Why Do Things Sink or Float? Activities and Fun Facts for Kids

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Updated on: Educator Review By: Michelle Connolly

Have you ever wondered why some things dive into the water while others bob on the surface like a little boat? Well, get ready to find out!

We’re going to play with water, test different things, and learn some really neat stuff along the way. So, put on your scientist hat and let’s discover the secrets of why things sink or float. 

The Science Behind Sinking and Floating

The fascinating phenomenon of objects sinking or floating in water is a result of several scientific principles, primarily density and buoyancy. Let’s learn these principles in detail. 

What is Density?

Imagine everything in the world is made up of tiny, invisible building blocks called molecules. In some objects, these molecules are packed tightly together like a room full of people, while in others, they’re spread out like you and your friends playing in a big park.

This “tightness” of molecules is called density. Objects with tightly packed molecules are dense, like a heavy rock, while those with loosely packed molecules are less dense, like a fluffy feather.

Let’s Talk Marshmallows and Chocolate:

Think of a fluffy marshmallow and a solid piece of chocolate. Marshmallows are like a big, airy cloud, so they’re not very dense. But chocolate is solid and packed together, making it denser.

What Is Buoyancy? 

Now, think about water. It has its own way of packing its molecules, too. When you put an object in water, it pushes on the water molecules around it. This push is called buoyancy. It’s like the water molecules are trying to push the object back up to the surface.

When we drop things into water, they push water out of the way. It’s like making space for themselves. This is called water displacement.

How do Things Sink or Float?

Whether something sinks or floats depends on how tightly its molecules are packed compared to the water’s molecules. Besides, if something is good at pushing water away (displacing a lot of water), it gets to float on the surface. If it doesn’t push much water (displaces a little), down it goes to the bottom.

  • If an object’s molecules are packed closer together than water’s, it’s denser. This means there are more molecules crammed into the same space, making it heavier. When you put a dense object in water, it pushes down on the water molecules harder than they can push back. So, it sinks. In other words, the weight of the object will be greater than the buoyant force of the displaced water. Imagine trying to hold a bowling ball underwater; you wouldn’t be able to keep it up for long!
  • If an object’s molecules are spaced further apart than water’s, it’s less dense. This means there are fewer molecules in the same space, making it lighter. When you put a less dense object in water, it pushes down on the water molecules, but they can easily push back with enough force to hold it up. It floats! In other words, the buoyant force of the displaced water will be strong enough to push the object up and make it float. Think of a beach ball; it’s light and airy, so it floats effortlessly on the water.

Shapes and Sizes Matter

Different shapes and sizes affect how good things are at displacing water. A big, flat object might be an awesome floater, while a tiny, heavy object might be a great sinker. Think of a boat—its wide hull displaces enough water to keep it afloat even though it’s made of heavy metal.

Archimedes’ Principle

Archimedes, a brilliant ancient Greek mathematician and inventor, discovered a fundamental principle that summarises buoyancy and density. It states that the weight of the fluid an object displaces equals the buoyant force acting on it. Put more simply, an object will float if its weight is less than the weight of the water it displaces, and it will sink if its weight is greater.

Imagine you have a tiny cupcake and a big cupcake. Even though they’re both cupcakes, the big one is heavier because it has more cake. In water, weight and volume work together to decide if something will sink or float.

Let’s Get Real

Archimedes’ principle has various applications in everyday life and technology.

  • Submarines: Submarines use ballast tanks to control their buoyancy. Filling the tanks with water increases their density, allowing them to submerge. Conversely, pumping out the water decreases their density, making them rise.
  • Hot air balloons: Hot air balloons rise because the heated air inside the balloon has a lower density than the surrounding air, creating an upward buoyant force.
  • Life jackets: Life jackets contain buoyant materials that help keep a person afloat, even if they are unconscious.

