(1) Formation of double bubble : If r₁ and r₂ are the radii of smaller and larger bubble and P₀ is the atmospheric pressure, then the pressure inside them will be

As excess pressure acts from concave to convex side, the interface will be concave towards the smaller bubble and convex towards larger bubble and if r is the radius of interface.

(2) Formation of a single bubble

(i) Under isothermal condition two soap bubble of radii ‘a’ and ‘b’ coalesce to form a single bubble of radius ‘c’.

If the external pressure is P₀ then pressure inside bubbles

(3) The difference of levels of liquid column in two limbs of u-tube of unequal radii r₁ and r₂ is

(4) A large force (F) is required to draw apart normally two glass plate enclosing a thin water film because the thin water film formed between the two glass plates will have concave surface all around. Since on the concave side of a liquid surface, pressure is more, work will have to be done in drawing the plates apart.

where T= surface tension of water film, t= thickness of film, A = area of film.

(5) When a soap bubble is charged, then its size increases due to outward force on the bubble.

(6) The materials, which when coated on a surface and water does not enter through that surface are known as water proofing agents. For example wax etc. Water proofing agent increases the angle of contact.

(7) Values of surface tension of some liquids.

 

Liquid	Surface tension Newton/metre
Mercury	0.465
Water	0.075
Soap solution	0.030
Glycerine	0.063
Carbon tetrachloride	0.027
Ethyl alcohol	0.022