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Why Don’t Oil and Water Mix?

Chemistry | 7-12 yrs | Interactive, Learning Pod
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Have you tried to wash oily hands in just water and discovered that your hands stayed oily? Or think of crude oil that spills from a tanker in the ocean and floats on top of the water.
There are two main reasons why don’t oil and water mix even if they are put in the same container. Both these reasons consider these two substances on a very small scale.
Everything around us is made up of tiny particles known as molecules. The way in which two substances interact depends on the molecules that make up the substances.

Oil and water have different densities

The first reason that water and oil don’t mix is because their molecules are packed differently. The molecules of water are packed very densely. (Would be good to show water molecules packed densely) In one glass of water, there are more molecules than the number of known stars in the universe!
This means if we take equal parts of water and oil, there will be more molecules of water than oil. This also means that it will always sink underneath the oil.

Oil and water charges

There is one more reason why they cannot mix with each other. Polarity. Polarity means a molecule is positively charged at one end and negatively charged at the other. Water is a polar molecule. Water molecules are made up of two hydrogen atoms and one oxygen atom each.
Since only opposites attract, the water molecules stick to each other.
Polar molecules only dissolve in polar solvents. Similarly, non-polar molecules only dissolve in non- polar solvents. Oil is made up of non-polar molecules. It has a shell of negative charges, or electrons, surrounding the molecule.
The oil molecules will ultimately stick to each other. Therefore, even if you stir a container with oil and water, they will eventually separate into two distinct layers.
Detergents and soaps help us to remove oil and dirt from utensils and our bodies. Oily feathers and coats help animals who live in seas and rivers to stay warm, as the oil keeps the cold water away from their skin.

THINGS TO DO

Watch this Density Test experiment and try to do it yourself. (Link to http://mocomi.com/density-test/)

Head on to Chemistry for Kids for more such interesting chemistry videos and interactive articles.

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Comments17 reactions

  1. Interesting information, but only part of a water molecule is positively charged (the part around the oxygen atom)

  2. This was very insightful. I plan on doing this for my upcoming Science Fair. I like how this is explained. Simple enough for kids to understand and complex enough to get people interested in science. Well done!