Surface Tension Lab Experiment PDF

NSEC/Laboratory Manual/B.Tech/1st Year/Chemistry(BESH) Experiment no: Date of Experiment: Name: Section: Stream: Roll No: Date of Submission: Record of marks Division Attendence...

NSEC/Laboratory Manual/B.Tech/1st Year/Chemistry(BESH) Experiment no: Date of Experiment: Name: Section: Stream: Roll No: Date of Submission: Record of marks Division Attendence Data Graph Calculation Submission Total recording in time Full marks 1 1 - 1 1 4 Marks obtained Determination of Surface Tension of a Given Liquid by Drop Count Method Using a Stalagmometer Theory: The net resultant force experienced by a liquid molecule in the interior of a liquid is zero as it is attracted equally from all sides by the surrounding molecules. Contrarily, at the surface of a liquid the molecules are attracted towards the bulk only as the vapor phase has lesser molecular density than the liquid phase. Thus, the surface of a liquid at rest behaves like a stretched membrane. This very property of liquid is called surface tension. At a particular temperature, surface tension of a liquid is defined as the force acting tangentially across the surface and perpendicularly to the unit length of the surface. It can also be defined as the work done in increasing the surface area of a static liquid by one unit. The unit of surface tension is dyne.cm -1 (C.G.S.) or N.m-1(S.I.) and the dimension is [MT-2]. Surface tension of a liquid decreases with increasing temperature and vanishes at a certain limiting value of temperature, known as critical temperature, which is a characteristic of the liquid concerned. The basis of determination of surface tension of a given liquid by drop count method using a stalagmometer is as follows: Suppose a liquid is flowing vertically drop by drop very slowly through a capillary orifice under the action of gravity. At the point of detachment of each spherical drop from the flat surface of the stalagmometer, there is a force balance. The net downward force (F1) = the weight of the drop = mg............................................... (1) The net upward force (F2) = the tangential force on the drop = surface tension () × circumference of the drop (2r) = 2r () Then, at the time of detachment, n (1) and (2) mg = 2r or, = or,..................................................(3) Let us consider now two liquids, one is water and another is any unknown liquid, having surface tension w and u respectively. following equation (3) it can be shown = or, = where v and d terms denote the respective volumes and densities of each drop of water and the unknown liquid. If nwand nu are the number of drops consisting a fixed volume (V) of both water and the liquid then, ( / ) = Or, = Or, =................................... (4) ( / ) where Su is the specific gravity of the unknown liquid. Then, surface tension of any unknown liquid can be calculated from the measurement of nw, nu and Su using equation (4). Apparatus Required: 1) Stalagmometer 2) Pipette 3) Specific gravity bottle 4) Beakers Procedure: 1) Record the room temperature. 2) Weigh the empty specific gravity bottle (W1). Then fill the bottle with distilled water to the brim, close the cap, wipe the excess water and weigh again (W2). 3) Repeat the entire process with the supplied liquid (W3). Then, find out the specific gravity of the liquid by calculation. 4) Fix the stalagmometer vertically in a stand. Put distilled water in the bulb by pipette and suck the water. 5) Allow the water to flow vertically drop by drop very slowly through the capillary tube inside the stalagmometer. 6) Adjust the water level at the upper mark. Allow the water to fall and count the number of drops for the volume between the upper mark and lower mark (nw). Repeat it twice.  7) Rinse the stalagmometer with the unknown liquid. 8) Repeat the entire process with the supplied liquid and count the number of drops (nu). 9) Find out the surface tension of the unknown liquid by calculation. Experimental Data: (1) Recording of room temperature Initial temperature Final temperature Mean temperature (2) Recording of weight for determination of specific gravity Weight of empty bottle (W1g) Weight of bottle + water Weight of bottle + unknown (W2g) liquid (W3g) (3) Recording of drop count Liquid No. of drops Mean No. of drops Water i) ii) iii) Unknown liquid i) ii) iii) Calculation: Put appropriate units where necessary by yourself. Use the chart given for the value of w and dw at the temperature closest to the recorded room temperature. Show detailed calculation on a separate sheet end attach it with this manual. Weight of water = (W2 – W1) = …………………………………….. Weight of unknown liquid (W3 – W1) = …………………………………… Specific gravity of unknown liquid (Su) = (W3 – W1) / (W2 – W1) = ………………………… Mean No. of drops for definite volume of water (nw) = …………………………….  Mean No. of drops for definite volume of unknown liquid (nu) = ………………………… Surface tension of unknown liquid = = = …………………………………….. Conclusion: Surface tension of the supplied unknown liquid is..............................................................................at ………….°C Discussion: 1) Surface tension is an intensive property and thermodynamic state function. 2) It is to be ensured that each drop of the liquid or water is fully formed before falling; the disc like flat surface at the end of the stalagmometer opening serves this purpose. 3) It is to be ensured that no water or liquid bubble is trapped inside the stalagmometer, otherwise the reading will be erroneous. 4) The buoyancy effect of surrounding air medium is neglected here in the force balance equation. 5) The actual force balance equation is mg = 2r where  is Harkins-Brown correction factor which is a characteristic of the instrument used and is therefore cancelled out finally in this experiment. Chart for densities and surface tensions of water at different temperature Temperature (°C) Density (g/mL) Surface tension(dynes/cm) 0 0.99987 75.60 10 0.99973 74.22 15 0.99913 73.49 20 0.998235 72.75 25 0.997073 71.97 30 0.995674 71.18 35 0.99406 70.38 40 0.99224 69.56