Why does oil float on water?

I'm going to post the occasional science question from time to time if thats ok with everyone. My first one asks why does oil float on water? This is super relevant to me, and to us as an oil and gas community.

We inject water into the reservoir to bring the oil higher up and easier to pump because we know that oil floats on water. God forbid if there's an accident, we know that we can deploy surface equipment to contain the slick. We also know that dispersants can make the oil sink to the bottom of the ocean.

So, I'm a driller and a damn good one but I never completed my education. I've made it my mission in life to keep on learning until the day I die. I'm actually really interested in science, but messed around in high school. Education is about motivation, and no-one needs a teacher standing at the front of the room.

I just thought that this topic might be a good one to discuss here, thanks!


  • Hi Kev,

    You bring up quite a few good talking points here, even the basic question can be answered on different levels. The high school answer would be a lot different to one offered by a physics professor.

    I'll get the ball rolling with the simple answer:

    The reason why oil floats on water is because of the relative densities of the liquids. Water is denser than oil so when the two are mixed together, the oil will rise and the water will sink.

    The topic of density relates to how close the molecules are from each other on an atomic level. This isn't about solidity, whilst all gases are less dense than liquids, solids and liquids can vary. For example, candles float on water because they are less dense, but are still more solid.

    Crude oil has a density of 58 pounds/26.3 kilos per cubic foot/30.48 cm.
    Sea water has a density of 64 pounds/29 Kilos per cubic foot/30.48 cm.

    Many liquids of different densities will mix together and stay that way.

    The old saying 'oil and water do not mix' relates to the polarity of the molecules.

    Water molecules are polar in nature, and oil molecules are non-polar. Polarity relates to the magnetic charge, meaning that there is a positive and a negative. More specifically, one end of a particular molecule is positively charged, while the other end is negatively charged. Polar solvents such as water will only dissolve other polar molecules, which excludes oil molecules.

    I don't know anything about oil dispersants and the effect that they have on the oil molecules, perhaps they bind to them?

    Anyone reading this page who has a deeper understanding of why oil floats on water, and the practical implications for the oil and gas industry might like to add to my answer?

  • It's funny how we all assume these types of facts, but don't really know why they are true!

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