Water is one of the important molecules of life. Hydrogen and oxygen combines to form water. Like many other molecules, water also has many properties that help them functional for the life. Among many, the two main property of water is adhesion and cohesion.
Cohesion involves attraction between molecules of same substance whereas adhesion is attraction between molecules of different substance. Likewise, this is the reason when adhesive force is greater liquid spreads in the surface and when cohesive force is greater, liquid gains certain shape of its own.
Adhesion is the property of water were it get attach to other substances and cohesion is the property of water due to which water molecule remain attached to other water molecules. These properties are very important for the water molecules. In the more explanatory way, when water molecule stick to one another, they gain spherical shape whereas when water is poured in any container, the attraction between water molecule and the surface molecule is more than two water molecules, so the water molecules that touch the surface of the container are at higher level. Likewise, this is the reason water spread to the surface rather than being stick to one another. Here, adhesive force is much more than cohesive force.
One more term is very important in this topic, i.e. surface tension. It is the tension cause by the surface film that minimizes the surface area. This can be more clearly explained with an example. Like, the water we see in the spherical shape is because the attraction or force between the water molecules is equal in all direction whereas that at the surface is unequal. Due to this uneven force, the surface molecules are pulled in the inner side and hence this gives water droplets the spherical shape. Therefore, from this it can be concluded that surface tension of liquid help it gain the spherical shape. Beside this, when a heavy object is thrown in water, sometimes it floats, like the log because the force that is cause due to surface tension is more than the actual weight of the object. Likewise, in the other example we can even see droplets of water in leaves of pine tree. It is because here, cohesiveness between the molecules of water help it remains in spherical shape and the adhesive property helps it remain stick to the surface of leaves. Likewise, the force of gravity rolls the droplets down the leaves. These are some of the forces and actions that come into play along with the property of water.
The cohesive property of water is because of the molecules that are excellently aligned to have the positive and negative charges. Oxygen lies between the two hydrogen atoms and among them hydrogen is slightly positive and oxygen is slightly negative. These positive and negative charges attract each other giving water molecule cohesive property.
Beside this there are many more examples that explain adhesive and cohesive property of water. When mercury is poured in the container, the cohesive forces between the molecule is greater and that of adhesive force between the molecule of mercury and the surface molecule, which is less in comparison to the cohesive force and this is the reason convex shape can be noticed. Likewise, in the other case when water is poured in the container, the cohesive force between water molecules is less than that of the adhesive force between the water molecule and surface molecule. Hence, in this case the surface of water curves up. Beside this, capillary action can also be seen due to the cohesive and adhesive property of water that acts against the force of gravity and this property is used in many processes like water from roots to tip, removal of sweat form the skin etc.
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