Cohesion and Adhesion of Water

Water is the basis of all life on this Earth. No living organisms can survive without water. If there had been no water on this Earth, then there would be no vegetation. Hence there would be no food for herbivores and therefore no food for carnivores also.  That means the entire food chain would collapse. It is clear that water supports life on Earth. Which property of water enables it to support life on Earth? The two properties of water which enable it to support life on Earth are Cohesion and Adhesion.

Water is sticky. Why?

Water is made up of two hydrogen atoms and one oxygen atom. The electric field around the Oxygen atom is stronger than that around the Hydrogen atom. The electrons from the Hydrogen atoms are attracted towards the Oxygen atom. This leaves the hydrogen atoms positively charged and oxygen atom negatively charged. In water molecule hydrogen atoms each share a pair of electron with the oxygen atom forming covalent bonds. Therefore, water is a covalent compound.

In a molecule of water, there are two ends. One end where oxygen atom is present and it is negatively charged. The other end with the hydrogen atoms is positively charged. Hence, the two ends of a water molecule have opposite charges. Due to this polarity, the positive hydrogen of one molecule is attracted to the negative oxygen of a different molecule of water forming a weak bond called hydrogen bond. Each water molecule is connected with the other molecules through hydrogen bond. This causes water molecules to stick to each other. Water sticking to itself is known as Cohesion. 

Cohesion:

Cohesion occurs because the partial negative charges on oxygen atom stick to the partial positive charges of hydrogen atoms of other water molecules. This causes stickiness in water. Hence, cohesion is the attraction between molecules of the same substance such as the attraction between two molecules of water. Water has high Cohesion.

Examples of Cohesion:

1.When water falls on lotus leaves or colocasia leaves water beads up on these leaves because water molecules hold each other together, forming the least surface area. The force of attraction between water molecules is greater than the force of attraction between the leaf and water. This force is called cohesive force. The cohesive force depends on the attractive force between the liquid molecules.

2.Force of attraction between the molecules of mercury is greater than that of other liquids. Mercury does not flatten on the surface of other substances.

3.Dip one end of a piece of paper towel into a glass of water. The water will climb up the fibres of the paper, getting it wet above the level of the water in the glass. We know that gravity is pulling down the water, but it still moves up. Why? Because the water molecules are positive. Negative charges present in the paper towel are attracted to the positive charges present in water.

4.Fill a glass of water to the very top and then slowly add a few more drops of water. Before it overflows, the water forms a dome-like shape above the rim of the glass. This dome-like shape is formed due to the cohesive properties of water molecules, or their tendency to stick to one another.

5.Cohesive forces between water molecules are responsible for surface tension of water. Due to the surface tension, small objects will "float" on the surface of the fluid, because the floating object cannot break the top layer of water molecules. When an object is on the surface of water, the top surface will behave like an elastic membrane. This is the reason why small insects can walk on water because their weight is not enough to penetrate the surface of water.

Adhesion:

Adhesion is also like cohesion. Adhesion   involves unlike (i.e. different) substances sticking together. Water is very adhesive; it sticks well to a variety of different substances. Water is polar so it is attracted to substances that have charges.  Adhesion is when water molecules stick to other substances.

Examples of Adhesion:

1.Place a thin glass tube inside a beaker containing water. Water will climb the glass tube without suction. Water adheres to the partial charges of glass. This is the reason why we see meniscus or a curved line in the glass tube. This is called capillary action.

2.Take two slides and wet it. Press it together. It is difficult to separate them. The force of attraction between water and the slides is more than the force of attraction between water molecules. So the force experienced here is adhesive force.

3.Wet the paper and stick to the wall.  The wet paper sticks to the wall because of the adhesive forces between the wall and the wet paper. The cohesive force between the water molecules is lesser than the adhesive force between the wet paper and the wall.

4.We wet our fingers when we want to count the currency notes because the adhesive force between the wet fingers and the currency is greater than the cohesive force between the currencies.

Cohesive forces and adhesive forces are important in our daily life and in Nature. They are:

Movement of water from roots to the top of the trees.

Absorption of water by roots of the plants and trees from soil.

Drainage of tears from the tear ducts in the corners of our eyes.

Some insects can walk on water and many more.... 

In the next post experiments on cohesion and adhesion will be discussed.

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