What Does “Float” and “Sink” Mean?
Before we start talking about why things float or sink, let’s make sure we know what those words mean!
Float: When something floats, it stays on top of the water. It doesnt go down or sink. It stays above the surface.
Sink: When something sinks, it goes to the bottom of the water. It doesn ât stay on top; it moves downward and rests at the bottom of the water.
Lets look at some examples:
A wooden block floats in the water. It stays on top.
A rock sinks in the water. It goes straight to the bottom.
Now that we understand what floating and sinking mean, lets learn why these things happen!
The Big Idea: Density
The reason things float or sink has a lot to do with density.
Density is a scientific word that helps us understand how heavy something is for its size.
It is all about how much “stuff” is packed into an object.
Imagine you have two things:
A big, soft pillow and a tiny, heavy rock.
Even though the pillow is bigger, its light and soft.
The rock, even though its small, is heavy.
Why? Because the rock has more â stuff packed into it, and that makes it denser than the pillow.
Density means how heavy or light something is compared to how much space it takes up.
If something is less dense (light for its size), it floats.
If something is more dense (heavy for its size), it sinks.
Things That Float
Wood
Have you ever seen a piece of wood floating in a pond or lake?
Wood floats because it is less dense than water.
Even though it is solid, the wood is full of tiny air pockets that make it lighter.
This is why a wooden toy boat can float in the water.
The air inside the wood helps it stay on top.
Plastic Bottle
Think about an empty plastic bottle.
If you fill it with air and put it in water, it will float! Why?
Because plastic is light and not very dense.
The air inside the bottle also makes it even less dense. So, it stays on top of the water.
Ice Cubes
Have you ever put an ice cube in your drink?
It floats! Ice is made of frozen water, and it is less dense than the liquid water around it
. So, it stays on top of the water, even though it is solid.
When the ice melts, it turns into water and becomes denser, which is why it no longer floats.
Cork
A cork from a bottle is a great example of something that floats.
Cork is made of a material that has lots of tiny air bubbles inside it.
These air bubbles make the cork very light, and thats why it floats on top of the water.
Things That Sink
Now, lets look at things that sink in water.
These objects are usually more dense than the water, so they dont float.
Instead, they go down to the bottom. Lets look at some examples:
Rock
A rock is a perfect example of something that sinks.
It is made of heavy, solid material, and that makes it more dense than water.
When you drop a rock in water, it sinks straight to the bottom because it is heavier compared to the amount of space it takes up.
Metal Spoon
Have you ever noticed that a metal spoon sinks in water?
Metal is heavy, and it is more dense than water.
Thats why the spoon doesnt float.
Even though the spoon might look small, the metal is very heavy for its size.
Coin
A coin is made of metal, which is very dense.
Thats why when you drop a coin into a glass of water, it sinks straight to the bottom. Its heavier than water, so it doesnt float.
Why Do Some Things Float and Others Sink?
Now, lets talk about why some things float and others sink.
Its all about density and buoyancy!
Buoyancy is a special word that explains why things float.
It means the force that helps objects stay on top of a liquid, like water.
When something is placed in water, the water pushes it up.
If the object is less dense than the water, the water will keep it on top.
If the object is more dense, it will sink because the water canât push it up.
Heres a simple way to remember it:
If something is lighter (less dense) than water, it will float.
If something is heavier (more dense) than water, it will sink.
Testing Things at Home:
The Float or Sink Experiment
You can test this idea at home by doing a fun experiment! Heres what youll need:
Materials:
A large bowl or container
Water
Several objects (like a coin, a leaf, a rubber ball, a spoon, a rock, and a piece of paper)
Steps:
1. Fill the bowl with water.
Make sure the bowl is large enough for the objects to float or sink without overflowing.
2. Drop the objects into the water.
Take one object at a time and carefully drop it into the water.
3. Observe what happens.
Does it float on top?
Does it sink to the bottom?
4.Record your results:
Write down which objects floated and which ones sank.
This is a great way to see for yourself how density works.
You can test all kinds of objects to see which ones float and which ones sink!
Fun Fact Time:
Why Does Ice Float?
Heres a fun fact you might not know!
Ice floats in water because ice is less dense than water.
Its a little strange because ice is made from water, but when water freezes, it actually becomes lighter.
This is why ice cubes can float in your glass of water, but the water inside the ice cubes is still there!
A Special Case:
Half Floating and Half Sinking
Have you ever noticed that sometimes an object is half-floating and half-sinking?
For example, when you put an ice cube in a glass of water, you can see that half of the ice cube is underwater, while the other half is above the water.
This happens because the ice cube is less dense than the water, but not enough to float completely.
The part that is underwater is the heavier part of the ice, while the part that above the water is lighter and floats.
This is a fun and interesting way to see how things can be both floating and sinking at the same time!
Other Liquids:
Things That Float or Sink in Oil
Did you know that different liquids can make things float or sink in different ways?
If you drop an object into oil, it might float or sink differently than it does in water!
Thats because oil is denser than water.
Some things that float in water may sink in oil, and some things that sink in water may float in oil.
For example:
A wooden block may float in water, but it may sink a little in oil, because oil is thicker than water.
Isnt that interesting?
Lets Review!
Lets quickly go over everything weve learned:
Float means to stay on top of the water.
Sink means to go to the bottom of the water.
Density helps us understand why things float or sink.
Objects that are less dense than water float, and objects that are more dense than water sink.
Buoyancy is the force that helps things float on water.
If an object is lighter than the water, the water pushes it up.
If its heavier, it sinks.
You can test floating and sinking by doing simple experiments with objects at home!
Conclusion
Now you know why things float or sink.
Isnt it cool to understand how this works?
Next time youre near water, you can be the expert and tell your friends why things are floating or sinking.
You can even test things yourself with your own experiments!
Weve learned that density and buoyancy help us figure out what floats and what sinks.
And remember, the next time you see an ice cube floating in your drink or a rock sinking to the bottom of a lake, youl know exactly why that happens!
