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

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, γ₁/γ₂ = 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.