Surface tension: oil and water in comparison

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Many physical concepts are difficult to understand, especially for children and teenagers. Experiments help to illustrate terms and theories, the "aha" effect is now only seconds away. The concept of surface tension is also easy to understand. Haven’t you often asked yourself why oil and water don’t mix?

Oil and water do not usually mix.
Oil and water do not usually mix. © A. Dreher / Pixelio

What you need:

  • Cooking oil
  • Washing-up liquid
  • water
  • dye
  • small glass bottle
  • Beaker

Surface tension - what is that anyway?

  • Water is made up of many small particles. Between these there are forces of attraction and repulsion (cohesion). This causes neighboring particles to form bonds.
  • However, interconnected water particles are also mobile. As they spread out in all directions, the bonds are loosened and new particles are sought with which a bond is possible.
  • This process is not so easily possible on the surface of the water, here bonds can only take place downwards or into the interior of the water. The surface then becomes tense and shrinks.
  • The surface tension between different liquids is very different. For example, water has a rather high value, whereas oil has a lower value. The following rule applies: the greater the attraction between the particles, the greater the value.
  • This physical concept is also related to interfacial tension. This occurs when two liquids with different values, e.g. water and oil, meet.
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  • This physical term should not be confused with density or viscosity. Viscosity indicates how flowable a liquid is. The density indicates the relationship between mass and volume. Oil has a slightly lower density than water. Therefore, the two liquids do not mix naturally.

Experiment - can oil and water mix?

  1. Fill a small bottle - this must fit into the beaker - with a little vegetable oil. For an even more exciting effect, you can color it with a little dye beforehand.
  2. Fill the beaker with water and place the oil bottle in it. Both liquids remain clearly separated from each other.
  3. Now add a little washing-up liquid to the water and stir it with your finger or a spoon.
  4. You can now observe that the oil runs out of the bottle to the surface of the water.
  5. The explanation: Due to the different surface tensions and the interfacial tension between the two liquids, there is no mixing. Since the vegetable oil also has a lower density than water, the liquids cannot penetrate one another.
  6. The soap works as an emulsifier, it lowers the high value of the water so that the two liquids can now mix.

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