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Why Does Oil Float on Water?

Updated on May 3, 2013

Have Fun With Science And Learn Why Oil Floats On Water

Why does oil float on water is just one of the fun things you can learn when you start playing with Science. Yes, Science is fun. You'll find answers to all your questions, like: Why is the sky blue? What is a rainbow? What are Sea Monkeys? Why do super balls bounce so high? And how do crystals form?

The cool thing about Science is, when you study it and perform experiments, you can discover the secrets of the world. You might even discover a new planet or a new species of animal - or the Lost City of Atlantis.

Science is a broad subject and some mysteries are easier to unravel than others. Let's take a look at the reason you're here: Why does oil float on water?

Image Credit: Wikimedia.com

Oil and water mixed in super slow motion - Video

Oil And Water Have Different Densities

There are two reasons that oil floats on water. The first has to do with density and bouyancy.

To understand density, think of a ping pong ball and a golf ball. They're both about the same size, but the golf ball is much heavier. That's because it's denser than the ping pong ball. The ping pong ball is only filled with air, which doesn't have any weight to it. The golf ball is packed with hard rubber, which weighs more than air.

Water has a certain density, too. It's made of hydrogen and oxygen molecules, packed together. But they're not packed as tightly as the rubber inside the golf ball. So water is a little bit heavy, but it's also still liquid - the molecules have room to move around.

Fill a glass half full of water and then drop in a golf ball. You'll notice that the ball sinks and the water level rises. That's because the golf ball is denser than water - it's heavier, so it sinks. And it also displaced some of the water, and moved it out of it's way so there would be room in the glass for the golf ball, too.

The reason oil floats on top of the water is because oil is not as dense as water, which means it's not heavy enough to sink to the bottom.

Fill another glass halfway with water and make a mark at the water line. Now pour in some oil and give it a minute to stop swirling around. You'll see that the oil floats on top of the water and the water line never rises.

Oil And Water Have Different Magnetic Charges

Not everything floats when you mix it with water and not everything sinks, either. Some things can be mixed with water and they end up sharing the same space. For example, if you add flour to your glass of water you can mix it around and the water will become thicker because it mixed with the flour.

But water and oil won't mix. Even if they both had the same density, you would still see bubbles of oil throughout they water but they wouldn't be mixed together like the flour mixture you made.

Everything on earth is made of atoms and each atom is like a magnet - it has a north pole and a south pole. When you put two magnets together, the north pole of one attaches itself to the south pole of the other.

However, the magnets have to be similar in strength. A very strong magnet would just push away a very weak magnet. And that's exactly what happens with oil and water. The atoms in the oil and water have very different magnetic strengths, so instead of being able to bond together they actually repel each other.

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    • profile image

      christian spooner 2 years ago

      this was my hmk and it gave me some good facts if i understood them

    • frances lm profile image

      frances lm 5 years ago

      That is a really good simple explanation. You should write a series!

    • LabKittyDesign profile image

      LabKittyDesign 5 years ago

      The next thing to investigate is why oil on water makes rainbow colors. The answer is quantum mechanics! (really!). Try this: http://www.youtube.com/watch?v=v1GdgD77AQ4

    • WriterJanis2 profile image

      WriterJanis2 5 years ago

      I love science and have a background in it.