Metric Units and Prefixes

For this course you need to know what properties are measured by the units meters, liters and grams. You should also become familiar with units having prefixes such as centimeters, milliliters and kilograms. These topics are presented in the pages of these sections as length, volume, mass, and metric system summary. If you are totally familiar with these units and prefixes, feel free to move on to the next topic.

Length

Meters and their derived units, such as kilometers, centimeters, and millimeters, all measure the property that we call length.

In the metric system, the basic unit of length is the meter. It is about the same as a yard. However, if you want to measure the distance between cities, the meter is too small. You wouldn't measure the distance to Portland in meters any more than you would measure it in yards. You need a larger unit. The metric system deals with different sized units by using prefixes.

Prefixes

The appropriate metric unit for distances between cities is a kilometer (km). That prefix, kilo-, means that we're talking about a unit that is 1,000 times larger than the basic unit of a meter. Thus a kilometer is 1,000 times the length of one meter.

If you do know how many meters it is between here and Portland, it's very easy to figure the distance in kilometers (divide by 1,000). That is much easier than with English units. In the metric system, the conversions are by factors of ten.

Sometimes a meter is too large a length for convenience. We have several smaller units. The two most common are the centimeter and the millimeter. The prefix centi- means "1/100th of", so a centimeter is 1/100th of the length of a meter. Therefore, there are 100 centimeters in one meter. Similarly, milli- means "1/1,000th of", so a millimeter is 1/1,000th of a meter. Therefore, there are 1000 millimeters in one meter.

The chart below shows some of the most commonly used units of length and their relationship to commonly used English units.

Volume

Another kind of property we frequently measure is volume. You can calculate the volume from length measurements. For example, if you have an object with a regular geometric shape, you might be able to measure the length, the width, and the depth (or height) and multiply them together to get the volume. If we measured those lengths in centimeters, we would calculate a volume in cubic centimeters when we multiplied the three numbers together.

The units of volume we will use are the liter (L) and the milliliter (mL). A liter is about the same volume as a quart. A milliliter is the same as a cubic centimeter or a cc.

All of the metric prefixes (kilo-, centi-, and milli-) can be applied to volume. The standard volume unit is a liter. If that's not large enough, then use a kiloliter which is 1,000 times as large as a liter. If a liter is too large for measuring the volume you are working with, you would use a smaller unit such as a milliliter (ml). A milliliter is the unit of volume that you'll use most often. The milliliter is 1/1,000th of a liter.

The chart below shows some of the most commonly used units of volume and their relationship to commonly used English units.

Mass

Grams measure the property of matter which we call mass. In chemistry you can also call it weight. There is a distinction between mass and weight. Let me dwell on that for a moment, even though it is something that chemists tend to ignore.

Mass and Weight

Mass measures the amount of matter in an object. It is related to inertia and to weight. Weight is the gravitational attraction between the earth and the mass of the object. The object's mass is attracted to the earth and the strength of that attraction is called the weight. Something can have mass and still be weightless by being so far away from the earth that the force of gravitational attraction is negligible.

Since most of our chemical reactions are done on or near the earth's surface, we don't have to worry about changes in weight due to varying distance from earth. Here on earth, if two things have the same mass, they will also have the same weight. If you took a billiard ball or a chemical sample to the moon or into outer space, its weight would be less but its mass would remain the same. Chemists usually refer to the weight rather than the mass. We treat mass and weight essentially the same although technically we should call it mass. If you would like to know more about the distinction between mass and weight, ask the instructor.

Metric Units of Mass

For weight or mass, our standard unit is the gram. This is the unit we will be using most. If we wanted to measure very small amounts of something, we would measure the mass in milligrams, in thousandths of grams. If we wanted to measure heavier objects, we would use the kilogram which is 1,000 times a gram. The SI or International System uses the kilogram as its standard rather than the gram.

Metric System Summary

In the previous sections you have a brief description of the metric system and its most common prefixes: milli-, centi-, and kilo-. They are related to whatever basic unit is attached to the prefix in the name by 1/1,000th or 1/100th or 1,000 times the size of that basic unit. Those units and prefixes are summarized in this table.

You can also look at the website for the National Institute of Science and Technology for an expanded list of metric prefixes.

Estimating Measurements

One objective of this lesson is to be able to estimate the sizes or the amounts of a meter, a centimeter, a millimeter, a liter, a milliliter, and a gram.

Let's start with length. A meter is approximately the same as a yard. A centimeter (cm) is a little less than half an inch, or about the thickness of the end of your little finger. A millimeter (mm) is 1/1,000th of a meter, the same as a tenth of a centimeter. If you take a dull pencil and draw a line with it, the thickness of that line is about one millimeter.

Next, volume. A liter is approximately the same as a quart. One liter is the same as 1000 milliliters. Therefore, one milliliter is 1/1,000th the size of a quart. It's about the size of the first segment of your little finger down to the fingernail bed. A milliliter is abbreviated ml or mL. When you see the abbreviation mL, you should think milliliter (thousandth of a liter). If you are familiar with fluid ounces, there are about 30 milliliters in one fluid ounce. In the lab area there are some containers of various volumes. In a moment, or when you are in the lab, you should check them out.

You can start getting familiar with metric units and prefixes by completing Practice Problem 1 in your workbook, and with estimating by completing Practice Problem 2.