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Calculating Density — Reading Comprehension

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MS-PS1-1
MS-PS1-2

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This comprehensive science passage introduces middle school students to the concept of density, aligning with NGSS standards MS-PS1-1 and MS-PS1-2. It begins with a real-world phenomenon—why objects float or sink—and guides students through the mechanisms of calculating density using mass and volume. The text explains how to measure mass with a balance, determine the volume of both regular and irregular objects, and apply the density formula (D = M/V) in various contexts. Worked examples and practice problems help reinforce learning, while advanced sections discuss real-world implications, such as environmental science and material engineering. The passage includes key vocabulary, glossary, Spanish translations, quizzes, writing activities, and graphic organizers, making it ideal for classroom or independent study. Audio integration ensures accessibility for diverse learners. This resource is perfect for building foundational science skills and connecting physical science concepts to everyday experiences.

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Cork floats, wood sinks partially, metal sinks fully in water. — Comparing object densities using water displacement and formula triangle.

Density is a key property that explains why some objects float in water while others sink. Scientists define density as the amount of mass packed into a certain volume. Understanding how to calculate density helps us design ships, identify substances, and solve real-world problems in science and engineering.

How Do We Measure Density?
To calculate density, we use the formula Density = Mass ÷ Volume (D = M/V). First, you need to find the object's mass, usually with a balance. For example, if a metal block weighs 200 grams, its mass is 200 grams. Next, determine the object’s volume. For regular shapes like a cube, measure its length, width, and height, then multiply them together (Volume = L × W × H). If the cube’s sides are each 2 centimeters, its volume is 2 cm × 2 cm × 2 cm = 8 cubic centimeters. Finally, divide the mass by the volume: Density = 200g / 8cm³ = 25g/cm³.

Measuring Volume of Irregular Objects
Not all objects are perfect cubes or rectangles. To find the volume of an irregular object, like a rock, scientists use water displacement. You fill a graduated cylinder with water and record its initial volume. Drop the rock in and record the new volume. The difference between the two readings is the rock's volume. For example, if the water level rises from 50 mL to 65 mL, the rock’s volume is 15 mL. If the rock’s mass is 45 grams, its density is 45g / 15mL = 3g/mL.

Applications and Rearranging the Density Formula
Density is used in many fields. Ships are carefully designed to have a lower density than water so they float. In recycling, density helps sort different plastics. Sometimes, you might know the density and volume, but need to find the mass. Rearranging the formula, Mass = Density × Volume. For instance, if a liquid has a density of 1.2g/mL and a volume of 100mL, its mass is 1.2g/mL × 100mL = 120g. Similarly, if you know mass and density, you can find volume: Volume = Mass / Density.

Understanding density connects to broader scientific principles. It helps explain natural phenomena, such as why oil floats on water or how fish control their position in lakes. Mastering density calculations builds skills needed for more advanced physical science topics.

Interesting Fact: The concept of water displacement was first described by Archimedes over 2,000 years ago, and his method is still used today to measure volume!

What is the formula for calculating density?

Density = Mass ÷ VolumeDensity = Volume × MassDensity = Mass + VolumeDensity = Volume ÷ Mass

Which scientific tool is used to measure the mass of an object?

A thermometerA rulerA balanceA beaker

How do you find the volume of a cube?

Measure the mass and divide by densityMultiply the length, width, and heightAdd the sides togetherUse water displacement

What does 'water displacement' help us measure?

The color of an objectThe temperature of waterThe volume of irregular objectsThe mass of a liquid

What is an example of an irregular object?

A cubeA rockA textbookA metal block

What does the passage say about ships and density?

Ships sink because their density is higher than waterShips float because their density is lower than waterShips float because of their colorShips always have the same density as water

If a rock’s mass is 45 grams and its volume is 15 mL, what is its density?

3g/mL30g/mL0.33g/mL60g/mL

If you know the density and volume of a substance, how can you find its mass?

Add density and volumeDivide density by volumeMultiply density by volumeSubtract volume from density

True or False: Archimedes discovered the method of water displacement.

TrueFalse

True or False: Density can help explain why oil floats on water.

TrueFalse