1.6: Measurements and Units
- Express quantities properly, using a number and a unit.
- State the different measurement systems used in chemistry.
Measurements
A coffee maker’s instructions tell you to fill the coffeepot with 4 cups of water and to use 3 scoops of coffee. When you follow these instructions, you are measuring. When the nurse checks your temperature, height, weight, and blood pressure at the doctor’s office, (Figure 1.6.1), the nurse is also measuring.
Chemists measure the properties of matter and express these measurements as quantities. A quantity is an amount of something and consists of a number and a unit. The number tells us how many (or how much), and the unit tells us what the scale of measurement is. For example, when a distance is reported as “5 kilometers,” we know that the quantity has been expressed in units of kilometers and that the number of kilometers is 5. If you ask a friend how far they walk from home to school, and the friend answers “12” without specifying a unit, you do not know whether your friend walks 12 kilometers, 12 miles, 12 furlongs, or 12 yards. Both a number and a unit must be included to express a quantity properly.
Identify the number and the unit in each quantity.
- one dozen eggs
- 2.54 centimeters
- a box of pencils
- 88 meters per second
Solution
- The number is one, and the unit is dozen eggs.
- The number is 2.54, and the unit is centimeter.
- The number 1 is implied because the quantity is only a box. The unit is box of pencils.
- The number is 88, and the unit is meters per second. Note that in this case the unit is actually a combination of two units: meters and seconds.
Units of Measurements
To understand chemistry, we need a clear understanding of the units chemists work with and the rules they follow for expressing numbers. How long is a yard? It depends on whom you ask and when you asked the question. Today we have a standard definition of the yard, which you can see marked on every football field. If you move the ball ten yards, you get a first down and it does not matter whether you are playing in Los Angeles, Dallas, or Green Bay. But at one time that yard was arbitrarily defined as the distance from the tip of the king's nose to the end of his outstretched hand. Of course, the problem there is simple: new king, new distance (and then you have to re-mark all of those football fields).
SI Base Units
All measurements depend on the use of units that are well known and understood. The English system of measurement units (inches, feet, ounces, etc.) are not used in science. The metric system is used because all metric units are based on multiples of 10, making conversions very simple. The metric system was originally established in France in 1795. The International System of Units is a system of measurement based on the metric system. The acronym SI is commonly used to refer to this system and stands for the French term, Le Système International d'Unités . The SI was adopted by international agreement in 1960. Five of the base units are shown in Table \(\PageIndex{1}\).
| Quantity | Base Unit | Symbol |
|---|---|---|
| Length | meter | \(\text{m}\) |
| Mass | kilogram | \(\text{kg}\) |
| Temperature | kelvin | \(\text{K}\) |
| Time | second | \(\text{s}\) |
| Amount of a Substance | mole | \(\text{mol}\) |
These units are frequently encountered in chemistry. All other measurement quantities, such as volume, force, and energy, can be derived from these base units.
The map below shows the adoption of the SI units in countries around the world. The United States has legally adopted the metric system for measurements, but does not use it in everyday practice. Great Britain and much of Canada use a combination of metric and imperial units.
Summary
- Identify a quantity properly with a number and a unit.
- The basic units in the SI system such as mass, length, temperature, time, and amount of substance are commonly used in chemistry.
Contributions & Attributions
This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:
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Henry Agnew (UC Davis)