Measurements and SI System

By the end of this lesson and the next few, you should be able to:

  • Explain the process of measurement
  • Identify the three basic parts of a quantity
  • Describe the properties and units of length, mass, volume, density, temperature, and time
  • Perform basic unit calculations and conversions in the metric and other unit systems

What are measurements?

Measurements provide the macroscopic information that is the basis of most of the hypotheses, theories, and laws that describe the behavior of matter and energy in both the macroscopic and microscopic domains of chemistry.

Every measurement provides three kinds of information: the size or magnitude of the measurement (a number); a standard of comparison for the measurement (a unit); and an indication of the uncertainty of the measurement. While the number and unit are explicitly represented when a quantity is written, the uncertainty is an aspect of the measurement result that is more implicitly represented and will be discussed later.

The number in the measurement can be represented in different ways, including decimal form and scientific notation. Scientific notation is also known as exponential notation, which is a mathematical concept. For example, the maximum takeoff weight of a Boeing 777-200ER airliner is 298,000 kilograms, which can also be written as 2.98 × 105 kg. The mass of the average mosquito is about 0.0000025 kilograms, which can be written as 2.5 × 10−6 kg.

Units

Units, such as liters, pounds, and centimeters, are standards of comparison for measurements. When we buy a 2-liter bottle of a soft drink, we expect that the volume of the drink was measured, so it is two times larger than the volume that everyone agrees to be 1 liter. The meat used to prepare a 0.25-pound hamburger is measured so it weighs one-fourth as much as 1 pound.

Without units, a number can be meaningless, confusing, or possibly life threatening. Suppose a doctor prescribes phenobarbital to control a patient’s seizures and states a dosage of “100” without specifying units. Not only will this be confusing to the medical professional giving the dose, but the consequences can be dire: 100 mg given three times per day can be effective as an anticonvulsant, but a single dose of 100 g is more than 10 times the lethal amount.

SI Units

We usually report the results of scientific measurements in SI units, an updated version of the metric system, using the units listed in the table below. Other units can be derived from these base units.

The standards for these units are fixed by international agreement, and they are called the International System of Units or SI Units (from the French, Le Système International d’Unités). SI units have been used by the United States National Institute of Standards and Technology (NIST) since 1964.

Base Units of the SI System

Property MeasuredName of UnitSymbol of Unit
lengthmeterm
masskilogramkg
timeseconds
temperaturekelvinK
electric currentampereA
amount of substancemolemol
luminous intensitycandelacd

Unit Prefixes

Sometimes we use units that are fractions or multiples of a base unit. Ice cream is sold in quarts (a familiar, non-SI base unit), pints (0.5 quart), or gallons (4 quarts). We also use fractions or multiples of units in the SI system, but these fractions or multiples are always powers of 10. Fractional or multiple SI units are named using a prefix and the name of the base unit.

For example, a length of 1000 meters is also called a kilometer because the prefix kilo means “one thousand,” which in scientific notation is 103 (1 kilometer = 1000 m = 103 m). The prefixes used and the powers to which 10 are raised are listed in the table below.

Common Unit Prefixes
PrefixSymbolFactorExample
femtof10−151 femtosecond (fs) = 1 × 10−15 s (0.000000000000001 s)
picop10−121 picometer (pm) = 1 × 10−12 m (0.000000000001 m)
nanon10−94 nanograms (ng) = 4 × 10−9 g (0.000000004 g)
microµ10−61 microliter (μL) = 1 × 10−6 L (0.000001 L)
millim10−32 millimoles (mmol) = 2 × 10−3 mol (0.002 mol)
centic10−27 centimeters (cm) = 7 × 10−2 m (0.07 m)
decid10−11 deciliter (dL) = 1 × 10−1 L (0.1 L )
kilok1031 kilometer (km) = 1 × 103 m (1000 m)
megaM1063 megahertz (MHz) = 3 × 106 Hz (3,000,000 Hz)
gigaG1098 gigayears (Gyr) = 8 × 109 yr (8,000,000,000 Gyr)
teraT10125 terawatts (TW) = 5 × 1012 W (5,000,000,000,000 W)

[Attributions and Licenses]


This is a lesson from the tutorial, Essential Ideas in Chemistry and you are encouraged to log in or register, so that you can track your progress.

Log In

Share Thoughts