The property of liquid by virtue of which it behaves like a stretched elastic membrane with natural tendency to contract ( occupy ) minimum surface area . Thus a liquid surface is always in a state of tension .
It is also defined as the force per unit length acting tangentially and right angle on either side of an imaginary line drawn at the liquid surface .
ST=F/L
example :
The shape of the liquid drop is determined by the potential energy due to surface tension . For any given volume of liquid , minimum area is occupied by the sphere . When the area of the surface is minimum the potential energy is minimum due to surface tension . Hence the liquid drop is spherical in shape.example :
| Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension. |
EFFECT OF TEMPERATURE :
The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with increase in temperature as shown in the graph.
Hot soup is more tasteful than cold one.
The surface tension of hot soup is less than that of cold one and therefore hot soup covers more area of the tongue.He nce hot soup is more tasty than the cold one
Iron needle can float on the surface of water.
End of a glass tube becomes round on heating.
Glass tube melts on heating and forms liquid.The liquid has tendency to occupy minimum surface area due to surface tension.
Since for any given volume a sphere has a minimum surface area hence;melted liquid at ends of the glass tube tends to assume spherical shape .Therefore ends of tube on heating becomes round
Angle of contact
The angle between the tangent drawn at the liquid surface at the point of contact and the solid surface inside the liquid is called the angle of contact for the pair of solid and liquid.
In figure( a) and( b) are shown the angle of contact for water and glass and for mercury and glass respectively.
The angle of contact for those liquids which wet the liquid is acute. It is zero for pure water and clean glass;for ordinary water and glass it is about 8
.The liquids which do not wet the solids have obtuse angle of contact.For mercury and glass the angle of contact is 135 .
Angle of contact
The angle between the tangent drawn at the liquid surface at the point of contact and the solid surface inside the liquid is called the angle of contact for the pair of solid and liquid.
In figure( a) and( b) are shown the angle of contact for water and glass and for mercury and glass respectively.
The angle of contact for those liquids which wet the liquid is acute. It is zero for pure water and clean glass;for ordinary water and glass it is about 8
.The liquids which do not wet the solids have obtuse angle of contact.For mercury and glass the angle of contact is 135 . 
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