# Surface Tension Experiment (Capillary Rise Method)

Surface Tension Experiment (Class 11)

Aim:

To find the surface tension of water by capillary rise.

Apparatus:

A narrow capillary tube of uniform bore of known radius, beaker containing water, travelling microscope, etc.

Principle:

Surface Tension, T = (1/2) x rhρg

where, h - capillary rise

r – radius of the bore

ρ – density of water

g – acceleration due to gravity

Procedure (Method 1):

A capillary tube is cleaned first with an acid, then with an alkali and finally with water. It is then passed through a hole in a cork and is arranged vertically with its lower end dipping in water contained in a beaker. A long pointer is passed through the same cork and fixed vertically so that its tip just touches the surface of water in the beaker. The water rises in the capillary tube. Least count of the vernier of the travelling microscope is noted. The microscope is kept horizontally and focussed on water level in the tube. Adjust the microscope till the horizontal crosswire just touches the lower meniscus. The total reading (R1) of the vernier is found out. The beaker containing water is carefully removed and the microscope alone is lowered till the horizontal cross-wire just touches the image of the tip of the pointer. The reading (R2) of the vernier is taken. The capillary rise, h = R1 — R2, of water in the tube is calculated, Surface tension (T) of water is calculated by the equation,

T = (1/2) x rhρg;

where r is the radius of the capillary tube and ρ is the density of water.

Procedure (Method 2):

A clean capillary tube is passed through a cork and is arranged vertically on a stand. Its lower end is arranged to be dipped in water taken in a beaker. A pointer is also passed through the same cork and its tip is made to touch the liquid surface. Due to capillarity water rises in the tube. The least count of the given travelling microscope is noted. The microscope is then focussed at the meniscus of the liquid in the capillary tube and the horizontal cross wire is made tangential to the meniscus. The microscope reading (MSR and VSR) is taken on the vertical scale. Total reading = M.S.R. + (V.S.R. x L.C.) is calculated. The beaker containing the liquid is carefully removed. The microscope is then focussed at the tip of the pointer and the microscope reading on the main scale and vernier scale are also noted. The difference between the two readings gives the capillary rise (h). The experiment is repeated by lowering the capillary tube to different depths in the beaker and the mean rise ‘h’ is determined. Knowing the values of r, ρ and g; surface tension (T) can be calculated.

To find capillary rise h

Least count of the vernier = ____ cm

 Trial Reading of meniscus Reading of tip of pointer Height, h=R1–R2 MSR VSR Total R1 MSR VSR Total R2 1 2

Mean height, h = _____m

Radius of the capillary tube = r = _____m

Density of water = ρ = 1000 kgm-3

Surface Tension of water, T = (1/2)rhρg = _____Nm-1

Result:

Surface tension of water =  ____