EFFECTS OF SALINITY AND TEMPERATURE ON THE

SURFACE TENSION OF WATER

I. INTRODUCTION

Background of the Study

Surface tension is a phenomenon that happens when the surface of a liquid, where the

liquid surface is in contact with gas, acts like a thin elastic sheet. The molecules in a liquid

(for example, water) are drawn together by intermolecular forces known as Van der Waals

force of attraction. The molecules on the surface of the water are not surrounded by water

molecules on all sides. They will cohere more strongly with neighboring water molecules, as

opposed to air molecules. This creates a “film” on the surface which requires a certain

amount of force to penetrate.

It is revealed, for example, in the floating of some objects on the surface of water,

even though they are denser than water, and in the ability of some insects (e.g. water striders)

to run on the water surface. This property is caused by cohesion of similar molecules, and is

responsible for many of the behaviors of liquids. An interesting thing about surface tension is

that it gives the shape that is formed by a drop of water. Ideally all liquids will form a

spherical shape in the absence of gravity in order to minimize their surface tension. This is

because the sphere has the smallest surface area for a given volume. However, the shape of a

water droplet is not spherical due to the force of gravity.

Statement of the Problem:

What are the effects of salinity and temperature in the strength of water surface

tension?

Salinity: Water with salt Water without salt

Temperature: 15°C 25°C 35°C 45°C 55°C

Strength of Water Surface Tension

Conceptual Framework:

Independent Variable/s Dependent Variable/s

Hypothesis:

When salt is added to water and the temperature of the water increased, the surface

tension of water is reduced.

Significance of the Study:

This study will verify if there are changes in the surface tension when there are

changes in the molecular level of the substance being used. If the formulation changes

due to the addition of another chemical, the addition of a surfactant, or if anything

contaminates the liquid in use, then the surface tension changes. Measuring surface

tension is a direct indicator of the quality of any liquid.

Scope and Limitation of the Study:

In this experiment, the changes in the strength of the surface tension of water was

deliberates. It is only limited to what are the effects when the water is added with salt.

Comparisons between the two solutions were observed. Also, temperature was varied

in order to further determine the changes in the strength of surface tension of water.

Definition of Terms:

Cohesion - phenomenon of intermolecular forces holding particles of a substance

together. Cohesion differs from adhesion in being the force of attraction between

adjacent particles within the same body; adhesion is the interaction between the

surfaces of different bodies. The force of cohesion in gases can be observed in the

liquefaction of a gas, which is the result of a number of molecules being pressed

together to produce forces of attraction high enough to give a liquid structure.

Intermolecular Forces - forces of attraction and repulsion between molecules of

matter. Molecular behavior depends to a great extent on the balance (or lack of it) of

the forces that pull the molecules together, or push them apart.

Salinity - measurement of the mass of dissolved solids, usually salts, present in a

given amount of water.

Sodium Chloride - chemical compound that has the formula NaCl. The term salt is

also applied to substances produced by the reaction of an acid with a base, known as a

neutralization reaction. Salts are characterized by ionic bonds, relatively high melting

points, electrical conductivity when melted or when in solution, and a crystalline

structure when in the solid state.

Surface Tension - condition existing at the free surface of a liquid, resembling the

properties of an elastic skin under tension. The tension is the result of intermolecular

forces exerting an unbalanced inward pull on the individual surface molecules; this is

reflected in the considerable curvature at those edges where the liquid is in contact

with the wall of a vessel. More specifically, the tension is the force per unit length of

any straight line on the liquid surface that the surface layers on the opposite sides of

the line exert upon each other.

Temperature - in physics, property of systems that determines whether they are in

thermal equilibrium

Water – common name for H2O, a chemical compound known to be the universal

solvent. Water can exist in three phases, solid, liquid, and gas.

II. REVIEW OF RELATED LITERATURE

Surface tension is a contractive tendency of the surface of a liquid that allows it to resist

an external force. It is revealed, for example, in the floating of some objects on the surface of

water, even though they are denser than water, and in the ability of some insects (e.g. water

striders) to run on the water surface. This property is caused by cohesion of similar molecules,

and is responsible for many of the behaviors of liquids.

Surface tension has the dimension of force per unit length or of energy per unit area. The

two are equivalent—but when referring to energy per unit of area, people use the term surface

energy—which is a more general term in the sense that it applies also to solids and not just

liquids.

The cohesive forces among liquid molecules are responsible for the phenomenon of

surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by

neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not

have other molecules on all sides of them and therefore are pulled inwards. This creates some

internal pressure and forces liquid surfaces to contract to the minimal area.

