# TO DETERMINE THE SURFACE TENSION OF A LIQUID (LIQUID PARAFFIN) USING STALAGMOMETER BY DROP WEIGHT METHOD AT ROOM TEMPERATURE

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## APPARATUS REQUIRED:

Stalagmometer, Beaker, Stand, Weighing

## CHEMICALS REQUIRED:

Distilled water, liquid paraffin

## THEORY:

Surface tension is the contracting force which contracts the surface molecules of a liquid towards the interior of the liquid. The surface tension is mainly a force that mainly acts to reduce the surface area. The SI unit of surface tension is Newton / meter & CGS unit is dyne/centimeter.

Surface tension = Force/length

There are a number of methods for determining Surface tension of liquids, among which the drop fall method is one of them.

### 1) Drop fall method.

i) Drop weight method:

This method is generally used to determine the surface tension of liquids in the laboratory. This method is based on the principle that the weight of a liquid falling from a capillary tube held vertically is approximately proportional to the surface tension of liquid. Hence, if the surface tension of two liquids be γ1 and γ2 and weight W1and W2 that drops falling from the capillary tube then, γ12 = W1/W2

## PROCEDURE:

The cleaned stalagmometer was fixed with a clamp & filled with water by sucking. Around 20-30 drops of water, falling from the stalagmometer (point A) was collected in a day & tared (preweighed) weighing bottle & weighed. The stalagmometer was then dried and the experiment was repeated with the liquid whose surface tension was to be determined. Density of the given liquid was measured by using a pycnometer.

## OBSERVATION AND CALCULATION:

Weight of empty clean dry weighing battle (W1) = 60.21gm

Weight of water (W₂1) = 63.39gm

Weight of water (W₂²) = 63.17gm

weight of water (W₂³) = 62.98 gm

Average weight of water with weighing bottle (W₂)=W₂1+W₂²+W₂³ / 3

=63.39 + 63.17+ 62.98 / 3

= 63.18gm

Weight of water (20 drops) = W₂ – W1

= (63.18-60.21) gm

= 2.97gm

wt. of liquid paraffin + weighing bottle (W31) = 61.40 gm

wt. of liquid paraffin + weighing bottle W3²) = 61.67 gm

wt. of liquid paraffin + weighing bottle (W33) = 61.83 gm

Average wt. of liquid paraffin + weighing battle (W3) = 61.40 + 61.67 +61.83/3

= 61.63gm

Therefore, wt.of liquid paraffin(20 drops) = (61.63-60.21) gm

= 1.42gm

Now, we know,

SI of liquid paraffin = weight of liquid paraffin(20 drops) x SI of water at room temperature/weight of water (20 drops)

= 1.42 × 70.4/2.97 [therefore, SI of water room temperature =70.4 dynes/cm]

=33.74 dynes/cm

## RESULT:

Hence, the surface tension of the liquid paraffin is found to be 33.74 dynes/cm.