Measurement

Learning Objectives

• I can determine the correct tool and measurements to use.
• I can convert metric unit measurements.

Introduction

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The device pictured above is called a pH meter. It is a scientific measuring device that measures the acidity of a liquid. Being able to use scientific measuring devices such as this is an important science skill. That’s because doing science typically involves making many measurements. For example, if you do lab exercises in science, you might measure an object’s length or mass, or you might find the volume of a liquid. Scientists use sensitive measuring devices to make measurements such as these. The measurements are usually made using SI (International System of Units) units of measurement, commonly known as the metric system.

Guided Learning

Measuring Length with a Metric Ruler

You’ve probably been using a ruler to measure length for a few years, but you may have made most of the measurements in English units of length, such as inches and feet. In science, length is most often measured in SI units, such as millimeters and centimeters. Many rulers have both types of units, one on each edge. The ruler pictured below has only SI units. It is shown here bigger than it really is so it’s easier to see the small lines, which measure millimeters. The large lines and numbers stand for centimeters. Count the number of small lines from the left end of the ruler (0.0). You should count 10 lines because there are 10 millimeters in a centimeter.

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Q: What is the length in millimeters of the red line above the metric ruler?

A: The length of the red line is 32 mm.

Q: What is the length of the red line in centimeters?

A: The length of the red line is 3.2 cm.

Not all objects should be measured in millimeters or centimeters. Larger objects like a school bus can be measured in a unit called meters. The distance that bus drives each day could be measured in kilometers. Any of these measurements can be converted to another unit. Take a look at the charts below.

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Can you use your math skills to convert metric units?

254 centimeters is equal to how many meters? The correct answer is 2.54 meters.

Most length measurements in science class will be in centimeters or meters.

Measuring Mass with a Balance

Mass is the amount of matter in an object. Scientists often measure mass with a balance. A type of balance called a triple beam balance is pictured in the figure below. To use this type of balance, follow these steps:

1. Place the object to be measured on the pan at the left side of the balance.
2. Slide the movable masses to the right until the right end of the arm is level with the balance mark. Start by moving the larger masses and then fine tune the measurement by moving the smaller masses as needed.
3. Read the three scales to determine the values of the masses that were moved to the right. Their combined mass is equal to the mass of the object.

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The figure below is an enlarged version of the scales of the triple beam balance in the figure above. It allows you to read the scales. The middle scale, which measures the largest movable mass, reads 300 grams. This is followed by the top scale, which reads 30 grams. The bottom scale reads 5.1 grams. Therefore, the mass of the object in the pan is 335.1 grams (300 grams + 30 grams + 5.1 grams).

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Q: What is the maximum mass this triple beam balance can measure?

A: The maximum mass it can measure is 610 grams (500 grams + 100 grams + 10 grams).

Q: What is the smallest mass this triple beam balance can measure?

A: The smallest mass it can measure is one-tenth (0.1) of a gram.

To measure very small masses, scientists use electronic balances, like the one in the figure below. This type of balance also makes it easier to make accurate measurements because mass is shown as a digital readout. In the picture below, the balance is being used to measure the mass of a yellow powder on a glass dish. To find an accurate measurement of just the powder, the mass of the dish alone would have to be measured first and then subtracted from the mass of the dish and powder together. The difference between the two masses is the mass of the powder alone.

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Mass can also be measured in different metric units. The gram is what you will typically be working with in science class. Other common units include the kilogram or milligram. Check the chart below to see how to convert units of mass.

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So how much is that? Need a common item for reference? A paper clip will weigh about 1 gram and a dictionary will weigh about 1 kilogram. Think about those items when you are trying to decide which unit is appropriate for your situation.

Can you convert 328 grams into kilograms? The correct answer would be 0.328 kilograms.

Measuring Volume with a Graduated Cylinder

At home, you might measure the volume of a liquid with a measuring cup. In science, the volume of a liquid might be measured with a graduated cylinder, like the one sketched in the figure below. The cylinder in the picture has a scale in milliliters (mL), with a maximum volume of 100 mL. Follow these steps when using a graduated cylinder to measure the volume of a liquid:

1. Place the cylinder on a level surface before adding the liquid.
2. After adding the liquid, move so your eyes are at the same level as the top of the liquid in the cylinder.
3. Read the mark on the glass that is at the lowest point of the curved surface of the liquid.^[8]

Q: What is the volume of the liquid in the graduated cylinder pictured above?

A: The volume of the liquid is 67 mL.

Q: What would the measurement be if you read the highest point of the curved surface of the liquid by mistake?

A: The measurement would be 68 mL.

Nearly all measurements in science class will be in mL, but many items in life are measured in liters (L). Have you ever purchased a 2-liter bottle of pop?

Can you convert mL and L? Sure! There are 1000 milliliters in every 1 liter. So how many mL are in that 2-liter bottle of pop? The correct answer is 2000 mL.

Review

• In science, length may be measured with a metric ruler using SI units such as millimeters and centimeters.
• Scientists measure mass with a balance, such as a triple beam balance or electronic balance.
• In science, the volume of a liquid might be measured with a graduated cylinder.