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

(d) cyclohexane is less dense than water.

Step 1: Understanding the Question

The question asks about why cyclohexane floats on water. We are given four options indicating different shapes of cyclohexane or a physical property related to its density.

Step 2: Revising Cyclohexane Structure

Cyclohexane can adopt various conformations, like chair, boat, and others. However, these shapes primarily affect its chemical stability and reactivity, not its ability to float or sink.

Step 3: Understanding Density Concepts

Objects float on a liquid due to density differences. If an object is less dense than the liquid, it will float. Conversely, if it is denser, it will sink.

Step 4: Comparing Densities of Cyclohexane and Water

The density of cyclohexane is approximately 0.78 g/cm³, whereas the density of water is 1.00 g/cm³. Since 0.78 is less than 1.00, cyclohexane is less dense than water.

Step 5: Concluding the Correct Answer

Based on density principles, cyclohexane floats on water because it is less dense than water, not due to its shape or form.

Key Concepts

Cyclohexane Conformations

Cyclohexane is a fascinating molecule often studied in organic chemistry because of its versatile and stable structures. It can morph into various 3D shapes or "conformations" to minimize strain and achieve stability. These conformations include the famous chair and boat forms, among others like the twist-boat. Here are some key details about cyclohexane conformations:

• Chair form: This conformation is the most stable due to minimal torsional strain and no angle strain, providing it maximum stability.
• Boat form: This is less stable compared to the chair form due to steric and torsional strain, making it higher in energy.

However, despite their significance, these shapes do not affect the ability of cyclohexane to float or sink in water. Its behavior in water is determined by other properties such as density.

Floating and Sinking Principles

Whether a substance floats or sinks in a liquid depends on the principle of density. In simple terms, density is the measure of how much mass is contained in a given volume. The basic rule of floating and sinking is:

• If an object is less dense than the liquid it is placed in, it will float.
• If an object is more dense than the liquid, it will sink.

This principle is why items like wood float in water while metals like steel sink. It’s also the reason why cyclohexane, despite its structural shapes, floats on water. This behavior is related to its density compared to that of water, not its conformation.

Density Comparison

Comparing densities is a straightforward way to determine whether one substance will float on another. Let's take a look at cyclohexane and water:

• Cyclohexane: Approximately 0.78 g/cm³
• Water: Known to be around 1.00 g/cm³ for its relatively high density.

Since the density of cyclohexane (0.78 g/cm³) is less than that of water (1.00 g/cm³), cyclohexane will naturally float when poured onto water. The floatation is purely due to this density difference, affirming that cyclohexane is less dense than water, and not related to its structural form.