A LABORATORY PROTOCOL ON THE PHYSICS  EXPERIMENT CARRIED OUT BY SPANISH  STUDENTS OF ESCUELA PROFESIONAL “LA SALLE” PATERNA (VALENCIA)

Surface tension

Introduction

This project deals with surface tension of water. A property shared by all kinds of liquids giving them a singular behavior of the surface: it seems like a thin elastic layer. The surface tension forces works all along the contact point between the surface and the liquid. That’s the reason why the insects can walk over the water without sinking. They can’t break the union between water molecules.

Objectives

Our goal is to visualize and measure the surface tension of water and maybe other liquids. We will build a balance with two Petri dishes in both extremes of the balance. The first dish will be in contact with water, and enhanced with her. In the second dish, we will put several little 1 gram masses. That’s the way we can measure the force of the surface tension. When the first dish leaves the water, the mass of the other dish will give us the force of the surface tension (we must multiply the mass in Kg for 9,8 in order to calculate the force in Newton). We will repeat the experience with alcohol. Then we can compare the differences between liquids.

We will check the effects of soap in surface tension. Usually, soap reduces the surface tension of liquids

We will invite the visitors to play with liquids, masses and soap effects in liquids. We also offer them to check the surface effects by putting water drops with a dropper in a coin, defying to guess the number of drops they can put on the coin’s surface before the water spills.

Materials and products

2 Petri dishes                                              

Fishing line

25 ct coins

Several wooden plates   

4 Pasteur pipette

1 g Masses 

Wools sticks

Water

alcohol

Water and  ethyl alcohol

Soap

Experimental procedure

Surface tension is the manifestation of the attractive forces between molecules of liquids. Water has a tension of γ= 72,75·10-3 N/m*. The polar nature of water molecules (composed of an Oxygen atom and two of hydrogen) is the reason of the dipole that increases the cohesion forces and the largest surface tension in water than other liquids (Olive oil 32·10-3 N/m; Alcohol 22.3·10-3 N/m; Benzine 28.9·10-3 N/m; Glycerin 63.1·10-3 N/m). The force (F) of surface tension is proportional to the perimeter of the object, not to its area. The coefficient of tension is γ=F/l (N/m).

*All values are for 293K.

Surface tension is altered by soap action that breaks the surface tension by attracting water molecules and reducing the cohesion forces.

In this experiment we will see:
1. The force of water will keep, by cohesion forces, the dish linked to water.
2. We will put, in the other dish, 1 g masses.
3. While weight in the second dish will not overcome surface tension in the other dish, this one will be linked to water.

We will repeat the experiment with alcohol and olive oil to compare the different tension surface of each liquid, showing that surface tension is a specific property of liquids.

We also will show surface tension with other elements like needles, rings and other small objects.

In a second moment we will dissolve a little bit of soap in water to reduce the surface tension and achieve the action of soap over the surface tension of water.

Results

We will prove that the surface tension of water is γ=72 dynes/cm (72·10-3 N/m) at 25 °C . It’s necessary a 72·10-3 N (72 din) force to break the water layer 1 meter long (remember: it depends on the perímeter of surface). If we use surfactants, the surface tension will be reduced.

Conclusions

Finally, we learn about this invisible and curious force of water that keeping a material adhered over water without effort is possible because of the polarity of water molecules and the enhance forces between them. Therefore, when we see an insect walking on the water, we will remember this theory..

We also see the action of surfactants on these specific properties, specifics to  each liquid.

Bibliography