Fun Facts about Sinking and Floating

  1. Air is a fluid, too! That’s why balloons filled with air can float in the atmosphere, just like boats float in water. The air molecules are less dense than the surrounding air, creating enough buoyancy to keep the balloon afloat.
  2. The density of liquids can vary. This means something might float in water but sink in another liquid, like oil or honey.
  3. Saltwater is denser than freshwater. The ocean’s higher salt content increases its density, making it easier for objects to float. That’s why you might feel more buoyant when swimming in the ocean compared to a freshwater lake.
  4. Ships can float even though steel is denser than water! Their secret lies in their hollow hulls and air pockets. These create a large volume with low density, allowing the ship to displace enough water for the buoyant force to keep it afloat.
  5. Scuba divers wear weighted belts to counteract their natural buoyancy and stay submerged. As they ascend, they release weight to maintain neutral buoyancy and avoid popping back to the surface.
  6. In space, there’s no gravity to pull astronauts down, so they float freely inside their spaceships. This can make everyday tasks like eating and sleeping quite a challenge!
  7. There are special “neutral buoyancy” pools used by astronauts to train for spacewalks!
  8. Some fish, like the pufferfish, can control their buoyancy! They can gulp air into their stomachs to inflate and become more buoyant when threatened. Imagine blowing up a balloon inside your body—that’s basically what they do!
  9. Even eggs can float! A fresh egg’s air pocket at the wider end makes it slightly buoyant in water. As the egg ages, the air pocket shrinks, and the egg sinks—a handy trick for checking the freshness of your eggs!
  10. A sponge can float or sink depending on how much water it absorbs!
  11. There’s a special kind of rock called pumice that is so light and full of air pockets that it actually floats in water!
  12. Ice floats in water! It’s because ice is actually less dense than liquid water. This is why ice cubes bob around in your drink and why frozen lakes don’t sink to the bottom of the Earth.
  13. An orange might float, but if peeled, the slices might sink. This is because the peel is light and full of air, making it buoyant, while the slices are denser. 
  14. Oil floats on water because it’s less dense! That’s why oil spills create big slicks on the surface.
  15. The Dead Sea is the saltiest body of water on Earth, making it so buoyant that you can easily float on the surface of the water without even trying.
  16. There’s a whole field of science called buoyancy engineering that uses the principles of sinking and floating to design things like submarines, aeroplanes, and even life jackets.

Sink or Float Activities to Do Yourself

It’s time for science experiments! Here are some sink or float activities you can try at home to further understand the concept of sinking and floating. 

The Big Sink or Float Investigation

  • Gather a variety of objects around the house, including some that you think will sink and others that you think will float. This could include fruits, vegetables, toys, kitchen utensils, and anything else safe for water.
  • Fill a large bowl or container with water.
  • Predict if each object will sink or float, and then test your predictions by dropping the objects one by one into the water.
  • Discuss why some objects sink and others float. Explain the concept of density and how it affects buoyancy.
  • You can also create a chart or table to record your findings and compare the densities of different materials.

The Cardboard Boat Challenge

  • Gather some cardboard, aluminium foil, tape, and other craft materials.
  • Design and build a boat that can float the most weight.
  • Experiment with different shapes, sizes, and materials to see what works best.
  • Once you have built your boat, test it out in a bathtub or pool to see how much weight it can hold before sinking.

The Blobbing Experiment

  • Fill a small bowl with water.
  • Add a drop of dish soap to the water.
  • Dip your finger in the water and then gently touch the surface.
  • You will see a thin film of soap spread across the water. This film is called a surface tension membrane.
  • Now, try dropping a small paperclip or other object onto the surface tension membrane.
  • The object will float on top of the membrane, defying gravity! This is because the surface tension of the water is strong enough to support the weight of the object.

The Raisins Experiment

  • Fill a tall glass with clear soda.
  • Gently drop in a few raisins.
  • Observe what happens! The raisins will sink at first, but then the bubbles in the soda will attach to them, making them rise to the surface.
  • As the bubbles pop, the raisins will sink again, and the cycle will continue.
  • This activity demonstrates the concept of buoyancy and how bubbles can affect an object’s density.

We’ve seen how the buoyant force pushes back against gravity, keeping some objects afloat on a liquid’s surface. So, keep asking questions, keep exploring, and keep your mind buoyant with the endless possibilities of this captivating scientific concept! Let your imagination guide you to new and exciting discoveries!

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