The key difference between cohesion and surface tension is that cohesion describes the intermolecular forces occurring between identical molecules, whereas surface tension describes the elasticity of the surface of a liquid.
Surface tension is a property of liquids, which arises due to the cohesion forces between identical liquid molecules. Cohesion can be described as the aggregation of similar molecules due to the intermolecular attraction forces between them.
CONTENTS
1. Overview and Key Difference
2. What is Cohesion
3. What is Surface Tension
4. Side by Side Comparison – Cohesion vs Surface Tension in Tabular Form
5. Summary
What is Cohesion?
Cohesion is a type of intermolecular force that occurs between two identical molecules. For example, the interaction between water molecules can be named as cohesion. This property of cohesion in water allows the water molecules to travel with consistency (in other words, continuous flow is maintained by cohesive forces). Furthermore, we can explain the shape of the raindrops or the existence of water droplets rather than single molecules using the concept of cohesion.
Figure 01: The Shape of Water Drops
Moreover, the ability to form hydrogen bonds between water molecules is the main reason behind the cohesion forces of water molecules. Each water molecule can form four hydrogen bonds with other water molecules; therefore, the collection of attraction forces is much stronger. Electrostatic forces and Van der Waals forces between similar molecules also cause adhesion. However, adhesion due to Van der Waals forces is somewhat weaker.
What is Surface Tension?
Surface tension is a phenomenon where the surface of a liquid, when the liquid is in contact with the gas, acts like a thin elastic sheet. This term is useful only when the liquid is in contact with a gas (ex: when opened to the normal atmosphere). The term “interface tension”, on the other hand, is important for the layer between two liquids.
Figure 02: Some Small Insects can Walk on the Surface of Water due to Surface Tension
Furthermore, the attraction forces between different chemical species cause the liquid molecules to unite together. Here, the liquid molecules in the surface of the liquid are attracted by the molecules in the middle of the liquid. Therefore, this is a type of cohesion. However, the attraction between liquid molecules and gaseous molecules in contact with the liquid (or the adhesive forces) are negligible. This allows this surface layer of liquid molecules to act as an elastic membrane. This surface layer of liquid molecules is under tension because there are not enough attraction forces to balance the cohesive forces act on them; therefore, this condition is called surface tension.
Formula for Calculation of Surface Tension:
Surface Tension (γ) = F/d
In the above formula, F is the surface force and d is the length along which surface force acts on. Therefore, the measurement of surface tension is given by the unit N/m (Newton per meter). It is the SI unit for the measurement of surface tension.
What is the Difference Between Cohesion and Surface Tension?
The key difference between cohesion and surface tension is that cohesion describes the intermolecular forces occur between identical molecules, whereas surface tension describes the property of elasticity of the surface of a liquid. In brief, surface tension can be observed due to cohesion.
Below infographic summarizes the difference between cohesion and surface tension.
Summary – Cohesion vs Surface Tension
Surface tension can be observed due to cohesion. The key difference between cohesion and surface tension is that cohesion describes the intermolecular forces occur between identical molecules, whereas surface tension describes the property of elasticity of the surface of a liquid.
Reference:
1. “Cohesion and Adhesion of Water (Article).” Khan Academy, Available here.
Image Courtesy:
1. “Cohesion” By Dave Nakayama (CC BY 2.0) via Flickr
2. “1221260” (CC0) via Pxhere