The scientific community
uses **SI
units** for measurement of such properties as mass, length, and temperature.
There are seven SI base units from which all other necessary units are derived.

Although the meter is the base SI unit used for length, it may not be convenient to report the length of an extremely small object or an extremely large object in units of meters. Decimal prefixes allow us to choose a unit that is appropriate to the quantity being measured. Thus, a very small object might best be measured in millimeters (1 millimeter = 0.001 meters), while a large distance might best be measured in kilometers (1 kilometer = 1000 meters).

The SI unit of temperature
is the kelvin, although the **Celsius
scale** is also commonly used. The **Kelvin
scale** is known as the *absolute* temperature scale, with 0 K being
the lowest theoretically attainable temperature.

K = ºC + 273.15

Figure 1.18 shows a comparison of the Kelvin, Celsius, and Fahrenheit scales.

Figure
1.18. |

Note that there are no
units of volume in Table 1.4. For measurements of volume, density, and other
properties, we must derive the desired units from SI base units. In the case
of volume, which has units of length cubed, (length)^{3}, the basic
SI unit for volume is the cubic meter (m^{3}). This is an extremely
large volume, though, and more often you will see volumes reported in liters,
L (1 cubic decimeter, or 1 dm^{3}), or milliliters, mL (which are the
same as cubic centimeters: 1 mL = 1 cm^{3}).

Density has units of mass
per unit volume and is often reported as grams per cubic centimeter, g/cm^{3}.

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