Density: Why Things Float or Sink

A puzzle about floating

Drop a tiny metal paperclip into a bowl of water and it sinks straight to the bottom. But a huge, heavy wooden log floats happily on a river. A giant steel ship — as heavy as thousands of cars — floats across the ocean, while a small coin sinks like a stone.

So floating is not just about being big or being heavy. There must be something else going on. That something is called density.

What is density?

Density is about how much stuff is packed into a certain amount of space.

Scientists use two words here:

• Mass — how much material, or "stuff", an object is made of.
• Volume — how much space the object takes up.

Density = how much mass is packed into a certain volume.

Imagine two boxes the same size. One is filled with feathers. The other is filled with rocks. They take up the same space (volume), but the rock box has much more stuff crammed in. So the rock box is more dense. The feather box is less dense.

Density is not about how big something is — it is about how tightly the material is packed.

Floating is a contest with water

Whether something floats or sinks depends on how its density compares to the density of water.

If something is less dense than water → it floats. If something is more dense than water → it sinks.

Water has its own density. When you put an object in water, it is like a contest:

• Cork, wood, and oil are less dense than water, so they float on top.
• Stone, metal, and glass are more dense than water, so they sink.

This finally solves our puzzle! A wooden log floats because wood is less dense than water, no matter how big and heavy the log is. A tiny steel nail sinks because steel is more dense than water, even though it is small and light. Size and weight on their own do not decide it — density does.

How do giant ships float?

Here is the trickiest part. Steel is more dense than water, so a solid lump of steel sinks. So how can a steel ship — far heavier than any log — float?

The secret is shape.

A ship is not a solid lump of metal. It is shaped like a big hollow bowl, full of air. When we think about the whole ship — the metal and all the air inside it — together they take up a huge amount of space. Spreading the heavy metal out over a big, air-filled shape makes the overall density of the ship less than water. So it floats!

If you crushed the same ship into a solid ball of metal, it would be more dense than water and sink. Same metal, different shape, different result.

Proving it with clay

You can see this shape idea with modelling clay:

• Roll clay into a tight ball and drop it in water → it sinks (the clay is more dense than water).
• Take the same clay and shape it into a wide, hollow bowl → it floats!

You did not add anything. You only changed the shape so it traps air and spreads out, lowering its overall density. That is exactly the trick boats use.

Try it yourself: a density tower

Make a colourful tower of liquids that float on each other, each layer less dense than the one below.

You will need: a tall clear glass, honey, washing-up liquid, water (with a drop of food colouring), and cooking oil.

1. Pour a layer of honey into the bottom of the glass.
2. Slowly pour washing-up liquid down the side on top.
3. Gently add the coloured water.
4. Finally, slowly pour the cooking oil on top.
5. Watch the layers settle without mixing! Honey is the densest, so it sinks to the bottom. Oil is the least dense, so it floats on top.

Now drop in small objects — a grape, a small bead, a piece of pasta. Each one stops at the layer that matches its density! Things float on liquids that are denser than themselves.

Stay safe: do not drink any of the liquids, pour slowly so the layers stay separate, and clean up spills so no one slips.

What we learned

Density is how much stuff (mass) is packed into a space (volume). An object floats if it is less dense than water and sinks if it is more dense — no matter its size or weight. Changing an object's shape can change its overall density, which is why hollow, air-filled ships made of heavy steel can float. Density rules the world of floating and sinking!

Want more on this idea? Explore Floating and Sinking for the pushing force of water, and discover energy and matter in Forms of Energy.