Measurement Units

Matter can be classifi ed and some physical or chemical properties can be observed
without making any measurements. However, the use of quantitative measurements
and calculations greatly expands our ability to understand the chemical nature of
the world around us. A measurement consists of two parts, a number and an identifying
unit. A number expressed without a unit is generally useless, especially in
scientifi c work. We constantly make and express measurements in our daily lives. We
measure the gallons of gasoline put into our cars, the time it takes to drive a certain
distance, and the temperature on a hot or cold day. In some of our daily measurements,
the units might be implied or understood. For example, if someone said the
temperature outside was 39, you would probably assume this was 39 degrees Fahrenheit
if you lived in the United States, but in most other parts of the world, it would be
39 degrees Celsius. Such confusion is avoided by expressing both the number and the
unit of a measurement.
All measurements are based on units agreed on by those making and using the measurements.
When a measurement is made in terms of an agreed-on unit, the result is expressed
as some multiple of that unit. For example, when you purchase 10 pounds of potatoes, you
are buying a quantity of potatoes equal to 10 times the standard quantity called 1 pound.
Similarly, 3 feet of string is a length of string 3 times as long as the standard length that
has been agreed on and called 1 foot.
The earliest units used for measurements were based on the dimensions of the human
body. For example, the foot was the length of some important person’s foot, and the biblical
cubit was the length along the forearm from the elbow to the tip of the middle fi nger.
One problem with such units is obvious; the size of the units changed when the person on
whom they were based changed because of death, change in political power, and so on.
As science became more quantitative, scientists found that the lack of standard units
became more and more of a problem. A standard system of units was developed in France
about the time of the French Revolution and was soon adopted by scientists throughout the
world. This system, called the metric system, has since been adopted and is used by almost
all nations of the world. The United States adopted the system but has not yet put it into
widespread use.
In an attempt to further standardize scientifi c measurements, an international agreement
in 1960 established certain basic metric units, and units derived from them, as
preferred units to be used in scientifi c measurements. Measurement units in this system
are known as SI units after the French Système International d’Unités. SI units
have not yet been totally put into widespread use. Many scientists continue to express
certain quantities, such as volume, in non-SI units. The metric system in this book is
generally based on accepted SI units but also includes a few of the commonly used
non-SI units.