Consider a molecule in the interior of liquid present in a container, it is attracted from all sides by other molecules so no net force is on it.

Now consider a molecule on the surface of a liquid. It is attracted from all sides except the upper side so it experiences a net downward force due to which the liquid surface stretches. This is surface tension.

Surface tension is defined as

“Stretching of a liquid surface due to net downward force”                      

OR

“The force acting perpendicularly downward at the right angle to the unit length of liquid surface”

OR

“The work done in increasing unit area of liquid surface”

Factors affecting Surface Tension:

1. Intermolecular forces:

Stronger are the intermolecular forces high will be the surface tension and vice versa. e.g.

Surface tension order for water, alcohol (spirit), mercury & ether is

   Hg > H₂O > C₂H₅OH > C₂H₅-O-C₂H₅

Ether has London forces; mercury has metallic bonding while the other two have Hydrogen bonding. London forces are weaker so ether has low surface tension. Water and alcohol have hydrogen bonding so their surface tension is higher than ether. Hydrogen bonding of water is stronger than that of alcohol so the surface tension of water is higher than alcohol. Mercury has metallic bondings which are stronger than hydrogen bonding so the surface tension of mercury is still higher.

2. Temperature:

            Higher is the temperature weak will be the forces and thus low will be the surface tension. e.g. surface tension of hot water is lower than that of cold water.

3. Soap or detergent:

The presence of soap or detergent decreases the surface tension of water because forces among water molecules become weaker.

4. Salt:

The presence of salt increases the surface tension of water because salt ions attract water molecules with greater force. e.g. surface tension of seawater is higher than that of fresh water.

Surface Tension Examples

Small insects called “water striders” are able to walk on water because it takes far less of their weight to pierce the water’s surface. There are other instances of surface tension in nature that are similar to this. Below are a few examples:

• insects moving through water
• floating a needle over the water’s surface
• Materials for rainproof tents where water’s surface tension will cause the pores to be bridged
  medical examination for jaundice
• disinfectants for surface tension (disinfectants are solutions of low surface tension).
• washing clothing with soaps and detergents that reduce the water’s surface tension
• using cold water to wash
• Water bubbles form in the form of rounded bubbles where the water’s surface tension acts as the wall tension.
  The shape of liquid droplets is also a result of this phenomenon.

Measurement of Surface Tension:

Units of Surface Tension:       

  S.I. Units →         N/m,           J/m²

 CGS System →      dyne/cm,         erg/cm²

How to Calculate Surface Tension?

Methods to Calculate Surface tension:

There are three methods of measurement of surface tension

1. Torsion balance method

2. Capillary rise method

3. Drop method using Stalagmometer:  

           In this method, relative surface tension is measured. For this purpose number of drops of reference and test liquid are counted for a volume of liquids present between two marks a & b on the stalagmometer.

Water is usually chosen as a reference liquid.

Its surface tension at 293K is 7.275 x 10-2 N/m or 72.75 dyne/cm           

Cohesive and Adhesive Forces:

 The forces of attraction among liquid molecules themselves are called cohesive forces.  The forces of attraction between liquid molecules and the wall of the container are called adhesive forces.

The liquids that have adhesive forces stronger than cohesive forces form concave meniscus and they are wetting liquids e.g. water.

The liquids that have cohesive forces stronger than adhesive forces, form convex meniscus and they are non-wetting liquids e.g. mercury.

The shape of the liquid surface is called the meniscus.           

Capillary Action:

The rise of liquid in a capillary tube is called capillary action.

 It is due to strong adhesive forces between the liquid and the wall of the capillary. e.g. spreading ink or water on blotting paper, absorption of water by cotton, water absorbed by roots from the soil is supplied to leaves through the xylem, supply of oil to a flame by wicks, all these are due to capillary action. These materials are porous and their pores act as capillaries.

The tubes of narrower bores show more height. Actually same volume of liquid rises through capillary tubes, so the less the area of the cross-section of the tube more will be the height gained by liquid as Volume = Area x Height.

Frequently Asked Questions – FAQs