Science Basic: 3. The Metric System: Units and Conversion |
Science Basic: 3. The Metric System: Units and Conversion
Just like you and your friend communicate using the same language, scientists all over the world need to use the same language when reporting the measurements they make. This language is called the metric system. In this lesson we will cover the metric units for length, mass, volume, density and temperature, and also discuss how to convert among them.
Metric Measurement
What do all of these words have in common: thermometer, barometer, diameter, odometer and parameter? All of these words end in -meter. You've probably heard this word before, but what does it mean? Meter at the end of a word means measure. You use all kinds of measurements each day. How much sugar is needed in the cookies you're baking? Will it be warm enough to leave your jacket at home? How fast are you driving? How much will a bag of apples cost? How much time will it take you to get home from work?
Most Americans are taught the English or standard system of measurement, but never get a really good dose of the metric system. Lucky for you, it's a much easier system to learn than the English system because all the measurements are base 10 - meaning that when you're converting from one to another, you will always be multiplying or dividing by a multiple of 10. This is much easier than trying to do calculations between ounces and pounds, and feet and miles.
Because you may not be used to thinking metrically, it may take a little practice using and working with the metric system before you gain a better understanding of it and become more fluent in the measurement language of scientists (and most non-Americans). I challenge you to sprinkle a little more metric in your life. Maybe read the milliliter measurement on your soda can or glance at the kilometer reading on your speedometer. Being able to picture metric quantities will really help with the rest of this course.
Length
We're going to start with the units of length so we can get back to this word meter that we started out with. The meter is the basic unit of length in the metric system. A meter is a tiny bit longer than a yard. For distances much longer than a meter, you'd add the prefix kilo- to make the measurement kilometer. A kilometer is the metric version of our mile, even though it's a bit shorter than our mile. A kilometer is equivalent to exactly 1,000 meters. Any unit that has the word kilo- in front of it is equivalent to 1,000 units. You can attach the prefix kilo- to just about anything. If something takes 1,000 seconds, it takes a kilosecond. If a forest has 1,000 trees, it has a kilotree. You get the idea.
For distances much shorter than a meter, we would use either a centimeter or a millimeter. A centimeter is about the width of your pinky. There are exactly 100 centimeters in a meter. In fact, anything that has the prefix centi- is one-hundredth the size of that base unit. This should be very easy to remember, because there are 100 cents in a dollar. One cent is one-hundredth of a dollar!
The last prefix you should be familiar with is milli-. There are exactly 1,000 millimeters in a meter. Anything that has the prefix milli-, is 1,000th the size of its base unit. This one is a bit more difficult to remember, but it is definitely the prefix you would use the most in a chemistry class.
Mass
Next on our list of important metric quantities is mass. This is one of the most important measurements a chemist makes. Mass is how much of something you have, or the amount of matter in an object. Don't confuse this with volume (which we'll get to in a bit). Mass is measured using a balance, and the basic unit for mass is the gram. To give you an idea of the relative size of a gram, the mass of a penny is about 2.5 grams.
Sometimes people get confused with the difference between mass and weight. They end up being quite similar because everything you and I do takes place on Earth. But, mass and weight differ because mass is how much of something you have and weight is the force of gravity on an object. Take a look at this example. Both of these blocks have the same mass (one kilogram, or 1,000 grams), but one is on Earth and the other is on the moon. Because the Earth has more mass than the moon, it is going to pull the block with more force. This is why things on Earth have more weight than things on the moon, even though both have the same mass. This may be difficult to imagine because it's not like you're going to the moon on a daily basis to check this stuff out.
Volume
If mass is a measure of how much you have, then volume is the measure of how much space it takes up. The basic unit for volume is the liter. Even in America, the liter is a common volume measurement. For example, we measure soda in 2-L increments. Often, we will be dealing with much smaller volumes, so the milliliter (or 1,000th of a liter) will be used. The liter is a three-dimensional unit. One interesting fact is that one milliliter is the exact same size as a cubic centimeter, so you may see these units used interchangeably.
Density
Our next common unit is the combination of the previous two units (mass and volume). This unit is called density and it is how tightly packed the particles of an object are. Solids have very high densities because their particles are super-close to each other. Gases have very low densities because their particles are very spread apart. That's why if you blow bubbles under water, the bubbles float to the surface. Things that are less dense float on things that are more dense.
Density can be calculated by taking the mass of an object divided by its volume. The units for density remain the same as the units you used for mass over the units you used for volume. For example, I may measure the mass of a rock as 8.3 grams. This rock also has a volume of 3.5 cubic centimeters (which is the same size as 3.5 milliliters). If I take 8.3 divided by 3.5, I get about 2.4. This means that the density of the rock is 2.4 grams per cubic centimeter. One very awesome thing about the metric system is that the density of water is about 1 g/mL, or 1 g/cm3. So if you have 300 mL of water, it will have a mass of 300 g.
Temperature
Our final unit in this lesson is the metric unit for temperature: degrees Celsius. This is the one you may be the most unfamiliar with, but this little rhyme helped me to better understand and remember temperature measurements in Celsius:
30 is hot
20 is nice
10 is cold
0 is ice
This also helps me remember that the freezing point of water is 0 degrees Celsius and the boiling point of water is 100 degrees Celsius. How easy is that to remember? Later on, you may be required to use a different unit for measuring temperature: the Kelvin. This is a very convenient scale to use because it is directly related to degrees Celsius and it's an absolute scale, meaning it doesn't go negative. Zero Kelvins is the lowest possible temperature ever! Zero Kelvins is about -273 degrees Celsius and each Kelvin is the same size as a degree Celsius. So, zero degrees Celsius is equal to about 273 Kelvins. Don't forget this - it will come in handy during the lessons on gases!
Lesson Summary
As we move through the course, we are going to encounter some other, more specialized units of measurement. But for now, make yourself more familiar with the metric units. It's easy to get used to because it's a base 10 system, meaning that everything is in multiples of tens, and it may come in handy if you are ever on vacation in a foreign country... unless you plan to visit Liberia (one of the only other countries that uses the Standard system).
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