Measurements – Base Units
All measurements consist of two parts: a. the number
b. the unit
There are two systems of measurements used in the United States today.
- English System – inches, feet, gallons, etc.
- The Metric System or SI (International System of Units) – meters, liters, kilograms,
etc.
In scientific work, the metric system or the revised version of this system called the SI is used.
The metric system is a decimal unit system, which means that conversion from one unit to
another can be accomplished by multiplying or dividing by multiples of 10.
There are seven base units; the other units are derived from these seven base units. During this
course, we will use the first five.
Base Units
Quantity
Length
Mass
Time
Temperature
Amount of Substance
Electric Current
Luminous Intensity
Quantity Symbol
l
m
t
T
n
I
Iv
Unit Name
meter
kilogram
second
kelvin
mole
ampere
candela
Unit Abbreviation
m
kg
s
K
mol
A
cd
The meter is the SI unit of length. Kilometer, centimeter, or millimeter can also be used to
measure lengths much larger or smaller than the meter. Length is a measure of distance.
The kilogram is the SI unit of mass. The gram or milligram can also be used for smaller
masses. Mass is not the same as Weight. Mass is the amount of matter in an object. Weight is
a measure of the gravitational pull on matter.
The second is the SI unit of time. In chemistry, chemical reactions may take seconds or
fractions of a second to complete, so time can be measured in seconds, milliseconds,
nanoseconds, or less. When measuring times much longer than about 100 seconds, we can
express the time in minutes or hours.
Temperature is the average kinetic energy of the particles that make up a substance. The Kelvin
is the SI unit of temperature. The Kelvin scale is an absolute temperature scale, which means
that lowest temperature is given the value zero. So, 0 Kelvin is the coldest temperature that
could exist. The Celsius scale is used by much of the world, as well as by the scientific
community. A third temperature scale is the Fahrenheit scale.
Derived units – combination of SI base units. Derived units are produced by multiplying or
dividing standard units.
Derived units commonly used:
Quantity
Derivation
Unit Abbreviation
Unit
Area
Volume
Density
Molar mass
Energy
length x width
length x width x height
mass/volume
mass/amount of substance
force x distance
m2
m3
kg/m3
kg/mol
N ∙ m or J
squared meter
cubic meter
kilograms per cubic meter
kilograms per mole
Joules
Area – a measure of the extent of a surface. Area is measured in squared units.
Square – side x side
Rectangle – length x width
Circle - п x radius2
Triangle – ½ base x height
Volume- the amount of space occupied by an object. The SI unit of volume is the cubic meter
(m3), however, this unit is inconvenient for expressing volume, so the cubic centimeter (cm3) is
often used in the laboratory. The liter (L) is also used in chemistry.
1 liter = 1 cubic decimeter (dm3) = 1000 cubic centimeters (cm3) = 1000 milliliters (mL)
So, 1 mL = 1 cm3
How to measure volume:
1. Mathematically (using a ruler)
a. rectangular solids – length x width x height
b. cube - side x side x side
c. cylinder – п x radius2 x height
d. sphere – 4/3 x п x radius3
* report your answer to the correct number of significant figures
2. Water displacement method
a. measure how much water is displaced by the object. i.e. how much did the
water rise after the object was introduced in the water.
b. the amount of water displaced by the object is equal to the volume of the
object
* report your answer to the correct number of significant figures
Density – is the ratio of mass to volume (mass/volume). The SI unit is kg/m3, however g/cm3 or
g/mL are often used. For gases, density can be reported in g/L.
How to find density:
1. measure the mass of the object to the correct number of significant figures
2. measure the volume of the object to the correct number of significant figures
3. divide the mass by the volume and round off the answer to the correct number of
significant figures.