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## Class 11 Physics (India) - Hindi

### Course: Class 11 Physics (India) - Hindi>Unit 8

Lesson 5: Surface tension (Hindi)

# Cohesion and adhesion of water

AP.BIO:
SYI‑1 (EU)
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SYI‑1.A (LO)
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SYI‑1.A.1 (EK)
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SYI‑1.A.2 (EK)
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SYI‑1.A.3 (EK)
Cohesion, adhesion, and surface tension of water and how they relate to hydrogen bonding.

## Cohesion of water

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This dome-like shape forms due to the water molecules’ cohesive properties, or their tendency to stick to one another. Cohesion refers to the attraction of molecules for other molecules of the same kind, and water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another.
Cohesive forces are responsible for surface tension, a phenomenon that results in the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water molecules at the surface (at the water-air interface) will form hydrogen bonds with their neighbors, just like water molecules deeper within the liquid. However, because they are exposed to air on one side, they will have fewer neighboring water molecules to bond with, and will form stronger bonds with the neighbors they do have. Surface tension causes water to form spherical droplets and allows it to support small objects, like a scrap of paper or a needle, if they are placed carefully on its surface.
Illustration of surface tension in a water droplet suspended in a spider's web. Water molecules inside the center of the droplet have more neighboring water molecules to interact with than water molecules at the surface. Thus, the water molecules at the surface form stronger interactions with the neighbors they do have.
Image credit: "Properties of liquids: Figure 2," by OpenStax College (CC BY 4.0).

Water likes to stick to itself, but under certain circumstances, it actually prefers to stick to other types of molecules. Adhesion is the attraction of molecules of one kind for molecules of a different kind, and it can be quite strong for water, especially with other molecules bearing positive or negative charges.
For instance, adhesion enables water to “climb” upwards through thin glass tubes (called capillary tubes) placed in a beaker of water. This upward motion against gravity, known as capillary action, depends on the attraction between water molecules and the glass walls of the tube (adhesion), as well as on interactions between water molecules (cohesion).
The water molecules are more strongly attracted to the glass than they are to other water molecules (because glass molecules are even more polar than water molecules). You can see this by looking at the image below: the water extends highest where it contacts the edges of the tube, and dips lowest in the middle. The curved surface formed by a liquid in a cylinder or tube is called a meniscus.
Illustration of water ascending a small tube via capillary action. The thin tube is inserted into a cup of water, and the water climbs up in the tube, reaching a higher level than it does in the cup. Also, the water extends the highest close to the sides of the tube, and dips down in the middle of the tube. This is because the water molecules are more strongly attracted to the sides of the tube than to each other. The curved surface of the water in the capillary tube is called the meniscus.
Image modified from "Water: Figure 5," by OpenStax College, Biology (CC BY 3.0). Modification of original work by Pearson-Scott Foresman, donated to the Wikimedia Foundation.
Why are cohesive and adhesive forces important for life? They play a role in many water-based processes in biology, including the movement of water to the tops of trees and the drainage of tears from tear ducts in the corners of your eyesstart superscript, 1, end superscript. A simple example of cohesion in action comes from the water strider (below), an insect that relies on surface tension to stay afloat on the surface of water.
Image of a water strider bug walking on the surface of water. This is possible thanks to the surface tension of the water.
Image credit: "Water: Figure 6, by OpenStax College, Biology (CC BY 3.0). Image by Tim Vickers.

## Want to join the conversation?

• So, in the case of the water strider, the strider is more dense than the water but stays afloat because of surface tension. Is this in any way the same as saying that the insect is less dense than the "film" on the surface of the water?
• I would not go as far as to say that the 'film' is denser than the water strider; seeing as this is a problem that more involves pressure and resistence, a better solution would be to form an equation to determine the pressures at the point of contact between the water strider and the 'film'. And Paul Tatman is completely justified in stating that the water strider would likely sink if all of its body weight were to be concentrated on one leg, or all four legs right next to one another (increasing the pressure), as opposed to it being spread out over four widly splayed legs (a greater area), if there are four legs in the above image that is.
• I was wondering if adhesion is affected by the temperature of a fluid
• Yes. Cold water will adhere to a glass container more than hot water, because I think the water molecules are moving around more slowly when they're cold and are therefore more able to stick to the glass than the faster moving hot ones. (Note: I actually tested this with some hot water and cold water. Try it!)
• Is an example of Adhesion when you drink a glass of water and there is still water droplets still stuck to the inside of the glass?
• If the cup is polished to the perfection then the droplet should roll all the way down to the bottom of the cup. This should happen because an isolated droplet stuck to the wall of the cup is surrounded by glass in all directions, so, not only the glass over the droplet is attracting it, but also glass under the droplet. The fact is that cups that we use to drink are not perfect. They have grooves, bumps and notches caused by production itself, or by washing it, or by entering in contact with other tableware, whatever... these imperfections plus adhesion are what makes the droplets that we see possible.
• I just had my blood donated and we are learning this in my Ap biology class, I was just wondering, does adhesion and/or cohesion take place when someone is donating their blood? I mean through the tube, or is it some other biophysics term?
• Yes. Adhesion happens because the water prefers to stick to the tube, causing capillary action. That then goes up the tube, because blood prefer sticking to the tube.
• ok, so does capillary action contribute to why bubbles work? just curious
• That's correct. Cohesive forces cause the water molecules to stick together with a lot of elasticity, allowing the water to function very much like a rubber balloon so that when it is filled with air it doesn't break and simply forms a blanket over the air pocket.
• will water eventually fill and the capillary tube and go out
• No, because the water can only rise as long as the adhesive forces are stronger than the force of gravity. Once the force of gravity is equal to the adhesive forces (which vary from substance to another, according to the degree of polarity) between the water molecules and the substance's, the water will cease to rise. This is why the water rises to different heights in capillary tubes made of different materials (substances).
Hope this helps! :)
• If cohesion is the cause of water moving to the tops of trees, (I'm guessing this is a also a capillary action) what is water adhering to in the tree that overcomes the cohesion in the water? (If this sounds like nonsense, apologies, I am trying to grasp the concepts in this article).