Part 1: The Surface Tension of Water


Surface Tension Defined

Surface tension is defined as the force that acts on the surface of a liquid and tends to minimize the surface area. Water will bead up because the surface tension is pulling it into a spherical shape, which has less surface area than other shapes. This is why rain drops and water drops are round.

The diagram on the right shows how surface tension pulls water into a round, or spherical shape. The red arrows show how the water is pulled in toward the middle of the water drop.

Water drop diagram

Surface Tension Pulls Water into a Spherical Shape.


The Surface of Water

Surface tension creates a thin elastic film on the surface of water. This film will support small objects so that they will sit on top of the water without sinking. It is possible to float paper clips and sewing needles on the top of the water if it is done carefully. This is not easy to do, however. One trick that works well is to take a small piece of tissue paper and lay it on the surface of the water. Put the pin, paper clip, or needle on top of the floating tissue. Lay them sideways on the tissue. The tissue will soak up water and sink leaving the pin, paper clip, or needle floating on the surface.


Floating paper clip

This image shows a paper clip floating on water. The film caused by surface tension can be seen along the edges of the paper clip.


Surface Tension at the Molecular Level

A water molecule has one oxygen atom (O) and two hydrogen atoms (H). There is a partial negative charge on the oxygen atom end and a partial positive charge on the hydrogen atom end as shown in the diagram to the right. Positive and negative charges attract each other and try to pull together. The negative end of the water molecule is attracted to the positive end of other water molecules. Under the surface of water these attractions balance out, but on the surface they are unbalanced. This is because the molecules on the surface have one surface exposed to the air and no other water molecules to balance out the charges. So, the surface molecules are pulled inward and surface tension is created. To learn more about how this works click the button below. It opens an animation that shows how surface tension works at the molecular level. The animation will open in a new window.


Water molecule diagram


Breaking Up Surface Tension

Surface tension can be broken by adding soap to water. Soap disrupts the surface tension causing the thin film to split and fly apart. The button below opens a short video that shows how soap affects surface tension. The video shows pepper floating on the surface of water in a bowl. The tip of a tootpick is dipped in liquid dish soap and touched to the surface of the water. The pepper moves quickly away from the toothpick and goes to the edges of the bowl. The video is short, but may take a few minutes to load on a 56 K modem.

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