# ANDC DU/Module on Surface Tension determination

 Work in progress, expect frequent changes. Help and feedback is welcome. See discussion page.

# DETERMINATION OF SURFACE TENSION

## To determine the surface tension of the given liquid, by using a stalagmometer by drop number method.

Objectives
 Learning Objectives To understand the cleansing action of certain substance like soap etc. To elucidate the structure, using the knowledge of surface tension and Parachor.

### REQUIREMENTS

A. CHEMICALS/SOLUTIONS

1. Distilled water
2. Given liquid

B APPARATUS

Stalagmometer, a piece of rubber tubing, pinch cock, pyknometer or specific gravity bottle, stand with a clamp, a water thermostat, and thermometer.

# Preknowledge

A molecule within the bulk of a liquid is surrounded by many molecules. It is equally attracted from all sides by these molecules so that the resultant force is zero. On the other hand, the surface molecules of the liquid are in a different condition with respect to the molecules in the interior of the liquid. These molecules are subjected to two kinds of pull:

• a downward pull by the molecules present directly below it in the bulk of the liquid, and
• a side-wise pull by molecules lying on their sides.

These forces result in a continuous tension on the surface molecules and thus, the surface of the liquid remains in a state of tension or strain. As a result, it behaves like a stretched membrane. This pull or tension at the surface of a liquid is known as SURFACE TENSION. Liquids tend to adopt shapes that minimize their surface area, for then the maximum number of molecules is in the bulk and surrounded by neighbours. Thus, the force of surface tension compels a freely falling drop of a liquid to take up a shape that minimizes their surface area. As the spherical shape has the smallest surface-to–volume ratio, the droplets of liquids tend to be spherical. Thus, it is the surface tension force which compels a freely falling drop of a liquid to take up a spherical shape.

Definition

Surface tension is the property of a liquid by virtue of which it behaves like elastic stretched membrane, with a tendency to contract so as to occupy a minimum surface area.

It may be defined as the force acting per centimeter along the surface of the liquid at right angles to any line taken in the surface of the liquid in any direction.

The surface tension is represented by

• the Greek letter gamma (),
• S.I Units- Newtons per meter,
• CGS units- Dynes per centimeter and
• Dimensions-MT-2

If two immiscible or partially miscible liquids are taken in a vessel, then the surface tension acting along their surface of separation i.e. along the interface is called INTERFACIAL TENSION. Its value is generally intermediate between the surface tension of the two liquids but sometimes it is less than both. This is due to the fact that at the interface, the molecules of one liquid are attracted by the molecules of the other. Surface tension is a type of interfacial tension; the two phases involved being the liquid and the vapour above it.

### SURFACE TENSION AND TEMPERATURE

The surface tension of liquids is greatly affected by the change in temperature of the liquids. It generally decreases with the increase in temperature and becomes zero at the critical temperature. This is because of the reason that rise in temperature raises the kinetic energy of the molecules. This results in the decrease of intermolecular attraction and consequently the surface tension decreases.

 Work in progress, expect frequent changes. Help and feedback is welcome. See discussion page.