Problem 86 - Master Resource Book in JEE Main Physics

Question

The force of cohesion is (a) maximum in solids (b) maximum in liquids (c) maximum in gases (d) same in solid, liquid and \(g a s\)

Step-by-Step Solution

Answer

The force of cohesion is maximum in solids (option a).

Step 1: Understanding Cohesion

Cohesion refers to the intermolecular forces that hold molecules together in a substance. These forces are strongest in solids because the molecules are closely packed and interact strongly with each other.

Step 2: Comparing States of Matter

In solids, molecules or atoms are in a fixed position and vibrate around mean positions due to strong cohesive forces. In liquids, the molecules are less tightly bound, allowing them to flow, making cohesive forces weaker than in solids. In gases, molecules are widely spaced and have minimal cohesive forces.

Step 3: Analyzing Answer Choices

With the understanding of cohesive forces in different states, we analyze the choices: (a) maximum in solids, (b) maximum in liquids, (c) maximum in gases, and (d) same in solid, liquid, and gas. Given the stronger intermolecular forces in solids, option (a) appears correct.

Step 4: Selecting the Correct Answer

The force of cohesion is strongest in solids because of their tightly packed molecular structure and strong intermolecular interactions. Therefore, we select (a) maximum in solids as the correct answer.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles such as atoms, molecules, or ions. These forces are essential for the cohesion of substances. They exist due to electric charge differences:

• Dipole-Dipole Interactions: Occur between molecules that have permanent dipoles (polar molecules).
• London Dispersion Forces: Present in all molecules, they are the weakest intermolecular forces and arise due to temporary dipoles created by electron movement.
• Hydrogen Bonds: A specific type of dipole-dipole interaction, known for being stronger, especially present in molecules with N-H, O-H, or F-H bonds.

The strength of these forces affects properties like boiling and melting points, solubility, and the physical state of a substance. In solids, intermolecular forces are generally the strongest, tightly holding the molecules close together. In liquids, these forces are weaker, allowing molecules to slide past each other, whereas in gases, the forces are weak or negligible.

States of Matter

The three traditional states of matter—solid, liquid, and gas—differ primarily in their molecular spacing and movement abilities:

• Solid: Molecules are tightly packed, keeping a fixed volume and shape due to strong intermolecular forces.
• Liquid: Molecules are less rigidly packed than in solids, allowing them to adopt the shape of their container while retaining a fixed volume.
• Gas: Molecules move freely and are widely spaced, filling any available volume, due to minimal intermolecular forces.

These properties are significantly influenced by the intermolecular forces present in each state. For solids, the strong forces create a structure where molecules are bound to specific positions. Liquids have weaker forces that enable fluidity. In gases, where forces are minimal, molecular collisions dominate their properties.

Solids

Solids are characterized by their defined shape and volume, a result of strong cohesive forces keeping the molecules or atoms locked in place. These forces result from close packing and often create a well-ordered structure. Several key features of solids include:

• Cohesion: The strong intermolecular forces ensure that solids maintain their shape and do not flow.
• Crystalline vs. Amorphous: Solids can be organized in an ordered lattice (crystalline) or disordered (amorphous) structure.
• Density: Solids usually have high density compared to liquids and gases due to their tightly packed particles.

Understanding solids' properties helps us analyze phenomena like melting and sublimation, where different energy levels overcome cohesive forces, leading to a change in state. Hence, when cohesion is mentioned, solids often serve as the ultimate example, given their molecular organization and interactions.