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 will not decrease the brown color intensity.

Step 1: Understand the Problem

We need to determine which substance, when added to a flask full of brown bromine vapours, will not decrease the intensity of the brown color. Bromine vapour is known for its strong brown color.

Step 2: Analyze the Options

Consider the nature of each substance listed: (a) Carbon disulphide can dissolve bromine. (b) A piece of marble is mostly composed of calcium carbonate, which is inert regarding bromine. (c) Carbon tetrachloride is a non-polar solvent and can dissolve bromine. (d) Animal charcoal is a form of carbon used for adsorbing substances.

Step 3: Determine Solubility and Adsorption Effects

Evaluate the effects each substance will have on bromine's color: Carbon disulphide and carbon tetrachloride can dissolve bromine, reducing its visibility. Animal charcoal can adsorb bromine, potentially reducing its intensity. Marble, however, is chemically inert and will not react with bromine or absorb it.

Step 4: Conclude Which Does Not Affect the Color

Since marble does not dissolve bromine nor does it have adsorbing properties, it will not reduce the intensity of bromine's color. Therefore, adding marble will not appreciably change the intensity of the brown color.

Key Concepts

Bromine Vapour

Bromine is a chemical element with the symbol \(Br\) and atomic number 35. In its elemental form, bromine is a brownish-red liquid at room temperature and readily evaporates to form brown vapour. The vapour consists of discrete \( Br_2 \) molecules. Bromine vapour is distinct due to its intense color, which poses a strong visual presence. It is highly volatile and toxic, and great care should be taken when handling it.

This element belongs to the halogen group, and its vapour has characteristics similar to other volatile halogens like chlorine and iodine. The dense brown color is due to the absorption of specific wavelengths of light, which results in bromine’s unique shading. Understanding these properties helps us predict how bromine vapour will interact with other substances such as solvents or solids in terms of chemical reactions or physical changes.

Solubility

Solubility is a crucial concept in understanding how substances interact with each other. It refers to the ability of a substance to dissolve in a solvent to form a homogenous solution. The extent to which bromine can dissolve in a substance depends on the solvent's nature, particularly its polarity.

Bromine is more soluble in non-polar solvents due to its non-polar molecular structure. This is why substances like carbon disulphide and carbon tetrachloride are effective in dissolving bromine vapour. Both solvents can interact with bromine molecules, allowing them to disperse evenly within the solvent, reducing the visible intensity of bromine's brown color.

• **Non-polar Solvents**: Substances like carbon tetrachloride dissolve bromine well because of similar intermolecular forces.
• **Polar Solvents**: Generally, less effective for non-polar substances like bromine.

Recognizing which solvents can dissolve bromine is vital in applications requiring the removal or dilution of bromine vapour.

Adsorption Effects

Adsorption is the process where molecules of a gas or liquid accumulate on the surface of a solid. This process can significantly affect the intensity and visibility of certain substances, such as bromine vapour.

When a solid like animal charcoal is introduced, it exploits the high surface area to bind bromine molecules on its surface. This interaction can lead to a reduction in the visible concentration of bromine in the vessel, as molecules are effectively "trapped" on the solid's surface.

• **Surface Phenomenon**: Unlike solubility, adsorption occurs at the interface between the solid and the gas or liquid.
• **Temperature and Pressure Dependence**: Adsorption capacity can change with varying conditions.

Therefore, understanding adsorption helps in predicting how materials can control or mitigate the presence and effects of volatile substances like bromine vapour in various environments.