Properties of liquids : Surface tension

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In a liquid,intermolecular forces are strong enough to confine the molecules to a specific volume, but they are not strong enough to keep molecules from moving from place to place within the liquid.

A small amount of a liquid tends to take a spherical shape:For example,mercury drops are nearly spherical and water drips from the faucet in nearly sphericl droplets.

A molecule within the body of a liquid tends to be attracted equally in all direactions,so that experiences a no net attraction by other molecules towards the interior of the liquid.As a result, there is a tendency for the surface area of a liquid to be reduced as much as possible.

Surface tension ,which measures the resistance of a liquid to an increase in its surface area,is the physical property responsible for all these types of behaviors.Energy is needed to reverse the tendency towards reduction of surface area in liquids."suface tension is the energy required to increase the surface area of a liquid by a unit amount".

The figure below explains why liquids at the molecular level exihibit surface tension.

Factors affecting surface tension:

  • surface tension can be affected by dissolved susbtances.
  • Soaps and detergents in particulary,drastically decrease the surface tension of water.

Molecules in contact with the surface of their container experience two sets of intermolecular forces.Cohesive forces attract  molecules in the liquid to one another.In addition,adhesive forces attract molecules in the liquid to the molecules of the container walls.

One result of the adhesive forces is the curved surface of a liquid, called meniscus.

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