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The following points highlight the top five experiments on diffusion. The experiments are: 1. Diffusion of Solid in Liquid 2. Diffusion of Liquid in Liquid 3. Diffusion of Gas in Gas 4. Comparative Rates of Diffusion of Different Solutes 5. Comparative rates of diffusion through different media.
Experiment # 1
Diffusion of Solid in Liquid:
Experiment:
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A beaker is almost filled with water. Some crystals of CuSO4 or KMnO4 are dropped carefully without disturbing water and is left as such for some time.
Observation:
The water is uniformly coloured, blue in case of CuSO4 and pink in case of KMnO4.
Inference:
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The molecules of the chemicals diffuse gradually from higher concentration to lower concentration and are uniformly distributed after some time. Here, CuSO4 or KMnO4 diffuses independently of water and at the same time water diffuses independently of the chemicals.
Experiment # 2
Diffusion of Liquid in Liquid:
Experiment:
Two test tubes are taken. To one 30 rim depth of chloroform and to the other 4 mm depth of water are added. Now to the first test tube 4 mm depth of water and to the other 30 mm depth of ether are added (both chloroform and ether form the upper layer).
Ether must be added carefully to avoid disturbance of water. The tubes are stoppered tightly with corks. The position of liquid layers in each test tube is marked and their thickness measured.
The tubes are set aside for some time and the thickness of the liquids in each test tube is recorded at different intervals.
Observation:
The rate of diffusion of ether is faster than that of chloroform into water as indicated by their respective volumes.
Inference:
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The rate of diffusion is inversely proportional (approximately) to the square root of density of the substance. Substances having higher molecular weights show slower diffusion rates than those having lower molecular weights.
In the present experiment ether (C2H5-O-G2H5, J mol. wt. 74) diffuses faster into water than chloroform (CHCI3, mol. wt. 119.5). This ratio (74: 119-5) is known as diffusively or coefficient of diffusion.
Experiment # 3
Diffusion of Gas in Gas:
Experiment:
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One gas jar is filled with CO2 (either by laboratory method: CaCO3 + HCL, or by allowing living plant tissue to respire in a closed jar). Another jar is similarly filled with O2 (either by laboratory method: MnO2 + KClO2, or by allowing green plant tissue to photosynthesize in a dosed jar). The gases may be tested with glowing match stick.
The oxygen jar is then inverted over the mouth of the carbon dioxide jar and made air-tight with grease. It is then allowed to remain for some time. The jars are carefully removed and tested with glowing match stick.
Observation:
The glowing match sticks flared up in both the jars.
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Inference:
The diffusion of CO2 and O2 takes place in both the jars until finally the concentrations are same in both of them making a mixture of CO2 and O2. Hence the glowing match sticks flared up in both the jars.
Experiment # 4
Comparative Rates of Diffusion of Different Solutes:
Experiment:
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3.2gm of agar-agar is completely dissolved in 200 ml of boiling water and when partially cooled, 30 drops of methyl red solution and a little of 0.1 N NaOH are added to give an alkaline yellow colour. 3 test tubes are filled three-fourth full with agar mixture and allowed to set.
The agar is covered with 4 ml portion of the following solutions, stoppered tightly and kept in a cool place:
(a) 4 ml of 0-4% methylene blue,
(b) 4 ml of 0.05 N HCl, and (4.2 ml of 0.1ml HCL plus 2 ml of 0-4% methylene blue.
The diffusion of various solutes is recorded in millimeters after 4 hours. The top of the gel should be marked before the above solutions are added.
Observation:
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The rate of diffusion of HCL alone (tube b) is faster compared to the combination of methylene blue and HCl (tube c) and minimum in case of methylene blue alone (tube a).
Inference:
Different substances like gases, liquids and solutes can diffuse simultaneously and independently at different rates in the same place without interfering each other.
HCL being gaseous in nature and of lower molecular weight can diffuse much faster than methylene blue which is a dye of higher molecular weight having an adsorptive property. Hence in combination, these; two substances diffuse more readily than methylene blue alone.
Experiment # 5
Comparative rates of diffusion through different media:
Experiment:
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Two apparatus for measuring comparative rates of diffusion through gas and liquid are set up (Figure 2). Tube 1 is filled with a 2% agar-sol containing 1 ml of methyl red indicator and 1 drop of 0.1 N NaOH (in alkaline medium methyl red is yellow and in acidic medium red).
When the agar sets in the tube, it is held over a small bottle containing conc. HCL. The distance to the diffusion front as indicated by the red colour line is measured at suitable time intervals and the rate of diffusion of HCL gas into agar-gel in millimeter per hour is recorded.
A strip of filter paper approximately of equal length and internal diameter of tube 2 is cut, soaked in methyl red indicator containing a little 0.1 N NaOH (the strip is coloured yellow) and suspended in tube 2.
The tube is held over a small bottle containing conc. HCL as in tube 1. The rate of diffusion of HCl through the gaseous medium surrounding the strip is recorded by noting the colour change of the strip.
Observation:
The rate of diffusion of HCl gas is faster in case of tube 2 containing filter paper strip and slower in case of tube 1 containing agar-gel.
Inference:
The rate of diffusion of gases through a medium is inversely related to the density of the medium. Hence HCL diffuses faster in gaseous medium (tube 2) than in semisolid medium (tube 1).
N.B. The presence of the agar in tube 1 does not materially retard diffusion of solutes; the rate of diffusion through this gel may be taken, therefore, to be equal to the rate of diffusion through water.
