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Density and Floating — Reading Comprehension

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MS-PS1-2

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This middle school science reading passage, aligned with NGSS standard MS-PS1-2, explains the scientific concept of density and how it determines whether objects float or sink in fluids. The passage begins by introducing the phenomenon of floating and sinking, then delves into the mechanisms behind density, buoyancy, and mass-to-volume ratios. Real-world examples, such as steel ships and floating ice, illustrate the importance of density in technology and nature. The passage also explores how density affects ecosystems, climate, and human engineering. Included are a glossary, comprehension questions, writing prompts, and graphic organizers to support diverse learners. The content is designed for audio integration, making it accessible for all students. This resource is ideal for teaching key science concepts and critical thinking skills in grades 6-8.

CONTENT PREVIEW

Density determines whether an object will float or sink when placed in a fluid such as water. This property is central to fields like engineering, environmental science, and even climate studies. When objects are placed in water, some float while others sink. Understanding the relationships between mass, volume, and density helps us explain these observations and design technology that interacts safely with water.

How Density Affects Floating and Sinking
Density is defined as the amount of mass in a given volume, usually measured in grams per cubic centimeter (g/cm³). Water has a density of 1 g/cm³. If an object’s density is less than water’s, it will float; if it is greater, it will sink. For example, a wooden block with a density of 0.7 g/cm³ floats, while a rock with a density of 2.5 g/cm³ sinks. The reason is that objects with lower density than water experience a greater buoyant force pushing them upward than their weight pulling them downward. This cause-and-effect relationship is a key principle in physical science and engineering.

Applications: Steel Ships and Floating Ice
It may seem puzzling that a steel ship floats while a solid steel ball sinks. Steel itself has a density around 7.8 g/cm³, much higher than water. However, ships are designed with hollow spaces filled with air, lowering their average density below 1 g/cm³. This allows the overall vessel to float. This application of density is crucial for transportation, trade, and economy. Another example is ice floating on water. Ice has a density of about 0.92 g/cm³, which is less than liquid water. That’s why ice cubes float in a glass and why polar ice forms on the surface of oceans and lakes. The floating ice insulates the water below, protecting aquatic life from freezing temperatures.

Density, Environment, and Scientific Understanding
The behavior of floating ice has major impacts on Earth’s climate and ecosystems. If ice sank, bodies of water would freeze from the bottom up, threatening aquatic life and disrupting global climate patterns. Scientists use precise tools to measure density and study its effects, from designing safer ships to predicting changes in polar regions. Understanding density and floating connects to larger scientific concepts such as matter properties, states of matter, and conservation of mass.

Density is not just a number—it shapes the world around us, from the ships we build to the survival of entire ecosystems. Mastery of this concept helps us solve real-world problems and reveals the interconnectedness of science and society.

Interesting Fact: Saturn’s average density is so low (about 0.7 g/cm³) that the planet would float in a gigantic bathtub of water!

What is the main factor that determines if an object will float or sink in water?

The object's density compared to the water's densityThe object's colorThe object's temperatureThe object's shape only

What is the density of water in grams per cubic centimeter (g/cm³)?

0.92 g/cm³1 g/cm³2.5 g/cm³7.8 g/cm³

According to the passage, why does a steel ship float while a steel ball sinks?

Because the ship's average density is less than water due to its hollow spaces filled with airBecause ships are lighter than waterBecause steel can float naturallyBecause ships move faster than balls

Which of these objects will sink in water based on its density?

A block of wood with 0.7 g/cm³ densityA rock with 2.5 g/cm³ densityAn ice cube with 0.92 g/cm³ densityA plastic ball with 0.6 g/cm³ density

What is the main function of floating ice for aquatic environments?

It insulates the water below and protects aquatic life from freezingIt makes water heavierIt increases the density of waterIt prevents ships from floating

What does the term 'buoyant force' mean as used in the passage?

The upward force from a fluid that helps objects floatThe weight of an objectThe color of waterThe size of an object

What might happen if ice sank instead of floated?

Lakes and oceans would freeze from the bottom up, harming aquatic lifeNothing would changeMore ships would floatWater would become less dense

Which statement best summarizes the passage's main idea?

Density determines floating or sinking, affecting nature and technologyAll objects float in waterOnly heavy objects sinkShips are made of steel

True or False: A wooden block with a density of 0.7 g/cm³ will float in water.

TrueFalse

True or False: The average density of a steel ship is higher than the density of water.

TrueFalse