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What is density?

The short answer

Density is how much stuff is packed into a space, or how heavy something is for its size. A brick and a sponge can be the same size, but the brick is packed far more tightly, so it is denser and much heavier. Things that are less dense than water float on it, and things that are denser than water sink.

Kenji and Chloe, in the kitchen

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The longer answer

Everything is made of tiny particles. In some materials they are squeezed close together, and in others they are spread out with lots of space or air between them. The closer they are packed, the denser the material.

That is why size alone doesn't tell you whether something will float. A huge log is light for its size, so it floats, while a small stone is heavy for its size and sinks.

Liquids have density too. Oil is less dense than water, so it floats on top, and syrup is denser, so it sinks to the bottom. Pour them carefully into one glass and they stack in layers.

Three things to know

  • A huge ship made of heavy metal can still float. Its hollow hull is full of air, which makes the whole ship light for the amount of water it pushes aside.
  • Oil floats on top of water. Oil is less dense than water, so it gathers in a layer on top instead of mixing in.
  • Salty water is denser than fresh water. Salt makes water heavier for its size, which is why it is easier to float in the sea than in a pool.

Try it at home: a layered drink tower

  1. Pour a little honey or syrup into a clear glass.
  2. Slowly pour some water down the side of the glass, over the back of a spoon.
  3. Slowly add a little cooking oil the same way.
  4. Look at the layers. Which liquid is the densest, and which is the least dense?

Drop in small things, like a grape, a cherry tomato or a cork, and see which layer each one stops in.

Other questions people ask

What is density in simple words?

Density is how heavy something is for its size. Heavy-for-its-size things, like stones and coins, are dense. Light-for-their-size things, like corks and sponges, are not very dense.

Why do things float or sink?

Water pushes up on anything in it. If an object is less dense than water, that push holds it up and it floats; if it is denser than water, it sinks.

Quick quiz

Watch the episode first, then tap an answer.

  1. Then I peeled my orange and put it back in. What happened?

  2. A big log floats and a small stone sinks. So is floating about how big something is?

  3. One orange has its peel and one does not. Which one floats in the bowl?

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