Surface tension of water is caused by an intermolecular force called hydrogen the water molecules thereby decreasing the surface tension. Note the correlation between the surface tension of a liquid and the strength of the intermolecular forces: the stronger the intermolecular forces. Surface tension is an effect within the surface layer of a liquid that causes the Surface tension is caused by the effects of intermolecular forces at the interface.
London dispersion forces also increase with chain length.
Due to a combination of these two effects, long-chain hydrocarbons such as motor oils are highly viscous. Viscosity increases as intermolecular interactions or molecular size increases.
Motor Oils Motor oils and other lubricants demonstrate the practical importance of controlling viscosity. Viscosity decreases rapidly with increasing temperatures because the kinetic energy of the molecules increases, and higher kinetic energy enables the molecules to overcome the attractive forces that prevent the liquid from flowing. So-called single-grade oils can cause major problems. If they are viscous enough to work at high operating temperatures SAE 50, for examplethen at low temperatures, they can be so viscous that a car is difficult to start or an engine is not properly lubricated.
These properties are achieved by a careful blend of additives that modulate the intermolecular interactions in the oil, thereby controlling the temperature dependence of the viscosity. Will the oil be pulled up into the tube by capillary action or pushed down below the surface of the liquid in the beaker?
What will be the shape of the meniscus convex or concave? Identify the cohesive forces in the motor oil.
Determine whether the forces interact with the surface of glass. From the strength of this interaction, predict the behavior of the oil and the shape of the meniscus. Solution A Motor oil is a nonpolar liquid consisting largely of hydrocarbon chains. The cohesive forces responsible for its high boiling point are almost solely London dispersion forces between the hydrocarbon chains. B Such a liquid cannot form strong interactions with the polar Si—OH groups of glass, so the surface of the oil inside the capillary will be lower than the level of the liquid in the beaker.
The oil will have a convex meniscus similar to that of mercury.
Will the ethylene glycol be pulled up into the tube by capillary action or pushed down below the surface of the liquid in the beaker? Answer Capillary action will pull the ethylene glycol up into the capillary.
11.4: Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action
The meniscus will be concave. Summary Surface tension, capillary action, and viscosity are unique properties of liquids that depend on the nature of intermolecular interactions. Surface tension is the energy required to increase the surface area of a liquid by a given amount. The stronger the intermolecular interactions, the greater the surface tension. Surfactants are molecules, such as soaps and detergents, that reduce the surface tension of polar liquids like water.
Capillary action is the phenomenon in which liquids rise up into a narrow tube called a capillary. It results when cohesive forces, the intermolecular forces in the liquid, are weaker than adhesive forces, the attraction between a liquid and the surface of the capillary.
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The shape of the meniscus, the upper surface of a liquid in a tube, also reflects the balance between adhesive and cohesive forces. The viscosity of a liquid is its resistance to flow. Liquids that have strong intermolecular forces tend to have high viscosities.
Forces within the body of a liquid How does temperature influence surface tension? In general, surface tension decreases when temperature increases because cohesive forces decrease with an increase of molecular thermal activity. The influence of the surrounding environment is due to the adhesive action liquid molecules have at the interface.
Effect of temperature on surface tension Surface tension of water Because intermolecular forces between water molecules are due to hydrogen bonds and these are high energy, surface tension for water is larger than many other liquids. Comparison of surface tension of water and other liquids Measurement of surface tension There are various methods to measure surface tension of a liquid.
One such method consists of a platinum ring placed over the surface of the liquid. The force required to separate the ring from the surface is measured with a high precision scale. Free body Diagram of Dunoy's Ring Surface Tension Statics Surface tension plays an important role in systems where small forces are involved wherever there is a liquid-air interface.
Sample Problem 1 A glass slide is in contact with a water surface along one of its sides as shown in the figure below. What is the magnitude of the force required to overcome surface tension as the slide is lifted from the water surface?
The surface tension of water, for water at 20 degrees Celsius, is 0.