Surface Tension

The attractive force which is exerted upon the surface molecules of a liquid by the molecules beneath the surface tends to draw the surface molecules into the bulk of the liquid and making the liquid assume the shape having the least surface area is called surface tension. Surface tension of water helps creatures mostly of insect class such as water striders to walk or to float on water. It also helps water to move up the xylem tissue of higher plants without breaking up, these are a few applications of surface tension in day to day life. 

Formula

Surface tension is given by the equation S = (ρhga/2) where S is the surface tension, ρ (or rho) is the density of the liquid that is to be measured, h is the height the liquid that rises in the tube, g is the acceleration due to gravity acting on the liquid which is 9.8 m/s2 and a is the radius of the capillary tube. 

Effect of detergents on surface tension

When detergent is applied to water, it lowers the water’s surface tension. Surfactants are compounds that lower the surface tension of water by separating the water molecules from one another. The surface tension in the area where the surfactant is injected is reduced in this activity, and the higher surface tension in front of the raft pulls the raft forward. The surface tension drops as the detergent distributes through the water, and the raft eventually stops moving since there is no longer a difference in surface tension.

Surface tension of water by capillary rise

Capillary rise is a phenomenon in which liquid spontaneously rises or falls in a narrow space such as a thin tube. Adhesion of forces of water to the walls of a vessel will create an upward force on the liquid at the edges and result in a meniscus which turns upward and then the surface tension will act to hold the surface as it is, and then the capillary action will occur when the adhesion to the walls is stronger than the cohesive forces of attraction between the liquid molecules, and hence water will rise inside the capillary tube due to adhesion between water molecules and the glass walls of the capillary tube. This adhesion, together with surface tension in the water, produces an effect which is called capillarity. The factors that are used to determine capillary rise are the diameter of the capillary tube, the contact angle between the liquid and the surface to which it adheres, the density of the liquid, and the viscosity of the liquid.

Conclusion

The surface tension of water is 72 mN/m at room temperature and it is one of the highest surface tension for any liquid. There is only one liquid having higher surface tension than water and that’s mercury which is a liquid metal with a surface tension of 500 mN/m. At the interface, the liquid molecules have only half of the neighbouring liquid molecules compared to the bulk of the liquid. This makes the molecule associate more strongly with the molecules at its sides and causes a net inward pull force towards the liquid. This force mainly resists the breakage of the surface and is called surface tension. The stronger the cohesive force of attraction, the stronger the surface tension. The water molecule has two hydrogen atoms bond to an oxygen atom with the help of covalent bonding and due to the high electronegativity of oxygen; it will have a large portion of the negative charge on its sides whereas the hydrogen will be more positively charged which will cause an electrostatic attraction between the hydrogen atom in one molecule and the oxygen atom in another molecule and the bonds formed are called hydrogen bonds which are mainly strong cohesive forces between the water molecules and high surface tension of water.