Cohesion is a term that describes how molecules stick to each other.
The force of cohesion is determined by the structure, shape, and distribution of electric charge. It is also known as an intrinsic property of molecules.
The simplest example of cohesion is water. In this, the particles of water stick together because of hydrogen bonding. It is entirely different from adhesion, which occurs because of intramolecular bonds.
The surface tension of water is also attributed to this property. The term surface tension can be defined as the tension in the surface layer of a liquid when it comes in contact with gas or air.
This can be explained by the phenomenon that the molecules present in water are pulled from all directions. This force is strongest in the middle and weakest at the surface.
Because molecules are pulled towards the middle at the surface, the fluid shrinks and forms a surface with minimum area, this is the reason behind the drops of water being spherical.
It is this surface tension that resists external force, and because of this, lighter substances float on the surface while heavier substances break through the top layer and shrink to the bottom of a fluid. It is because of this surface tension of water that some insects can walk on water.
Cohesion forces are strongest in solids, considerable in liquids, and weakest in gases. This can be best explained with an example. Water molecules are more attracted to each other than they are to air molecules.
Water consists of HOH particles, meaning one oxygen and two hydrogens. Though the net charge in a water molecule is zero, water is polar because of its shape. This water molecule consists of two hydrogen atoms and one oxygen atom.
The hydrogen ends of the molecule are positive, and the oxygen end is negative. This makes water a polar molecule. Because of this polarity, it possesses the properties of cohesion, adhesion, and surface tension.
Adhesion and cohesion in water-based processes play an integral role. It includes the procedure of passing water to the top of the tree, which lets every part, like leaves, buds, flowers, stems, and others, get a sufficient amount of water.
This water behavior is what you can term cohesion in simple words, and strong attraction makes molecules sticky, which will help them draw together.
This attraction of molecules allows for another phenomenon known as capillary action. Take a glass of water and place a thin straw. After a few seconds, you will find that the water gets attracted to it.
However, at the same time, this fluid wants to stick to other molecules. If the attraction of adhesion between the straw and water is strong, then due to this attraction of cohesion, the fluid will move up without any assistance. This discovery was made after some experts did experiments in labs.
In 1895, J Joly and HH Dixon, Irish plant physiologists, said that water is pulled up by plants and transported to different parts through negative pressure or tension. Also, you can see that water is being lost from leaves and stems by transpiration.
Both Joly and Dixon believed that the water loss in these leaves exerts pull because of which more water is drawn into the leaf.
But the question that remains is how water is being transported from ground level to these leaves or the other parts of a plant. The answer lies in the concept of cohesion of water molecules. This property of water allows the molecules to stick to one another with the help of hydrogen bonds.
Importance
Did you ever fill a glass of water fully and try adding a few more drops from the top? If not, you should do it to find out the outcome.
Before the fluid starts to overflow, you will find a dome-like shape forming on the glass. It is just about the rim of the glass, which happens because of molecules present in cohesion.
As you already know, it happens because of surface tension. It is a tendency of a liquid surface that can resist rupture when placed under stress or tension.
The water molecules form hydrogen bonds at the surface with their neighbors. Here, the molecules in touch with the air will have fewer water molecules to bond with. But with the other molecules, they will have stronger bonds. Because of this surface tension, the fluid takes the shape of droplets and allows it to support small objects.
Because of cohesion, the water molecules let plants absorb water from the soil with the help of their roots. Cohesion also leads to a high boiling point of water, which will help regulate animals' body temperature.
Also, the molecules in water can form bonds surrounding both their negative and positive regions. To understand it better, you can take the example of sugar and water.
Both sugar and water are polar, and the individual molecules of water surround the individual molecules of sugar, breaking them apart. A similar thing happens when you add salt to water due to cohesion.
In addition to that, it is because of this phenomenon that a substance can withstand an external force and does not rupture easily under stress or tension because of this phenomenon. Furthermore, it is the reason why water forms droplets on a dry surface before breaking apart due to excessive tension.
This property of cohesion is also responsible for water's high boiling point. As previously noted, it also helps animals regulate their body temperature.
Did you know that it is possible to float a needle on water, given it is placed very gently without breaking the surface tension of water?
Reasons For Cohesion
Cohesion makes water sticky, and it happens because of hydrogen bonds. Naturally, water has the property to stick to other substances or to its own molecules.
Cohesion describes the capability to attract, making water a sticky liquid. Hydrogen bonds attract due to electrostatic energy that causes the difference in charge between negative and positively-charged ions.
The hydrogen bonds form between these neighboring oxygen and hydrogen atoms of water molecules present in them. In other words, the attraction that leads to the creation of water molecules is known as hydrogen bonds.
Water has higher negative charges, which indicate that it needs more electrons. Cohesion in water is so strong that hydrogen causes more water molecules to bind tightly. That's why you will find that water has formed a tight membrane on the surface.
