Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When cyclohexane is poured on water, it floats, because: (a) cyclohexane is in 'boat' form [1997-1 Mark] (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 (d).

Step 1: Understanding the Problem

We need to determine why cyclohexane floats when poured on water. The options given involve different forms of cyclohexane and its density compared to water.

Step 2: Evaluating the Forms of Cyclohexane

Cyclohexane can exist in various conformations like 'chair', 'boat', or 'crown'. However, these forms affect its structural stability, not its density.

Step 3: Comparing Densities

To determine why a substance floats, compare its density to the liquid it's in. Substances less dense than water (density ~1 g/cm³) will float.

Step 4: Concluding with the Correct Choice

Since the density of cyclohexane ( 0.78 g/cm³ ) is less than that of water, it floats on water. This means the correct reason for it floating is related to its density, not its molecular conformation.

Key Concepts

Cyclohexane Conformations

Cyclohexane is a fundamental component in organic chemistry with unique structural versatility. It possesses multiple conformations like the 'chair', 'boat', and 'crown', each named after their distinctive shape. The 'chair' conformation is the most stable due to minimal steric hindrance and torsional strain. This stability arises because the hydrogen atoms are staggered, reducing interaction.

• Chair Conformation: This is the most commonly encountered form of cyclohexane in nature due to its lower energy arrangement.
• Boat Conformation: Less stable compared to the chair due to more significant steric hindrance.
• Crown Conformation: It is less frequently seen and generally of higher energy.

While these conformations define the cyclohexane's energy states and reactivity, they do not directly influence its ability to float on water. Understanding these forms helps us appreciate the unique versatility of cyclohexane structures.

Density Comparison

Density is a crucial factor in understanding why substances float or sink in water. It's defined as mass per unit volume and is crucial in determining whether a material will float on a specific liquid. Cyclohexane has a density of about 0.78 g/cm³, while water has a density of approximately 1 g/cm³.

• Since cyclohexane's density is lower than that of water, it naturally tends to float.
• Comparing densities helps predict floating and sinking behavior effectively.

This simple property of density being less than that of water enables cyclohexane to exist buoyantly on the water surface, regardless of its conformation. Thus, density plays a key role in understanding the interaction between cyclohexane and water.

Floating Phenomenon

The floating phenomenon can intrigue many students and is explained by principles of density and buoyancy. When a substance like cyclohexane, which has a lower density than water, is placed on the water, it experiences an upward buoyant force greater than its weight. This buoyancy causes cyclohexane to float.

• Without enough downward force to counteract this, the cyclohexane stays on the water's surface.
• The concept is echoed in various scientific scenarios beyond just organic chemistry.

Understanding floating through density and buoyancy principles is critical for grasping more complex physical and chemical interactions in science. This knowledge is not just confined to textbooks but crucial in real-world applications of materials in different environments.