Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

When cyclohexane is poured on water, it floats because (a) cyclohexane is in boat form (b) cyclohexane is in chair form (c) cyclohexane is in crown form (d) cyclohexane is less dense than water

Step-by-Step Solution

Answer

Cyclohexane floats on water because it is less dense than water (option d).

Step 1: Understanding the Problem

The question is asking why cyclohexane floats on water. We know that substances float on water if they are less dense than water.

Step 2: Analyzing the Options

We have four options. Options (a), (b), and (c) involve specific conformations of cyclohexane, namely the boat, chair, and crown forms. However, these conformations describe the molecular structure, not the density of the molecule.

Step 3: Focusing on Density

Option (d) states 'cyclohexane is less dense than water'. Density is a property that affects whether a substance floats or sinks in a liquid.

Step 4: Conclusion

The given options suggest that only one, option (d), addresses the floatation of cyclohexane on water based on a relevant physical property: density. Cyclohexane's density is less than that of water, which results in it floating.

Key Concepts

Density

Density is a measure of how much mass is contained in a given volume. It can be thought of as how "packed" or "compact" a material is. The formula to calculate density is:

• Density = Mass / Volume

This concept is crucial in determining whether an object will sink or float in a fluid. When comparing materials, a substance with a lower density will float on a denser one. For liquids like water and cyclohexane, these principles allow us to predict behaviors in mixtures. Cyclohexane, with a density of approximately 0.779 g/cm³, is less dense than water, which has a density of about 1.0 g/cm³. This difference in density explains why cyclohexane floats on water. In applications, understanding density helps us in various fields, from designing products to be buoyant to creating solutions and mixtures.

Floatation

Floatation is a physical property that describes the ability of an object to remain on the surface of a liquid. It relies heavily on the concept of density but also involves the physics of buoyancy. When an object is placed in a fluid, the fluid exerts an upward force, known as buoyant force, on the object. This force is equal to the weight of the fluid displaced by the object. If the object's density is less than that of the fluid, the buoyant force will be greater than the object's weight, allowing it to float. In the case of cyclohexane and water, cyclohexane's lower density leads to it displacing less water weight than its own weight, resulting in floatation. Understanding floatation helps in designing vessels and various applications in shipping, swimming, and separation processes.

Molecular structure

The molecular structure of a substance pertains to the arrangement of atoms within a molecule. In chemistry, structural conformation plays a significant role in physical and chemical properties. Cyclohexane is a classic example, known for its ability to adopt different conformation forms such as chair, boat, and twist-boat. Although these shapes describe how cyclohexane's atoms are arranged in space, they don't directly influence density. Rather, they affect the molecule's interactions, stability, and energy levels. Cyclohexane's flexibility in structure minimizes steric strain and enhances stability, particularly in its chair form. While the molecular structure is crucial for chemical reactivity and physical interactions, in floatation analysis, it serves more to explain why cyclohexane behaves the way it does, rather than affecting its density outright. Grasping molecular structures enables students to predict reactivity and understand major concepts in organic chemistry.