Measurements provide the information that is the basis of most of the hypotheses that describe the behavior of matter and energy. Every measurement provides both the size or magnitude of the measurement (a number) and a standard of comparison for the measurement (a unit).
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.
We usually report the results of scientific measurements in SI units, an updated version of the metric system, using the units listed in Table 1. Other units can be derived from these base units.
|Table 1. Base Units of the SI System|
|Property Measured||Name of Unit||Symbol of Unit|
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 Table 2.
|Table 2. Common Unit Prefixes|
|micro||µ||10−6||1 microliter (μL) = 1 × 10−6 L (0.000001 L)|
|milli||m||10−3||2 millimoles (mmol) = 2 × 10−3 mol (0.002 mol)|
|centi||c||10−2||7 centimeters (cm) = 7 × 10−2 m (0.07 m)|
|deci||d||10−1||1 deciliter (dL) = 1 × 10−1 L (0.1 L )|
|kilo||k||103||1 kilometer (km) = 1 × 103 m (1000 m)|
|mega||M||106||3 megahertz (MHz) = 3 × 106 Hz (3,000,000 Hz)|
|giga||G||109||8 gigayears (Gyr) = 8 × 109 yr (8,000,000,000 Gyr)|
SI Base Units
The initial units of the metric system, which eventually evolved into the SI system, were established in France during the French Revolution. The original standards for the meter and the kilogram were adopted there in 1799 and eventually by other countries.
The standard unit of length in both the SI and original metric systems is the meter (m). A meter was originally specified as 1/10,000,000 of the distance from the North Pole to the equator. It is now defined as the distance light in a vacuum travels in 1/299,792,458 of a second. A meter is about 3 inches longer than a yard (Figure 1); one meter is about 39.37 inches or 1.094 yards. Longer distances are often reported in kilometers (1 km = 1000 m = 103 m), whereas shorter distances can be reported in centimeters (1 cm = 0.01 m = 10−2 m) or millimeters (1 mm = 0.001 m = 10−3 m).
The standard unit of mass in the SI system is the kilogram (kg). A kilogram was originally defined as the mass of a liter of water (a cube of water with an edge length of exactly 0.1 meter). It is now defined by a certain cylinder of platinum-iridium alloy, which is kept in France (Figure 2). Any object with the same mass as this cylinder is said to have a mass of 1 kilogram. One kilogram is about 2.2 pounds. The gram (g) is exactly equal to 1/1000 of the mass of the kilogram (10−3 kg).
The SI unit of temperature Celsius (°C). Water freezes at 0 °C and boils 100 °C by definition, and normal human body temperature is approximately 37 °C.
Volume is the measure of the amount of space occupied by an object. The standard SI unit of volume is liter (L). One liter is about 1.06 quarts.
A cubic centimeter (cm3) is the volume of a cube with an edge length of exactly one centimeter. The abbreviation cc (for cubic centimeter) is often used by health professionals. A cubic centimeter is also called a milliliter (mL) and is 1/1000 of a liter.