What do cohesion and adhesion mean

Similarly, the term "adhesive forces" refers to the attractive forces between unlike substance, such as mechanical forces sticking together and electrostatic forces attraction due to opposing charges.

In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glasses surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules. When a liquid is placed on a smooth surface, the relative strengths of the cohesive and adhesive forces acting on that liquid determine the shape it will take and whether or not it will wet the surface.

If the adhesive forces between a liquid and a surface are stronger, they will pull the liquid down, causing it to wet the surface. However, if the cohesive forces among the liquid itself are stronger, they will resist such adhesion and cause the liquid to retain a spherical shape and bead the surface. The meniscus is the curvature of a liquid's surface within a container, such as a graduated cylinder.

However, before we explain why some liquid have a concave up meniscus while others share a concave down meniscus, we have to understand the adhesive forces at work of surface tension.

Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens and a partial negative charge on the oxygen.

Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge. This is the origin of the cohesive forces within the water. Water molecules buried inside the liquid is then being pulled and pushed evenly in every direction, producing no net pull. Meanwhile, the molecules on the surface of the liquid, lacking pulling forces in the upward direction, thus encompass a net downward pull.

But in this case, the cohesive force of water molecules provides a very strong pull. Figure 10 shows one device for studying negative pressure. Some experiments have demonstrated that negative pressures sufficient to pull sap to the tops of the tallest trees can be achieved. Figure The density of oil is less than that of water, yet a loaded oil tanker sits lower in the water than an empty one.

Birds such as ducks, geese, and swans have greater densities than water, yet they are able to sit on its surface. Explain this ability, noting that water does not wet their feathers and that they cannot sit on soapy water.

Water beads up on an oily sunbather, but not on her neighbor, whose skin is not oiled. Explain in terms of cohesive and adhesive forces.

What effect does capillary action have on the reading of a manometer with uniform diameter? Explain your answer. Pressure between the inside chest wall and the outside of the lungs normally remains negative.

Explain how pressure inside the lungs can become positive to cause exhalation without muscle action. What is the pressure inside an alveolus having a radius of 2. You may assume the pressure is the same as that created by a spherical bubble. Assuming the alveolus acts like a spherical bubble, what is the surface tension of the fluid? You may need to extrapolate between values in Table 1. What is the gauge pressure in millimeters of mercury inside a soap bubble 0.

Calculate the force on the slide wire in Figure 3 shown again below if it is 3. Figure 9 a shown again below shows the effect of tube radius on the height to which capillary action can raise a fluid. We stated in Example 2 above that a xylem tube is of radius 2.

What fluid is in the device shown in Figure 3 shown again below if the force is 3. If the gauge pressure inside a rubber balloon with a Calculate the gauge pressures inside 2. Which liquid forms the most stable bubbles, neglecting any effects of evaporation? Suppose water is raised by capillary action to a height of 5.

Is this value consistent with that for most organic liquids? When two soap bubbles touch, the larger is inflated by the smaller until they form a single bubble. Calculate the ratio of the heights to which water and mercury are raised by capillary action in the same glass tube. What is the ratio of heights to which ethyl alcohol and water are raised by capillary action in the same glass tube?

Alcohol forms the most stable bubble, since the absolute pressure inside is closest to atmospheric pressure. Skip to main content. Fluid Statics. Search for:. Define surface tension. Understand capillary action.

Cohesion and Adhesion in Liquids. Adhesive Forces Attractive forces between molecules of different types are called adhesive forces. Surface Tension Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. Making Connections: Surface Tension Forces between atoms and molecules underlie the macroscopic effect called surface tension. These attractive forces pull the molecules closer together and tend to minimize the surface area.

This is another example of a submicroscopic explanation for a macroscopic phenomenon. Table 1. Example 1. Cohesion : Water is attracted to water, and Adhesion : Water is attracted to other substances. I used to wake up in a cold sweat because I could not get the concepts of water adhesion and cohesion clear in my mind. If you have that problem, too, then read on to learn about these important properties of water Cohesion : Water is attracted to water Adhesion : Water is attracted to other substances.

Adhesion and cohesion are water properties that affect every water molecule on Earth and also the interaction of water molecules with molecules of other substances. Essentially, cohesion and adhesion are the "stickiness" that water molecules have for each other and for other substances.

A water drop is composed of water molecules that like to stick together-an example of the property of cohesion. In the picture of pine needles above, the water droplets are stuck to the end of the pine needles-an example of the property of adhesion. Also noticeable in this picture is the effect that gravity has on the water drops.

Gravity is working against both adhesion and cohesion, trying to pull the water drop downward. Adhesion and cohesion are winning the battle so far, as the drops are sticking to the pine needles. If you just look at the picture of the water drop sitting of the leaf, you might think the water drop has a "skin" holding it into a sort of flattened sphere although there is nothing flat about a water drop in outer space. It turns out that this surface tension is the result of the tendency of water molecules to attract one another.

The natural form of a water drop occurs during the "lowest energy state", the state where the atoms in the molecule are using the least amount of energy. For water, this state happens when a water molecule is surrounded on all sides by other water molecules, which creates a sphere or ball perfectly round if it was in outer space.

On Earth, the effect of gravity flattens this ideal sphere into the drop shape we see. Although you may have heard of a "skin" where water meets the air, this is not really an accurate description, as there is nothing other than water in the drop.

What is the shape of a raindrop? Are they really tear-shaped? Maybe not. Find out here. Water is highly cohesive—it is the highest of the non-metallic liquids.

Water is sticky and clumps together into drops because of its cohesive properties, but chemistry and electricity are involved at a more detailed level to make this possible. More precisely, the positive and negative charges of the hydrogen and oxygen atoms that make up water molecules makes them attracted to each other. If you've played with bar magnets you will know that the north pole of one magnet will repel the north pole of another magnet, but it will attract the south pole of another magnet.

Opposite magnetic poles attract one another much like positively charged atoms attract negatively charged atoms in water molecules. The adhesion and cohesion forces are one of both phenomena. Although they sound to be similar, they are completely known to occur in different terms. Surface tension is one of the important physical properties of water that describes the relationship between adhesion and cohesion. Coming to the definitions, the tendency of either two or more different molecules to bond with each other is called Adhesion.

In contrast, the attraction force between the same molecules is referred to as Cohesion. Adhesion forces can be one of the electrostatic forces' results exerted on various substances. Simultaneously, the cohesive forces are associated with the Van der Waals forces and hydrogen bonding that cause liquids like water to withstand the separation. When a glass surface is poured with water, the forces of both adhesive, cohesive act on the water's surface.

Also, a strong adhesive force tends the liquid to spread over the surface; on the other side, a strong, cohesive force is responsible for forming water droplets on the water surface. Both the adhesion and cohesion forces vary in their strengths. For example, if the cohesion forces between water molecules are stronger compared to the adhesion forces between them, then the individual molecules present in them will attract towards each other hence resulting in settling.

If the adhesion forces of the water surfaces are stronger than the water molecules' cohesion forces, the water tends to disperse then. Let us discuss the adhesion and cohesion difference.