SI System of Measurement

Système International d’Unités, or SI, is the set of units for scientific measurements. This set of units is based on the metric system. Many students who are more comfortable with the English system really hate the metric system. This is largely because they have had to do too many English to metric conversions. The real value of the SI system is in converting units within the system. Calculating energy and pressure from SI base units is much easier than calculating these using base units from the English system.

SI Base Units

The base units are the fundamental units representing the most straightforward measurements.

SI System Base Units Commonly Used in Chemistry Lab
Measurement	Unit	Notes
Mass	Kilogram, kg	A one liter bottle of water has one kilogram of mass (not counting the bottle, that is).
Length	Meter, m	If you extend your arm to the side at shoulder height, the length of your hand, arm, and across your chest is about one meter.
Amount of Substance	Mole, mol	This unit is very important in chemistry. The amount of matter in a mole varies a lot depending on what it is. If you cup one hand, a mole of water will just about fit. You could blow up at least ten balloons with a mole of breath.
Time	Second, s	If you don’t have a watch, you can estimate time with slow, deep breaths or by chanting “one Mississippi, two Mississippi.”
Temperature	Kelvin, K	Zero on the Kelvin scale is the temperature at which molecular motion stops. Water freezes at 273K and boils at 373K. Room temperature is 298K.

The base units can be scaled up (or down) using multiples (or factors) of ten. The scale is indicated by prefixes familiar to those who have worked with the metric system.

Prefixes

Metric System Prefixes Commonly Used in Chemistry Lab
Prefix	Scale	Example
Kilo-, K	1000	1000 grams = 1 kilogram
Centi-, c	10^-2	100 centimeters = 1 meter
Milli-, m	10^-3	1000 milliliters = 1 liter
Micro-, µ	10^-6	1 micrometer = 0.001 millimeter =1x10^-6 meter
Nano-, n	10^-9	1 nanometer = 1x10^-6 millimeter = 1x10^-9 meter

SI and Metric Derived Units

Derived units are more complex units that require more than one dimension or type of measurement. Length is an example of a base unit. Volume is an example of a derived unit, because it is a three dimensional representation of length.

Derived Units Used in Chemistry Lab
Measurement	Unit	Notes
Mass	Gram, g	A gram is 1/1000 of a kilogram. A typical solid chemical sample the size of a pea has a mass of about a gram. One milliliter of water has a mass of one gram.
Length	Centimeter, cm	A centimeter is 1/100 of a meter. Your thumbnail is about one centimeter wide.
Length	Angstrom, Ǻ	An Angstrom is 1x10^-10 meter. This unit is used to measure the radii of atoms.
Volume	Milliliter, mL	A milliliter is 1/1000 of a liter. It is the volume that would occupy a cube with sides one cm long, so one milliliter is equivalent to one centimeter cubed. A milliliter is 22 drops from an eyedropper or buret.
Heat	Calorie, cal	One calorie is the amount of thermal energy needed to raise the temperature of one gram of water by one degree Kelvin. A nutritional calorie is one thousand calories.
Energy	Joule, J	The Joule is the SI unit of energy and can be related to base SI units (1 J = 1 kgm²/s²). One calorie is equal to 4.184 J.
Force	Newton, N	One Newton is the force needed to accelerate one kilogram of mass by one meter per second squared (1 N = 1 Kg m/s²). This unit is not often used in chemistry.
Pressure	Pascal, Pa	One pascal is the pressure exerted by one Newton of force acting on an area of one square meter (1 Pa = 1 N/m²). This unit is not used as often as the other pressure units.
Pressure	Atmosphere, atm	The typical air pressure at sea level is 1 atmosphere. 1 atm = 101.325 kPa
Pressure	Torr	One Torr is the air pressure needed to support a column of mercury one millimeter high (1 Torr = 1 mmHg). This unit is based on devices used to measure air pressure. One atmosphere is 760 Torr.

This site was last updated 08/01/06