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 conversion problems. 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 system of measurement
Système International d’Unités, or SI, is the set of units for scientific measurements.
SI base units
The base units are the fundamental units representing the most straightforward measurements. The table below lists the base units along with a rule of thumb to help you estimate amounts.
|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. The following table lists the prefixes most commonly used in the general chemistry lab along with some conversion factors.
|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 =1×10-6 meter|
|Nano-, n||10-9||1 nanometer = 1×10-6 millimeter = 1×10-9 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. The following table lists the derived units most commonly used in the general chemistry lab along with a rule of thumb to help you estimate amounts.
|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 1×10-10 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 burette.|
|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.|
For more information about SI units see the National Institute of Standards and Technology web site.