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

Step 1: Understand the Physical Properties

To solve this question, we need to consider the physical property of density. Substances with a density less than water (1 g/cm³) will float on it. Cyclohexane’s conformation does not affect its tendency to float.

Step 2: Analyze Cyclohexane's Density

We should check if cyclohexane's density is less than water. The density of cyclohexane is approximately 0.78 g/cm³.

Step 3: Compare Densities

Since cyclohexane's density (0.78 g/cm³) is less than water's density (1 g/cm³), cyclohexane floats on water.

Step 4: Verify the Answer

The question asks for the reason cyclohexane floats on water. The correct reason based on our analysis is that cyclohexane is less dense than water, corresponding to option (d).

Key Concepts

Physical Properties of Cyclohexane

Cyclohexane is a common organic compound. It is often used as a non-polar solvent in chemical reactions and processes. Understanding its physical properties is crucial for science and engineering applications. One essential physical property is its density, which is a measure of mass per unit volume.

When discussing organic molecules like cyclohexane, its density is approximately 0.78 g/cm³. This is significantly lower than the density of water, which is 1 g/cm³. Since cyclohexane is less dense than water, it will naturally float when placed in it. Unlike solid objects that rely on shape, the floating behavior of liquids like cyclohexane is dictated by their density. When designing experiments or chemical processes involving cyclohexane, knowing its lower density is essential. This knowledge helps predict and explain its behavior when in contact with other substances like water.

Floating and Density

The concept of floating and density is an interesting topic in physical sciences. It's based on the principle that an object will float if its density is less than the fluid in which it's submerged. Imagine a piece of wood or a balloon; these materials float in water because they have, on average, a density lower than water.

For liquids, the principle is the same. Cyclohexane serves as a perfect example. With a density of approximately 0.78 g/cm³, cyclohexane is less dense than water, allowing it to float. Even though cyclohexane can take on various forms or conformations, its floating capability stems entirely from its density. This aspect is unrelated to its molecular geometry like chair or boat forms. Thus, when cyclohexane is observed floating on water, it purely demonstrates the predictable outcome of density comparison.

Hydrocarbon Density Comparison

Hydrocarbons are organic compounds consisting of solely carbon and hydrogen. They come in varying structures and properties. An important physical property for hydrocarbons is their density, as it influences how they interact with other substances, such as water.

When comparing densities among various hydrocarbons, cyclohexane (0.78 g/cm³) is generally less dense relative to many other substances, such as water. Other common hydrocarbons like hexane and octane also share similar density traits, which are typically less than 1 g/cm³. This density property is why many hydrocarbons are immiscible and float on top of water.

Knowing the density comparisons across different hydrocarbons can help in practical applications such as oil spill clean-ups, where understanding the interaction with water is vital. Hence, cyclohexane and its lighter density play a role in numerous chemical and environmental contexts.