Investigatory Project on Viscosity

Investigatory Project on Viscosity

Aim:


Comparison of viscosities of different liquid by rising bubble method


Materials and Apparatus:


(i) a tall glass jar (measuring jar)

(ii) a large flask

(iii) a funnel

(iv) a clean narrow capillary tube (drawn tube)

(v) glass tubes

(vi) rubber tubes

(vii) pinch-cock

(viii) different transparent liquids (eg: water, clear kerosene oil, sodium chloride solution, dilute copper sulphate solution etc)

(ix) stopwatch etc.


Theory:


When an air bubble of radius r moves up through a liquid of viscosity η with a terminal velocity v, then,


(4⁄3) πr3ρg + 6πrηv = (4⁄3) πr3dg


Where ρ = density of air, d = density of the liquid

(Neglecting ρ the density of air)


But v = s/t ; 

η = 2r2dgt/s


If the radius r of the bubble and depth s of the liquid through which the bubble rises up are constants,

η dt

η1 : η2 : η3 : … = d1t1 : d2t2 : d3t3 : …


Procedure:


Experimental setup is as shown in the figure. Water is taken in the flask. The flask is fitted with a two-holed cork. A long glass tube and a short glass tube are introduced through the cork. The lower of the long tube is dipped in the water. The upper end of it is connected to a funnel by means of a short rubber tube. The rubber tube is closed with a pinch-cock. Water is taken in the funnel. The lower end of the short glass tube should not touch the water surface. Its upper end is connected to the capillary tube using a long rubber tube. The capillary tube is kept vertically inside the liquid taken in the glass jar.


The densities of the liquids are measured using a Hare’s apparatus, if they are not known. Pour the liquid ‘l’ of density d1 in the jar. The mouth of the capillary tube should be sufficiently deep in the liquid. The depth of immersion of the capillary tube is marked on it.


The pinch-cock is gradually opened and the water in the funnel is allowed to trickle down slowly into the flask. Then the pressure of the air within the flask is slowly increased. Bubbles of air are formed in the liquid at the end of the capillary tube. These bubbles rise up through the liquid.


Time (t1) of rise of the air bubble through the liquid is noted using a stop-watch. d1t1 is calculated.


The experiment is repeated with the other liquids 2,3,… keeping the mouth of the capillary tube at the same depth.


Observations


To find the densities of the liquid


Comparison of viscosities

η1 : η2 : η3 = d1t1 : d2t2 : d3t3

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