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Surface Tension. Surface tension is a measurement of the cohesive energy present at an interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal attractive force in all directions.Surface tension of liquids
The molecules of liquids attract each other by cohesive forces resulting into small distances
between the molecules (on the order of 0.1 nm). Thus the compressibility of liquids is lower then
that of gas, while the density is much higher. On the other hand, these cohesive forces are not
strong enough to result into the fixed position of molecules that can be seen in solid matter.
Liquids do not keep a fixed shape, but adapt the shape of a container. Attractive cohesive forces
are short range forces which are based on the electronic interactions. They affect molecules in
their close vicinity only (zone of molecular interaction). In the bulk of the liquid, each molecule is
attracted equally in all directions by the neighboring molecules, hence zero net force A stalagmometer is a device for investigating surface tension using the stalagmometric method. It is also called a stactometer or stalogometer. The device is a capillary glass tube whose middle section is widened.The stalagmometric method is one of the most common methods used for the surface tension determination. If the weight of each drop of the liquid is known, we can also count the number of drops which leaked out to determine the surface tension. The drops are formed slowly at the tip of the glass capillary placed in a vertical direction. The pendant drop at the tip starts to detach when its weight (volume) reaches the magnitude balancing the surface tension of the liquid. The weight (volume) is dependent on the characteristics of the liquid
Requirements:
Stalgmometer, specific gravity bottle, a small rubber tube with a screw pinch
cork, distilled water, experimental liquid.
Theory:
In the drop number method, the number of drops formed by equal volumes
of two liquid is counted. If m1 and m2 is the mass of one drop of each of the
liquid having densities d1 and d2 respectively. If n1 and n2 is the number of
drops formed by volume v of the two liquids, then their surface tensions are
related as
¥1/¥2= (d1/d2)*(n2/n1)
One of the liquid is water its surface tension and density are known.Then te
surface tension of the given liquid can be calculated.
Procedure:
1. Clean the stalgmometer with chromic acid mix, wash with water and dry it
2. Attach a small piece of rubber tube having a screw pinch cock at the upper end of the stalgmometer.
3. Immerse the lower end of the stalgmometer in distilled water and suck the water 1-2cm above mark A. adjust the pinch cork so that 10-15 drops fall per minute
4.Clamp the stalgmometer allow the water drops to fall and start counting the number of drops when the meniscus crosses the upper mark A and stop counting when the meniscus passes mark B
5. Repeat the exercise to take three to four readings
6. Rinse the stalgmometer with alcohol and dry it
7. Suck the given liquid in the stalgmometer and count the drops as in case of water
8. Take a clean dry weighing bottle weighs it with water as well as with liquid.
9. Note the temp of water taken in a beaker.
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