2.1: Taking Measurements
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Learning Objectives

To express quantities properly, using a number and a unit.
A coffee maker’s instructions tell you to fill the coffeepot with 4 cups of water and use 3 scoops of coffee. When you follow these instructions, you are measuring. When you visit a doctor’s office, a nurse checks your temperature, height, weight, and perhaps blood pressure (Figure \(\PageIndex{1}\)); 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 he or she walks from home to school, and the friend answers “12” without specifying a unit, you do not know whether your friend walks—for example, 12 miles, 12 kilometers, 12 furlongs, or 12 yards. Both a number and a unit must be included to express a quantity properly.
To understand chemistry, we need a clear understanding of the units chemists work with and the rules they follow for expressing numbers. The next two sections examine the rules for expressing numbers.
Example \(\PageIndex{1}\)
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.
Exercise \(\PageIndex{1}\)
Identify the number and the unit in each quantity.
 99 bottles of soda
 60 miles per hour
 32 fluid ounces
 98.6 degrees Fahrenheit
 Answer a:
 The number is 99, and the unit is bottles of soda.
 Answer b:
 The number is 60, and the unit is miles per hour.
 Answer c:
 The number is 32, and the unit is fluid ounces.
 Answer d:
 The number is 98.6, and the unit is degrees Fahrenheit.
Key Takeaway
 Identify a quantity properly with a number and a unit.
Contributors
Henry Agnew (UC Davis)