Surface Tension Of Water

The capillary rise method was used to measure the surface tension of aqueous solutions of sodium chloride and acetic acid. Molar concentrations of 0. 5 M and 1. 0 M for each solution were tested. The results were then compared to the surface tension of pure water to observe how solute properties affect the surface tension of water. The experimental results showed that adding acetic acid solute in water lowers the surface tension of water. As the concentration of acetic acid was increased, the surface tension of water decreased even more.

However, the experiment has failed to accurately show the effect of sodium chloride solutes on the surface tension of water. Nevertheless, previous studies revealed that surface tension of electrolyte solutions, such as NaCl solution, increase with increasing concentration. The experiment revealed several disadvantages of the capillary rise method, such as difficulties in firmly holding the equipment, in reading the liquid level inside the capillary, in keeping the temperature constant, and in accurately marking the liquid level with a pen.

Nevertheless, the capillary rise method proved to be comparatively reliable, provided that proper technique is observed. Results of the capillary rise experiment for the effect of acetic acid somehow agreed with results from previous studies.

Surface tension occurs due to cohesive forces (attractive forces acting between like molecules) pulling the surface molecules toward the body of the liquid. Furthermore, these surface molecules exhibit stronger cohesive forces upon neighboring molecules compared to the rest of the liquid.

Thus, a droplet of water on a table will assume a roughly spherical or globular shape.

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From experience we also observe that it is more difficult to move an object through the surface of a liquid than to move it when it is completely submersed. In this study, the surface tension of dilute aqueous solutions will be measured. The study aims to examine how the surface tension of these solutions compare with that of pure water. The surface tension of water can vary due to the factors solute type (or substance dissolved), concentration of solution (i.e. amount of substance dissolved), and temperature. This study will focus particularly on the effects of solute type and concentration. Solute types of particular interest in this study are strong electrolytes, and low molar mass organic acids. (Another type of substance are surfactants, or surface active compounds, but they will not be covered in this experiment). Past scientific studies have concluded that electrolytes increase the surface tension of water slightly, while organic acids slightly decrease surface tension.

These studies have made use of advanced methods with the help of special equipment called tensiometers. One such method is called the du Nuoy Ring Method, which measures the force needed to pull a ring up from the surface of a liquid. However, in this experiment the capillary rise method will be used for measuring surface tension. This procedure relates the elevation of the liquid inside the capillary tube with the surface tension of the solution. With this method we can observe how fast or by how much the liquid will rise inside the capillary.

Then we can compare the results for the solutions to that of pure water. Thus, the method gives us a direct way to observe the effects of certain factors on the surface tension of water.

Methodology: Capillary Rise Method

Calibration of the Capillary Tube Using De-ionized Water The inner diameter of the capillary tube was measured by performing the capillary rise method using de-ionized water. The tube was first cleansed with hot nitric acid, and was rinsed with water afterwards. Then, using a marker or pen, a zero mark was placed near one end of the tube.

The capillary tube was then dipped into a test tube containing de-ionized water, until the zero mark coincided with the water level. The initial water level in the test tube was about an inch below the edge of the test tube. Upon immersion of the capillary tube into the test tube, the water level inside the capillary was elevated over that of the surrounding water. This height difference was then measured using a Vernier caliper. Five trials were made for this procedure, and the inner radius of the capillary tube was calculated using the formula:

Measuring Surface Tension of Aqueous Solutions of Sodium Chloride and Acetic Acid The effect on surface tension of factors such as concentration and solute type was examined by performing the capillary rise procedure on 0. 5 M and 1. 0 M solutions each of sodium chloride (NaCl) and acetic acid (HOAc). Each solution was placed in a separate test tube, with the liquid level rising to about an inch below the edge of the test tube. The test tubes were immersed in a water bath to ensure that they had roughly the same temperature. The same procedure employed during calibration was used.

The capillary tube was dipped in the test tube until the zero mark coincided with the liquid level. The height difference between this initial level and the elevated level inside the capillary was measured using a Vernier caliper. Five trials were performed on each sample. A more general form of [Eq. 1] above was used to relate the surface tension of each sample to the height difference, as well as to the inner radius of the capillary. Equation 2 above, together with [Eq. 1], was used to calculate the surface tension of each sample solution.