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To determine the surface tension of water with different concentrations of sodium chloride, capillary rise was examined. This was done by examining the height a liquid rose in a capillary depending on the concentration. The liquid was but in an apparatus containing the capillary, and the system was disrupted and the let sit to return to equilibrium. The heights at equilibrium for each sampe was taken at each concentration (water, 1M NaCl, 2M NaCl, 3M NaCl, 4M NaCl).
Surface tension is defined by the work needed to expand the surface by a unit area, and is a measure of free energy within the system. This difference in surface tension between five solutions, was determined by the difference in the rise of the solution in a capillary tube in comparison to one another. The equation used to determine surface tension from this capillary rise is defined as
γ_1=1/2 (h+ r/3)rρg
Where γ_1 is the surface tension
h is the height recorded
r is the radius of the capillary tube used
ρ is the density of the liquid
g is gravitational acceleration
With this information the surface tension can be found for different concentrations, along with the general trend of surface tension versus the concentration of ions in a solution.
A dried capillary tube is placed into an apparatus containing a 250mL test tube capped with a rubber stopper. There is two holes in the stopper, one for the capillary and another for an adapter as a means to disrupt the system. The five solutions are placed in the tube for different trials, where the capillary height would be measured. After the liquid is placed in the container the system is disrupted using a particular plunger to suck up the liquid and then releasing to have the liquid return to equilibrium. After this is done the readings are taken. Five trials is done for each of the samples of different concentration.
Fig 1. Diagram of Apperatus used
Overall the results for the surface tension calculated from the capillary height were inconsistent with a high standard deviation. The mean values of the height of the tube at different concentrations were used to create a graph following the change of NaCl concentration to the change in surface tension. As an overall trend, the surface tension was show to increase with the increase of ions present and gave the equation y = 2850.9x + 69.812 (graph 2), where the slope is positive 2850.9 suggesting this increase. Another factor influencing the increased surface tension was due to the density increasing while the concentration was raised.
For the height recorded in the capillary at each different concentration (Table 1), the mean calculated values are as follows, DI Water (control): 3.6 ± 0.2 cm, 1M NaCl: 3.5 ± 0.4 cm, 2M NaCl: 3.0 ± 0.2 cm, 3M NaCl: 3.6 ±0.7 cm, 4M NaCl: 3.3 ±0.2 cm. This data suggests a lower in capillary height due to the increased NaCl concentration
For the calculated surface tension the average and standard deviation (Table 2) for each concentration (Mol/ cm
3) were as follows: 0.000 Mol/ cm
3: 71.99 ± 5.34 dyn/cm, 0.001 Mol/ cm
3: 73.91 ± 10.54 dyn/cm, 0.002 Mol/ cm
3: 66.03 ± 5.05 dyn/cm, 0.003 Mol/ cm
3: 84.87 ± 17.72 dyn/cm, 0.004 Mol/ cm
3: 80.77 ± 5.74 dyn/cm. the general slope when these points were plotted against concentration was 2850.9. this value was then used to calculate the layer thickness, which was calculated to be 1.15cm.
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