Surface Tension

# Materials required

• A clean and dry capillary tube
• A tipped pointer
• A beaker containing water
• A travelling microscope
• Adjustable wooden stand
• Clamps and stand

# Real lab Procedure

## To set up the apparatus :

• Place the adjustable height stand on the table and make its base horizontal by leveling the screws.
• Fix the capillary tube and the pointer in a cork, and clamp it in a rigid stand so that the capillary tubes and the pointer become vertical.
• Adjust the height of the vertical stand, so that the capillary tubes dip in the water in an open beaker.
• Adjust the position of the pointer, such that its tip just touches the water surface.

## To find the capillary rise :

• Find the least count of the travelling microscope for the horizontal and the vertical scale.
• Make the axis of the microscope horizontal.
• Adjust the height of the microscope using the height adjusting screw.
• Bring the microscope in front of the capillary tube and clamp it when the capillary rise becomes visible.
• Make the horizontal cross wire just touch the central part of the concave meniscus.
• Note the reading of the position of the microscope on the vertical scale.
• Now, carefully remove the beaker containing water
• Move the microscope horizontally and bring it in front of the pointer.
• Lower the microscope and make the horizontal cross wire touch the tip of the pointer.
• Corresponding vertical scale readings are noted.
• The difference in the two readings (i.e., height of water meniscus and height of the tip of pointer) will give the capillary rise of the given liquid.
• We can repeat the experiment by changing the height of the wooden stand.

## To find the internal diameter of the capillary tube :

• Place the capillary tube horizontally on the adjustable stand.
• Focus the microscope on the end dipped in water.
• Make the horizontal cross- wire touch the inner circle at A (fig i). Note microscope reading on the vertical scale.
• Raise the microscope to make the horizontal cross wire touch the circle at B (fig ii). Note the vertical scale reading.
• The difference between the two readings will give the vertical internal diameter (AB) of the tube.
• Move the microscope on the horizontal scale and make the vertical cross wire touch the inner circle at C (fig iii). Note microscope reading on the horizontal scale.
• Move the microscope to the right to make the vertical cross wire touch the circle at D  (fig iv). Note the horizontal scale reading.
• The difference between the two readings will give the horizontal internal diameter (CD) of the tube.

• We can calculate the diameter of the tube by calculating the mean of the vertical and horizontal internal diameters. Half of the diameter will give the radius of the capillary tube.

# Simulator Procedure (As performed through Online Labs)

• Select the liquid for which the coefficient of viscosity is to be measured, from the 'Select Solution' drop down list.
• Select the environment to perform the experiment from the 'Select Environment' drop down list.
• Use the ‘Select Temperature’ slider to change the temperature of the liquid.
• Use the ‘Capillary tube diameter’ slider to change the diameter of the capillary tube.
• Use the ‘Height of wooden stand slider to change the height of the wooden stand.
• Use the ‘Level of retort stand’ slider to change the height of the retort stand.
• Use the ‘Focus of microscope’ slider to focus the microscope.
• Use the arrows on the focusing screw also to focus the microscope.
• Use the ‘Height of microscope’ slider to change the height of the microscope.
• Use the arrows on the fine adjusting screw also to adjust the height of the microscope.
• Click on the beaker to see the zoomed view of that portion, to see whether the pointer tip just touches water level.
• Use the ‘Remove beaker’ button to remove the beaker.
• Click on the microscope to see the zoomed view of the vertical scale. Drag the scale to take the main scale and vernier scale readings.
• Now, calculations are done as per the real lab procedure and the surface tension of the selected liquid can be found out.
• Click on the ‘Reset’ button to redo the experiment.

# Observations

Least count of the travelling microscope = Value of one MSD / Number of divisions on the Vernier

$=\frac{0.5 mm}{50}$

=  0.01 mm

= 0.001 cm

## To find the capillary rise :

 No Radius of the water meniscus Reading at tht tip of pointer Height, h = h1-h2  (cm) M.S.R. (cm) V.S.R. (div.) Total = MSR+(VSR×LC)  h1 (cm) M.S.R. (cm) V.S.R. (div.) Total = MSR+(VSR×LC)  h2 (cm) 1 2 3 4 5

Mean h = ……….......... cm

= ............. ×10-2 m

## To find the internal diameter of the capillary tube :

 Microscope readings for cross wire in position, Internal diameter Internal radius, $r=\frac{d}{2}$    (cm) A (cm) B (cm) C (cm) D (cm) Vertical, Y= B-A (cm) Horizontal, X=D-C (cm) Mean, $d=\frac{(X+Y)}{2}$ (cm)

# Calculations

Density of water at observed temperature, ρ = ............ kgm-3

Angle of contact of water in glass, θ            = 8o

$\therefore$           Cos θ            = 0.99027

$\approx$   1

Note the values of  h in the first table and r in the second table for each capillary tube separately  and find the value of T in each case.

Surface tension ,

$T=\frac{r(h+\frac{r}{3})\rho g}{2 cos\theta }$

= .................. N/m

# Result

Surface tension of water  is ………………… N/m

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