Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.
Attractive forces between molecules of the same type are called cohesive forces. Liquids can, for example, be held in open containers because cohesive forces hold the molecules together. Attractive forces between molecules of different types are called adhesive forces. Such forces cause liquid drops to cling to window panes, for example.
Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. This general effect is called surface tension. Molecules on the surface are pulled inward by cohesive forces, reducing the surface area. Molecules inside the liquid experience zero net force, since they have neighbors on all sides.
The model of a liquid surface acting like a stretched elastic sheet can effectively explain surface tension effects. For example, some insects can walk on water (as opposed to floating in it) as we would walk on a trampoline—they dent the surface as shown in Figure (a). Figure (b) shows another example, where a needle rests on a water surface. The iron needle cannot, and does not, float, because its density is greater than that of water. Rather, its weight is supported by forces in the stretched surface that try to make the surface smaller or flatter. If the needle were placed pointed down on the surface, its weight acting on a smaller area would break the surface, and it would sink.
Surface tension is proportional to the strength of the cohesive force, which varies with the type of liquid. Surface tension γ is defined to be the force F per unit length L exerted by a stretched liquid membrane:
γ= F/L
Surface tension is the reason why liquids form bubbles and droplets. The inward surface tension force causes bubbles to be approximately spherical and raises the pressure of the gas trapped inside relative to atmospheric pressure outside.
Practice Questions
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MCAT Official Prep (AAMC)
Practice Exam 4 C/P Section Question 44
Key Points
• Surface tension is a fundamental property of the surface of liquid.
• Surface tension is responsible for the curvature of the surfaces of air and liquids.
• Surface tension is responsible for the ability of some solid objects to “float” on the surface of a liquid.
• Surface tension is responsible for the shape of the interface between two immiscible liquids.
Key Terms
Cohesive forces: Attractive forces between molecules of the same type.
Adhesive forces: Attractive forces between molecules of different types.
Surface tension: The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.