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Why Do Things Float? — Reading Comprehension

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This Grade 4-5 science passage, 'Why Do Things Float? Understanding Buoyancy,' explores the scientific reasons behind floating and sinking. Students will learn about key concepts such as buoyancy, density, and displacement, all aligned with NGSS standards. The passage defines important vocabulary in bold and provides real-world examples, including boats and ice cubes. Activities include a multiple-choice quiz and writing prompts to deepen understanding and encourage scientific thinking. A Spanish translation and glossary are included to support diverse learners. This resource is ideal for classroom or home use, and is audio integrated for accessibility. Targeted at upper elementary, it helps students connect science to everyday life while building comprehension and critical thinking skills.

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small boat, single crew boat, man in boat, boat, lake, river, boat on lake, reflection, nature, evening, travel, trees, pleasure craft, sail boat, sailing, landscape, outdoors, water, tourism, relaxation, scenic — Boats are a great real-world example of buoyancy.Image by TheOtherKev, Pixabay.

Density is a property that describes how heavy something is for its size. Some materials are heavy for their size, like a rock. Other materials are light for their size, like a foam pool noodle. Understanding density helps us predict whether objects will float or sink in water.

To understand density, we need to think about two things we already know: mass and volume. Mass is the amount of matter in an object. Volume is how much space an object takes up. When we compare an object's mass to its volume, we can figure out if it's heavy or light for its size. A bowling ball and a beach ball might have similar volumes, but the bowling ball has much more mass packed into that space. The bowling ball is heavy for its size, while the beach ball is light for its size.

Water has its own density. Objects that are lighter for their size than water will float. Objects that are heavier for their size than water will sink. Think about a wooden block and a metal coin. The wooden block floats because wood is light for its size compared to water. The metal coin sinks because metal is heavy for its size compared to water. Even though the coin is smaller, it has more mass packed into less space.

Here's a helpful way to think about it: imagine two boxes that are the same size. One box is filled with feathers, and the other is filled with rocks. The box with rocks is much heavier, even though both boxes take up the same amount of space. The rocks are packed more tightly together, making that box heavier for its size. This is similar to how density works with floating and sinking.

The size of an object doesn't always tell us if it will float. A large log floats because wood is light for its size. A tiny pebble sinks because rock is heavy for its size. What matters is whether the object is lighter or heavier for its size compared to water. This is why huge ships made of steel can float—they are designed with lots of empty space inside, making them light for their overall size.

Interesting Fact: The Dead Sea has water so dense that people float easily without even trying! The water contains so much salt that it becomes heavier for its size than regular water, making it easier for objects and people to float on top.

What is density?

How heavy something is for its sizeHow big something isHow much water something holdsHow fast something moves

What two things help us understand density?

Mass and volumeLength and widthColor and shapeSpeed and temperature

Why does a wooden block float?

It is light for its sizeIt is very smallIt is smoothIt is dry

What happens to objects heavier than water?

They sink to the bottomThey float on topThey stay in the middleThey disappear

Why can huge steel ships float?

They have empty space insideThey are made of woodThey are very smallThey use special paint

Which has more mass: bowling ball or beach ball?

Bowling ballBeach ballThey have the same massNeither has mass