale is
commonly used in the united States outside the laboratory. The Celsius scale is the
temperature scale in general scientific use. On this scale, the freezing point of water is
0^{o}C and the boiling point of water at normal atmospheric pressure is 100^{o}C.
The *SI base unit of temperature *is the **kelvin (K)**. The kelvin scale is an
absolute temperature scale, that is a scale on which the lowest temperature possible is
zero. The Celsius and the Kelvin scales have equal-sized units, but 0^{o}C is
equal to 273.15 K. This makes it easy to convert from one scale to the other.

Converting Units

Suppose we wish to convert 23 mm to an equivalent length expressed as cm. First we construct a conversion factor.

1000 mm = 1 m = 100 cm

1000 mm = 100 cm

10 mm = 1 cm

We can make two conversion factors from this. First divide each side by 10 mm.

~~10 mm~~ / ~~10 mm~~ = 1 = 1 cm / 10
mm ( this factor will convert from mm to cm --- it converts
from unit on bottom to unit on top)

We could have divided each side by 1 cm and obtained this conversion factor.

10 mm / 1 cm = ~~1 cm~~ / ~~1cm ~~= 1 ( this
factor will convert from cm to mm)

Next we take the quantity to be converted and multiply it times the conversion factor which will cause all units except the desired one to cancel.

1 cm | ||||

23 mm | x | ----- | = | 2.3 cm |

10 mm |