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

Question

A one litre flask is full of brown bromine vapours. The intensity of brown colour of vapour will not decrease appreciably on adding to the flask some (a) carbon disulphide (b) piece of marble (c) carbon tetrachloride (d) animal charcoal powder

Step-by-Step Solution

Answer

Adding a piece of marble (b) will not decrease the color intensity.

Step 1: Understand the Exercise

We are given a flask with brown bromine vapors and must determine which substance, when added, will not significantly decrease the intensity of the color of these vapors.

Step 2: Analyze Option (a) Carbon Disulphide

Carbon disulphide is a non-polar solvent and can dissolve bromine, which might decrease the vapor concentration if added. This would decrease the intensity of the color.

Step 3: Analyze Option (b) Piece of Marble

A piece of marble is insoluble in bromine vapors and does not interact with bromine in a way that would change the vapor's color intensity. Thus, the bromine vapor's color should remain unchanged.

Step 4: Analyze Option (c) Carbon Tetrachloride

Carbon tetrachloride is also a non-polar solvent like carbon disulphide. It can dissolve bromine, decreasing the concentration of bromine vapors, and hence the color intensity.

Step 5: Analyze Option (d) Animal Charcoal Powder

Animal charcoal, also known as activated carbon, has strong adsorption properties and can reduce the concentration of bromine vapors, hence decreasing the color intensity.

Step 6: Select the Correct Answer

After analyzing all options, the piece of marble (Option b) is the only substance that does not react with or dissolve bromine vapors and does not alter the intensity of the brown color.

Key Concepts

Intensity of Color

The intensity of color in a chemical substance, like bromine vapors, depends on the concentration of that substance in the observed environment. In the exercise, the browning of bromine vapors signifies a high concentration of bromine atoms within the flask. When we say that the intensity of the brown color is "appreciable," we mean it is noticeable and potentially impactful. If a new substance is introduced that changes the concentration of bromine, the intensity of the color will also change. However, if a substance does nothing to alter concentration, the color intensity will remain the same. A clear understanding of how concentration correlates with color intensity can help predict outcomes when various substances are added to chemical environments.

Non-polar Solvents

Non-polar solvents, such as carbon disulphide and carbon tetrachloride, have a unique set of attributes that enable them to dissolve other non-polar substances. Bromine vapors are non-polar, making them particularly susceptible to being dissolved by non-polar solvents.

• Non-polar solvents and solutes share similar properties which facilitate dissolution.
• When bromine vapors are dissolved, their concentration lowers, potentially decreasing the intensity of the color.

Understanding the nature of non-polar solvents is key when predicting how these substances will interact in chemical reactions. This is why, in the exercise, both carbon disulphide and carbon tetrachloride have the potential to decrease the bromine vapors' color intensity.

Adsorption

Adsorption is a surface-based phenomenon, wherein atoms, ions, or molecules adhere to a surface. It differs from absorption, where a substance is taken up completely. Animal charcoal, also known as activated carbon, exhibits a distinctive porosity and large surface area, perfect for adsorbing molecules like bromine.

• Activated carbon traps bromine vapors on its surface.
• This reduces the concentration of bromine in the gaseous phase.

As a result, adsorption significantly reduces vapor concentration, thus dulling the intensity of the color in such scenarios. The exercise demonstrates how adsorption plays a crucial role in altering vapor intensity by decreasing bromine vapors.

Chemical Reactions

Chemical reactions are processes where substances interact to form new products. In our scenario, we want to determine if any interaction between added substances and bromine vapors could result in chemical changes.

• A chemical change could alter the color by forming a new substance.
• The piece of marble doesn't chemically interact with bromine vapors.

In the absence of a chemical reaction, bromine stays in its current form, and its concentration remains unchanged. Thus, the intensity of the bromine vapors' color persists. Understanding when and how chemical reactions alter substance properties helps discern the outcome of adding various substances to bromine vapors.