Adhesion vs Cohesion
In Science, particularly Chemistry and Physics, there are a lot of technicalities discussed that are very applicable to man’s life. These subjects often explain why certain phenomena occur in everyday living. With regard to the naturally occurring forces, adhesion and cohesion are often confused with each other.
Although these terms sound the same, they are actually the opposite ends of the same coin. Basically, adhesion is the attraction shared between several dissimilar molecules. It is then the joining of different forms of matter. Cohesion, on the contrary, is the force of attraction between similar molecules. It is simply the force that holds all molecules of one type of matter or object altogether.
It is best to explain these forces using the unique properties of water. As a liquid form of matter, water is composed of several molecules that are not that loosely packed from each other as compared to gas. Also, water is made of two hydrogen atoms and one oxygen atom. The molecules of water carry two charges (a positive and a negative one) making it a dipole. The electrons, that carries the negative charge clump near oxygen and at the same time distancing itself from hydrogen. As a result, one negative end of the water molecules will bind together with another positive end which gives its cohesive characteristic.
In practical application, you will not be surprised why water falls as rain (or raindrops) and not in the form of individual molecules. On hydrophobic surfaces (i.e. waxed surfaces of the car or floor) and similar areas, water gathers like beads because of this property. Water molecules just pull together towards each other.
Once again because of the water bearing two different charges, it still has another ability ‘“ to get attracted to other surfaces or molecules that carry different charges. This is when adhesion sets in. if you dip a piece of paper on a basin filled with water, you will notice that it (the paper) will slowly become wet going up from the point of contact with water. This happens even with the natural action of gravity because the adhesion forces of the paper molecules are strong enough to attract the differing charges of the water molecules.
Both adhesion and cohesion forces vary in strength. For example, if the cohesion force of the water molecules is stronger than any other adhesion forces surrounding them then its individual molecules will stick toward each other resulting to clumping or settling. In cases where the adhesion forces of other matter or surfaces are stronger than the cohesion force of the water molecules, then the water will get dispersed.
1. Adhesion is the molecule attraction between dissimilar molecules.
2. Cohesion is the molecular attraction between similar types of molecules.