Question

I is surface molecule whereas II is interior molecule. Choose the correct one. (a) I results net attraction into the liquid. (b) II are attracted in all directions. (c) Both (a) and (b). (d) Neither (a) nor (b).

Short answer

(c) Both (a) and (b) are correct statements regarding surface and interior molecules.

Step-by-step solution

Understanding Surface Molecules

Surface molecules are those that are located on the surface of the liquid. They are not surrounded by molecules on all sides, and as a result, they experience a net force of attraction back into the liquid. This is due to the imbalance in the intermolecular forces acting on a surface molecule.

Analyzing Interior Molecules

Interior molecules are surrounded by other molecules in all directions. These molecules experience forces of attraction from neighboring molecules in all directions. Therefore, there's no net force of attraction pulling them in any specific direction since these forces generally cancel out.

Key concepts

Surface Molecules

Surface molecules hold a unique position in the structure of liquids. Unlike their counterparts in the interior, surface molecules are exposed to an environment that is part liquid, part air (or another phase). This placement results in an asymmetrical distribution of intermolecular forces. Since they're only surrounded by other molecules within the liquid on one side, they experience these forces predominantly from the interior. Consequently, these molecules are pulled inward, creating what is known as surface tension.

This can be likened to a game of tug-of-war, where the molecules in the middle are evenly pulled in all directions, but those at the end of the line - the surface molecules - are being tugged on mostly from one direction. This imbalance creates a sensation as if the molecule is attracted into the liquid, which indeed it is due to the presence of attractive forces that are uncompensated by air molecules.

Interior Molecules

Moving beneath the surface, interior molecules find themselves in an entirely different situation. Encased by a symmetry of neighboring molecules, these units do not experience a net pull towards any particular direction because the forces of attraction they feel are uniformly distributed in the sphere around them. This equilibrium of forces ensures that an interior molecule remains in a state of dynamic stability.

Consider an office worker in a cubicle, surrounded by colleagues. This worker receives input from all directions, but no single direction overpowers another, creating a balanced work environment. This analogy closely mirrors the situation of interior molecules; the forces around them create an isotropic condition where the summation of individual interactions leads to a null net force on any given molecule at a stable temperature and pressure.

Forces of Attraction

The forces of attraction between molecules, known as intermolecular forces, are essential for understanding the behavior of matter in different states. These forces can be of several types, such as Van der Waals forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding. They play a pivotal role in governing the macroscopic properties of substances, like boiling points, melting points, viscosity, and surface tension.

The strength and nature of these forces depend on various factors including the size of the molecules, their shape, and their ability to form permanent or temporary dipoles. For example, a molecule with a larger surface area can experience stronger dispersion forces. In contrast, molecules capable of hydrogen bonding will demonstrate significantly higher boiling points due to the strength of these specific interactions. These phenomena are observable in daily life, such as water droplets forming beads on a waxed car surface, demonstrating high surface tension due to strong hydrogen bonding between water molecules.