How To Use stalagmometer to detrermine surface tension of a liquid.

 

Experiment:

 

Determine the surface tension of a given liquid by stalagmometer (Drop Number Method) at a given temperature.


Theory:

Surface Tension is defined as “The force acting upon unit length of the surface liquid”.

It is also defined in terms of work done (or energy required)

“Surface tension is the work done (or energy required) to produce a unit increase in the surface area.”

Surface tension may be explained by considering two molecules A in the interior of the liquid and B on the surface of the liquid. Due to intermolecular forces, the molecule A is pulled equally on all sides by the neighbouring molecules whereas B is pulled down-ward and laterally only. The resultant force acting on A is zero but B on the other hand, is subjected to an unbalanced force acting in-wards molecule, present in the surface.

 

 Units (or Dimensions) of Surface Tension:

 

According to CGS System, the units of surface tension are dynes.com-1 or erg.com-2 According to SI unit, surface tension is expressed in newton per meter (N.m-1) or joules per square meter (J.m-2).

Units in two systems are related to each other as

Dynes.cm-1 = 10-3.m-1

 

Apparatus:

 

Stalagmometer, specific gravity bottle, screw type pinch cock, beakers, rubber tube, balance,

stand etc.

 

Procedure:

·         Take a stalagmometer, wash it with ether, dry it and then fill it with water up to the level of the upper mark a by sucking through rubber tube.

·         Release water at a very slow rate and count the number of drops of water from the upper mark A to the lower mark B.

·         Also count the number of drops for each of the given liquids in the same way.

·         Take a specific gravity bottle and weigh it .Then fill it with water and weigh it.

·         Find the weight of water by subtracting the weight of empty bottle from the total weight of bottle filled with water.

·         Also find the weight of each liquid in the same way. Then calculate the density of each of the liquid as follows:

Density of liquid = (Weight of liquid / Weight of water) x Density of water as density of water is usually taken as 1, therefore

Density of liquid = (Weight of liquid / Weight of water)

 

Observations:

 

Liquid

No. of Drops (n)

Surface tension (g)

Water

 

 

Given Liquid

 

 

 

 

Room temperature = oC

Weight of empty specific gravity bottle = W1 g

Weight of empty specific gravity bottle + water = W2 g

Weight of water = W2 – W1 = W3 g

Weight of specific gravity bottle + liquid 1 = W4 g

Weight of liquid = W4 – W1 = W5 g

 

Calculations:

Surface tension of a liquid is calculated using the following relationship

gl = gwx (nw / nl) x (dl/ dw)

 

Surface tension of water at 30oC is 71.22

 

 

 

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