Learning Objectives Express quantities properly, using a number and a unit. A coffee maker’s instructions tell you to fill the coffee pot with 4 cups of water and to 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 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. 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 a 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. Key Take Away Identify a quantity properly with a number and a unit.