Places Where Cohesion Takes Place In Nature
Cohesion and adhesion are natural forces that occur around us all the time. Water molecules sticking to each other or molecules of mercury attracting each other are examples of cohesion.
If you observe mercury in a container, then the surface of the fluid appears to be convex. This is because of the strength of cohesion in mercury. The surface tension of water is also because of cohesion. In addition to that, cohesion plays a crucial role in facilitating water transportation in plants.
Another example of cohesive force is the pressure present in biomolecules like DNA. For example, in meiosis and mitosis, the cohesion event gets mediated by several protein complexes.
These are known as cohesins. After the duplication of DNA, cohesion is responsible for holding the sister chromatids together while preparing for cell division. Cohesion is utilized by both meiosis and mitosis, which helps in keeping the sister chromatids together.
Cohesion Vs. Adhesion
Cohesion and adhesion are both forces of attraction, and both are important for determining the movement of a watery substance or a fluid over a solid surface. However, cohesion is of an intermolecular attraction type, while adhesion is of an intramolecular type.
Cohesion is the force that exists between the same molecules of the same kind. For example, the energy existing between two water molecules that makes a water drop is because of cohesion. The same energy is witnessed among molecules of mercury. In water molecules, the cohesive force is more rampant.
on the other hand, adhesion is the tendency of two or more different molecules to bond with each other. This force is responsible for giving the water its stickiness.
A water droplet sticking on the surface of a stem against gravity is an example of adhesion. In adhesion, the force of attraction is present between walls of xylem cells and water molecules.
Cohesion is the force that gives water drops a spherical shape. In other words, in a water molecule, the hydrogen and oxygen atoms are held together by this force. In comparison, adhesion gives the water its property to spread over a surface.
Cohesion is related to weak Van der Waals forces and surface tension. In contrast, adhesion involves electrostatic or mechanical forces. This force acts as a natural glue that helps different molecules to stick to each other. In most cases, cohesion exists among liquid substances, while adhesion is seen between a solid and a liquid substance.
The effects of cohesion are capillary action, meniscus, and surface tension. Capillary action is the curved surface that is formed by any liquid present in a cylinder, and the meniscus is the effect of adhesion.
Both cohesion and adhesion vary in their strengths. If cohesion between molecules is very strong, then it results in the settling of a substance. But if the adhesion force is stronger, then it results in dispersion.
Cohesion is a concept that works against gravity, just like adhesion. But these two forces have different roles to play. Cohesion is a natural force determined by several properties of a liquid. It helps in several day-to-day activities, many of which go unnoticed. It would have been difficult for plant life to survive without this pressure.
FAQs
Who discovered cohesion?
Joly and Dixon discovered cohesion in 1894 and Boehm in 1893. Later, this theory was supported by Galston and Bonner in 1952, Clark and Curtis in 1951, Renner in 1911, and Kozlowski and Gramer in 1960.
What is a force of cohesion?
The force of cohesion is a strong mutual bond formed among similar molecules and cannot be separated without an external force.
What are the different types of cohesion?
Different types of cohesion that will help a science student understand why molecules are tightly bound to each other are discussed below.
Sequential cohesion is where a wide range of molecules are categorized into a series of activities. In functional cohesion, molecules perform similar or related functions. Communicational cohesion is a situation where every molecule shares common data.
Temporal cohesion is a process where activities happen in the same period. In procedural cohesion, molecules share the exact procedural implementation. The start-up activities or functions responsible for initialization, like control flags or setting programs, exhibit temporal cohesion.
Another type is logical cohesion, where the same categories of activities are grouped. Coincidental cohesion is another type that includes instructions with no or little relationship to each other. It is always better to avoid coincidental cohesion as much as possible.
How do you observe cohesion?
Cohesion is a simple principle due to which water is attracted to water particles. So, if you observe a water drop, you will see that water particles stick together.
Which cohesion is best?
Functional cohesion is the best type of cohesion because it features the highest degree of cohesion. The molecules functionally are grouped into logical units, and it helps in promoting reusability and flexibility.
What is cohesion used for?
Cohesion helps develop surface tension, which is why it takes the shape of drops when they are kept on a dry surface. They do not get flattened because of gravity.
Why is cohesion important to life?
Cohesion is important in life because it helps plants transport water from their roots to leaves and other parts. Also, it contributes to the high boiling point of water and helps animals regulate their body temperature.
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Joan AgieBachelor of Science specializing in Human Anatomy
With 3+ years of research and content writing experience across several niches, especially on education, technology, and business topics. Joan holds a Bachelor’s degree in Human Anatomy from the Federal University of Technology, Akure, Nigeria, and has worked as a researcher and writer for organizations across Nigeria, the US, the UK, and Germany. Joan enjoys meditation, watching movies, and learning new languages in her free time.
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