What intermolecular forces are present in #NH_3#? Such foams are now used universally to fight large-scale fires of organic liquids. Capillary action is the phenomenon in which liquids rise up into a narrow tube called a capillary. 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. Liquids consisting of long, flexible molecules tend to have higher viscosities than those composed of more spherical or shorter-chain molecules. Because they affect the surface properties of a liquid, soaps and detergents are called surface-active agents, or surfactants. Cohesion is the attraction in a molecule of liquid to each other due to intermolecular forces.
Viscosity (η) is the resistance of a liquid to flow. Others, such as motor oil, molasses, and maple syrup, flow very slowly and have a high viscosity. Enroll in one of our FREE online STEM summer camps. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. In this diagram, the solid connections are chemical bonds and the dashed connections are intermolecular forces. The stronger the intermolecular interactions, the greater the surface tension. The height to which the water rises depends on the diameter of the tube and the temperature of the water but not on the angle at which the tube enters the water. The cohesive forces responsible for its high boiling point are almost solely London dispersion forces between the hydrocarbon chains. A Motor oil is a nonpolar liquid consisting largely of hydrocarbon chains. So-called single-grade oils can cause major problems. When a glass capillary is is placed in liquid water, water rises up into the capillary. See all questions in How Intermolecular Forces Affect Phases of Matter. Adding soaps and detergents that disrupt the intermolecular attractions between adjacent water molecules can reduce the surface tension of water. Mercury is an apparent anomaly, but its very high surface tension is due to the presence of strong metallic bonding. Although you have been introduced to some of the interactions that hold molecules together in a liquid, we have not yet discussed the consequences of those interactions for the bulk properties of liquids. What does “like dissolves like” mean? How does viscosity depend on intermolecular forces?
For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. The meniscus will be concave.
Such phenomena are manifestations of surface tension, which is defined as the energy required to increase the surface area of a liquid by a specific amount. Water has both strong adhesion to glass, which contains polar SiOH groups, and strong intermolecular cohesion. 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? Many insects take advantage of this property to walk on the surface of puddles or ponds without sinking. The upper surface of a liquid in a tube is called the meniscus, and the shape of the meniscus depends on the relative strengths of the cohesive and adhesive forces.
Determine whether the forces interact with the surface of glass. Some liquids, such as gasoline, ethanol, and water, flow very readily and hence have a low viscosity. The viscosities of some representative liquids are listed in Table 11.3.1 and show a correlation between viscosity and intermolecular forces. Because a sphere has the smallest possible surface area for a given volume, intermolecular attractive interactions between water molecules cause the droplet to adopt a spherical shape. Attractive interactions between the polar substances and water cause the water to spread out into a thin film instead of forming beads. Weaker than hydrogen bonding, but still affects viscosity, boiling point, and density. The surfactants reduce the surface tension of water below that of fuel, so the fluorinated solution is able to spread across the burning surface and extinguish the fire. Capillary action is the net result of two opposing sets of forces: cohesive forces, which are the intermolecular forces that hold a liquid together, and adhesive forces, which are the attractive forces between a liquid and the substance that composes the capillary. When filling a glass with water, the glass can be overfilled so that the level of the liquid actually extends above the rim. We now turn our attention to three unique properties of liquids that intimately depend on the nature of intermolecular interactions: If liquids tend to adopt the shapes of their containers, then, do small amounts of water on a freshly waxed car form raised droplets instead of a thin, continuous film? The viscosity of a liquid is its resistance to flow. Bigger molecules have stronger London Dispersion Forces. Surface tension, capillary action, and viscosity are unique properties of liquids that depend on the nature of intermolecular interactions. The longer the molecules, the easier it is for them to become “tangled” with one another, making it more difficult for them to move past one another.
Explanation: As the pressure increases the intermolecular distance decreases; consequently, the intermolecular force increases. Cohesive forces bind molecules of the same type together, Adhesive forces bind a substance to a surface. What other factors affect viscosity? Register now! How do intermolecular forces affect solvation? Surface tension is the energy required to increase the surface area of a liquid by a given amount.
How do intermolecular forces affect freezing point? 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. 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. The addition of a second hydroxyl group to ethanol, for example, which produces ethylene glycol (HOCH2CH2OH), increases the viscosity 15-fold. In the 1960s, US Navy researchers developed a method of fighting fires aboard aircraft carriers using “foams,” which are aqueous solutions of fluorinated surfactants. This maximizes the number of attractive interactions and minimizes the number of water molecules at the surface. Let's take honey as an example. To describe the unique properties of liquids. Surface tension is the energy required to increase the surface area of a liquid by a unit amount and varies greatly from liquid to liquid based on the nature of the intermolecular forces, e.g., water with hydrogen bonds has a surface tension of 7.29 x 10-2 J/m2 (at 20°C), while mercury with metallic bonds has as surface tension that is 15 times higher: 4.86 x 10-1 J/m2 (at 20°C). For example, water, with its strong intermolecular hydrogen bonding, has one of the highest surface tension values of any liquid, whereas low-boiling-point organic molecules, which have relatively weak intermolecular forces, have much lower surface tensions. In liquids such as water, the meniscus is concave; in liquids such as mercury, however, which have very strong cohesive forces and weak adhesion to glass, the meniscus is convex (Figure \(\PageIndex{4}\)). Cotton towels are also made of cellulose; they absorb water because the tiny tubes act like capillaries and “wick” the water away from your skin. How does polarity difference affect intermolecular forces when two different chemicals interact to form a solution? Weaker than chemical bonds.
Thicker liquids tend to have a higher viscocity. When a glass capillary is put into water, the surface tension due to cohesive forces constricts the surface area of water within the tube, while adhesion between the water and the glass creates an upward force that maximizes the amount of glass surface in contact with the water. Consequently, there is no net force on the molecule that would cause it to move in a particular direction. Legal.
The two most common methods for evaluating the viscosity of a liquid are (1) to measure the time it takes for a quantity of liquid to flow through a narrow vertical tube and (2) to measure the time it takes steel balls to fall through a given volume of the liquid. Plants contain tiny rigid tubes composed of cellulose, to which water has strong adhesion. Motor oils and other lubricants demonstrate the practical importance of controlling viscosity. Viscosity is expressed in units of the poise (mPa•s); the higher the number, the higher the viscosity. Have questions or comments?
London Dispersion Forces: happens in simple hydrocarbons and other non-polar molecules; momentary charge differences cause momentary attractions. Liquids with high viscosity, like honey, are commonly called “thick.”. Surfactants are molecules, such as soaps and detergents, that reduce the surface tension of polar liquids like water.
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