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What Causes Surface Tension?

Sometimes the smallest forces in nature are responsible for dramatic results. Surface tension is one such example. This characteristic of liquids, caused by molecular attraction, is responsible for the shape of rain drops and the formation of bubbles, as well as impacting the way you wash your clothes. While all liquids exhibit this natural characteristic, water possesses an unusually high degree of surface tension.

  1. General Surface Tension

    • All liquids display surface tension to some degree. The tension is created by molecular cohesion. This cohesion causes like molecules to be attracted to one another. Below the surface of a liquid, molecules are pulled equally in all directions by the cohesive force. However, at the surface, the molecules of the liquid are only pulled in a sideways and downward direction. The sideways attraction between the molecules creates surface tension.

    Surface Tension of Water

    • In addition to the natural attraction between molecules, water molecules exhibit another type of force that impacts surface tension. Each water molecule consists of two hydrogen atoms and one oxygen atom. The hydrogen atoms form weak hydrogen bonds between molecules. These bonds are high energy, resulting in a higher surface tension for water than most liquids.

    Results of Surface Tension

    • Surface tension causes the surface of a liquid to behave like a thin elastic film. The surface stretches until the cohesive force between molecules is exceeded. This is similar to a marble suspended on a stretched piece of toilet paper. This is how insects are able to walk on water. They exert less downward force than is required to break the water's surface tension. Surface tension also causes drops of a liquid to form into a spherical shape, which represents the smallest possible surface area.

    Measuring Surface Tension

    • The surface tension of a liquid is measured in units of force per unit of length. Typically, dynes per centimeter are used. This measure represents the force required to break the surface tension of a liquid. Water has a surface tension of 72.8 dynes/cm, while ethanol has a surface tension of only 22.1 dynes/cm.

    Effects of Temperature and Additives

    • Surface tension decreases as the temperature of a liquid increases. This is because the cohesive forces between molecules decreases as molecular activity increases. This is why hot water cleans clothes better than cold water. The lower surface temperature of the hot water allows the water to be more easily absorbed into the fabric and stain. Additives, such as soap, further reduce water's surface tension by reducing the strength of the hydrogen bonds.

    Uses of Surface Tension

    • Liquids with higher surface tensions are easily transformed into aerosols. This is important for industrial and health applications. Surface tension is also used to create material forms in industry. For example, lead's high surface tension enables it to be easily shaped into round pellets.

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