Purpose
This experiment sought to determine the value of the surface tension of water and paraffin oil from the downward pull on a horizontal ring fastened to a spring.
Apparatus
The following materials were used in the experiment:
Platinum-iridium ring equipped with stirrup (circumference L=4cm)
Spring
Paraffin oil
Distilled water
Petri dish
Weight pan
Striker
Level
Jolly balance with adjustable platform
Scale to determine the paraffin oil density
Suitable weights
Bunsen burner
Tweezers
Introduction
Surface tension refers to the energy required to expand a liquid’s surface area (Ozment, John, and Conrad, 391). Liquid molecules are attracted to one another by adhesive forces. Molecules under the surface of a liquid are bordered by other molecules from all directions (Flammer, Maneli, and H Bebie, 222; “Journal of Scientific & Industrial Research”, 65). Therefore, they experience a net force of zero since other molecules exert forces on them from all directions. On the other hand, molecules on the surface of a liquid are only bordered by molecules occurring below them and those occurring on their sides. Therefore, the adhesive forces linking them to their neighboring molecules are stronger than those linking the molecules that are found below the surface of the liquid. The surface molecules cling to one another relatively more strongly, hence creating a strong barrier between the liquid surface and the atmosphere. This layer is so strong that it can allow some objects whose densities are higher than that of the liquid to float on the liquid. For example, a steel needle can float on water yet the density of steel is higher than that of water. Furthermore, an insect can walk on water despite having a density higher than that of water. As long as the surface tension is not broken, the two phenomena can occur (needle floating on the surface of water and an insect walking on water). Figure 1 shows an illustration of surface tension.
Surface tension is applied widely in solving various problems. For example, the clinical test for jaundice is based on changes in the surface tension of urine. One of the common tests for jaundice is called Hay test. This test involves the spraying of the powdered sulfur on the surface of urine. Bile is said to be present in the urine if the sulfur sinks in the urine. This test is based on the fact that bile lowers the surface tension of urine. Surface tension is also of significance to the use of soaps and detergents. Soaps and detergents lower the surface tension of water so that water can soak all the soiled parts of the cloths.
Procedure
Before starting the experiment, the students familiarized themselves with the setup. In particular, the students familiarized themselves with taking measurements using the Jolly balance.
The students then verified that the Jolly balance conforms to the setup shown in figure 2. The weight pan was then suspended from the spring and the apparatus aligned. Next, the wheel, W, shown in figure 2, was used to adjust the position of the spring support so that it is in line with the glass tube.
Next, with the weight pan in place, 5 independent “zero” readings of the balance position were taken. Next, 5 readings of the balance with 0 to 1 gram weight in the pan were taken as the representatives of the 5 “zero readings”. Next, the following steps were repeated at least five times:
Removal of any dirt or grease from the ring by placing it in the flame of Bunsen burner
Filling the petri dish with distilled water and placing it on the support p shown in figure 2.
Hanging the ring from the spring of the apparatus and moving the dish of water up towards the ring until the ring just touches the water and is pulled under
Taking a reading of the balance position
Raising the balance using the wheel W until the ring just breaks free from the pull of the distilled water.
Taking a reading of the balance position
Next, another petri dish was filled with paraffin oil. The steps 1 to 5 outlined above were then repeated with the paraffin oil in the petri dish instead of water.
Figure 1: Illustration of the surface tension of water
Figure 2: Apparatus setup
Results
The data concerning 5 “zero” readings are shown in table 1 shown below:
Data obtained when 5 readings of the balance with 0 to 1 gram weight in the pan were taken are shown in table 2 below:
Balance positions due to the surface tension of water are shown in table 3 shown below:
The balance positions due to surface tension of paraffin oil are shown in table 4 below:
Data Analysis
Total downward force due to surface tension of water
The total downward force due to surface tension is given by the formula shown below:
F=2LT
This force is associated with the displacement of the balance. It can be determined using the equation of Hooke’s law;
F=-kx
Figure 3: Graph of force against displacement
Average displacement=0.31+0.35+0.28+0.3+0.335=0.314cm=0.00314m
Therefore, the force is calculated as follows:
F=-3.623*0.00314+0.0003=-0.0117N
The total downward force due to surface tension is 0.0117N
Surface Tension of Water
T=F2LG
G= 1.0813
Therefore, T=0.01172*0.041.08=0.01260.08=0.158Nm-1
Total downward force due to the surface tension of paraffin
The total downward force due to surface tension is given by the formula shown below:
F=2LT
This force is associated with the displacement of the balance. It can be determined using the equation of Hooke’s law;
F=-kx
Average displacement=(0.07+0.17+0.21+0.1+0.16)5=0.142cm=0.00142m
Therefore, the force is calculated as follows:
F=-3.623*0.00142+0.0003=-0.0054N
The total downward force due to surface tension is 0.0054N
Surface Tension of paraffin
T=F2LG
G= 1.003
Therefore, T=0.00542*0.041.003=0.0054160.08=0.0677Nm-1
Therefore, the surface tension of paraffin oil is 0.0677N/m
Surface tension for water at 20ºC
T20℃=20*0.154=3.08dyne/cm
Surface tension for paraffin at 20ºC
The ration of the surface tension of water to that of paraffin = 2.33
T20℃=3.082.33=1.32dyne/cm
Answer to the question: The G factor is introduced to account for the errors created by effect of the atmospheric pressure.
Works Cited
Flammer, J, Maneli Mozaffarieh, and H Bebie. Basic Sciences in Ophthalmology: Physics and Chemistry. Berlin: Springer, 2013. Internet resource.Bottom of Form Bottom of FormTop of Form
Ozment, Judy, John W. Moore, and Conrad L. Stanitski. Student Solutions Manual: Chemistry: the Molecular Science, Fifth Edition [by] John W. Moore, Conrad L. Stanitski. , 2015. Print.Top of Form
