Problem 25 - Chemistry and Chemical Reactivity

Question

What is surface tension? Give an example illustrating the phenomenon of surface tension. Explain why surface tension is the consequence of intermolecular forces.

Step-by-Step Solution

Answer

Surface tension is caused by cohesive forces, allowing insects to walk on water.

Step 1: Defining Surface Tension

Surface tension is the tendency of the surface of a liquid to resist an external force due to the cohesive forces between the molecules at the surface. It causes the surface to behave like a stretched elastic membrane.

Step 2: Example of Surface Tension

An example of surface tension is the ability of small insects, like water striders, to walk on the surface of a pond without sinking. This occurs because the surface tension creates a supporting surface capable of holding the weight of the insect.

Step 3: Intermolecular Forces Explained

Surface tension is the result of cohesive forces between liquid molecules. Molecules at the surface experience a net inward force due to stronger attractions to other similar molecules inside the liquid than to the surrounding air.

Key Concepts

Cohesive Forces

Cohesive forces are the attraction forces that occur between like molecules. These forces are what make a liquid molecule want to cling to another molecule of the same type. For example, water molecules are strongly cohesive, which means they are more attracted to each other than to air molecules around them.

This strong attraction among water molecules is primarily due to hydrogen bonds. Hydrogen bonds are special types of bonds where a hydrogen atom is attracted to a more electronegative atom, such as oxygen. In water, this results in a kind of 'stickiness' that we term cohesive force.

In a container of liquid, these cohesive forces pull the molecules together. These forces are stronger near the surface, as they are not balanced by other liquid molecules on all sides. This is why the surface behaves like a stretched elastic membrane.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together in a substance. These forces are present in all states of matter: solids, liquids, and gases, but are especially interesting in liquids where they contribute to surface tension.

The strength of intermolecular forces determines how strongly the molecules hold onto each other. Types of intermolecular forces include:

• Dipole-Dipole Forces: Occur between polar molecules.
• Hydrogen Bonds: A special dipole-dipole interaction involving hydrogen.
• London Dispersion Forces: Present in all molecules, especially significant in non-polar molecules.

Each of these intermolecular forces contributes to the overall cohesive force within a liquid, making its molecules cling together. The stronger the intermolecular forces, the greater the surface tension the liquid exhibits.

Liquid Surface Behavior

Liquid surface behavior explains how the surfaces of liquids react to external forces. Due to cohesive forces, the molecules at the surface tend to stick together more tightly, which is not the case deeper in the liquid where molecules are surrounded uniformly on all sides.

This creates a unique behavior where the surface acts like a thin, elastic sheet. It resists small forces due to the internal pull of cohesive forces. This phenomenon is most noticeable in small objects trying to break through the surface of a liquid.

For example, water striders can walk on water because their feet cannot penetrate this "sheet" owing to their minimal weight and the significant surface tension. Liquids with high surface tension will display greater resistance to breaking this surface molecular "film" than those with lower tension. This characteristic is critical for many natural and industrial processes, from droplet formation to the function of detergents.