Surface tension is dependent on temperature. For that reason, when a value is given for

the surface tension of an interface, temperature must be explicitly stated. The general trend is

that surface tension decreases with the increase of temperature.

The tendency of any liquid surface is to become as small as possible as a result of this

tension, as in the case of mercury, which forms an almost round ball when a small quantity is

placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the

result of the distribution of tension on the thin film of soap, is another example of this force;

surface tension alone can support a needle placed horizontally on a water surface. See also

Capillary Action.

Surface tension is important at zero gravity, as in space flight: Liquids cannot be stored in open containers because they run up the vessel walls.

III.METHODOLOGY

Materials:

- 2 beakers or any containers of similar size

- 400ml distilled water

- 2 tablespoons of salt

- Rice grains

- 2 pieces of aluminum foil measuring 1cm x 1cm each

- Refrigerator

- Hot plate

- Thermometer

Procedure:

1. For this experiment, the independent variable was the salinity of the water and its

temperature – 15°C, 25°C, 35°C, 45°C and 55°C. The dependent variable was the

number of rice grains placed on the aluminum foil before it sinks. This was

determined by gradually adding grains of rice on the surface of the aluminum foil.

The constant variables were the size of the aluminum foil, the size of the grains of

rice and the room temperature.

2. The 2 beakers were labeled “pure water” and “salt water”. The beakers were each

filled with 200ml of distilled water. In the beaker labeled “salt water” 2 tablespoons

of salt were added and mixed into the water. The 2 beakers were then placed in the

refrigerator until the contents in the 2 cups reach 15°C. The beakers were taken out

of the refrigerator and the required temperature was confirmed using a thermometer.

3. The 1cm x 1cm piece of aluminum foil was made to float on the surface of the water

in the beaker labeled “pure water”. The grains of rice are placed one at a time on the

aluminum foil until the aluminum foil sinked into the water. The number of rice

grains placed on the foil was recorded.

4. Procedure 3 was repeated with the beaker labeled “salt water” and the results were

also recorded.

5. The 2 beakers were brought to a room temperature of 25°C and Procedure 3 was

repeated on these 2 beakers.

6. The 2 beakers were then placed on a hot plate and heated to temperatures 35°C, 45°C

and 55°C. Procedures 3 and 4 are repeated at each required temperature level and the

results were recorded.

IV. RESULTS AND DISCUSSION

Table 1. Results obtained from the experiment

Water SolutionNumber of Rice Grains

15°C 25°C 35°C 45°C 55°C

Water 21 19 16 13 11

Water + Salt 9 8 8 7 5

Table 1 shows the amount of rice grains needed before the foil collapsed. This data

represented the strength of the surface tension in each set-up. Also, a descending trend in the

number of rice grains was shown in both water solutions. However, water alone gave a higher

number of rice grain needed. The relationship of salinity, temperature, and strength of surface

tension was represented in Figure 1.

Figure 1. Relationship of salinity, temperature, and strength of surface tension

0 5 10 15 20 25 30 35 40 45 50 55 600

5

10

15

20

25

Effect of Salinity and Temperature on Water Surface Tension

WaterWater + Salt

Temperature, °C

Num

ber o

f Gra

ins

Results show that as the temperature of a solution increases, its surface tension decreases.

This means that there is an inverse relationship between temperature and surface tension. Based

on these data, we can say that hot water is a better cleaning agent because the lower surface

tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them

with surface tension.

On the other hand, salinity further lowers the surface tension of water. Salt readily

dissolves in water thus, it completely dissociates into their separate ions in water. These ions can

lower the surface tension by small amounts.

V. CONCLUSION AND RECOMMENDATION

Based on the experiment, we can say that the hypothesis made was correct. Indeed,

higher temperature will cause the lowering of the surface tension as well as the addition of salt.

Surface tension in liquids is caused by a phenomenon known as cohesion. Cohesion is when the

molecules of a substance cling together more tightly to each other than to molecules of other

substance. The water molecules at the surface have fewer molecules around them than the ones

in the middle, so the bond between these surface molecules is stronger than the bonds of other

water molecules. This attraction of the surface molecules to each other is also much greater than

their bond to the air molecules surrounding them.

It is highly recommend to further test the effects of other substances when added to water

like detergent, oil, etc.

VI. REFERENCES

Freedman, Roger A.; Young, Hugh D. University Physics with Modern Physics,

Cambridge, Mass. Addison-Wesley Publishing. Com., Inc., 10th ed., 2000

Jones, Andrew Zimmerman, 2010, Surface Tension, About.com Guide in Physics

Lite, David R. CRC Handbook of Chemistry and Physics, 75th Ed.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

White, Harvey E. (1948). Modern College Physics. van Nostrand.