Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Separation of two substances by fractional crystallization depends upon their difference in. (a) viscosity (b) density (c) solubility (d) crystalline shape

Step-by-Step Solution

Answer

Solubility differences.

Step 1: Understanding Fractional Crystallization

Fractional crystallization is a method to separate substances based on their solubility differences. When a solution containing multiple solutes is cooled, the solute with lower solubility at a cooler temperature tends to crystallize out first.

Step 2: Evaluating the Options

The options provided are: (a) viscosity, (b) density, (c) solubility, and (d) crystalline shape. Given that fractional crystallization is fundamentally about how substances crystallize out of solution, we focus on solubility—the ability of a substance to dissolve in a solvent at various temperatures.

Step 3: Selecting the Correct Option

The correct factor that fractional crystallization depends on is (c) solubility. This process specifically relies on the different solubilities of substances in the solvent, causing them to crystallize at different temperatures as the solution cools.

Key Concepts

Solubility

Solubility is an important concept when understanding how substances dissolve in solvents. It refers to the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature. Each substance has its own solubility properties, which can change with temperature. For example, if you dissolve sugar in water, as the water is heated, more sugar can dissolve. This is because the solubility of sugar in water increases with temperature.

Several factors influence solubility:

• Temperature: Typically, solubility increases with temperature for solid solutes in liquid solvents.
• Nature of the solute and solvent: "Like dissolves like"; polar solvents dissolve polar solutes well, and nonpolar solvents dissolve nonpolar solutes.
• Pressure: For gases, solubility in liquids usually increases with pressure.

In fractional crystallization, the differing solubilities of substances at various temperatures allow for the selective separation of components in a mixture.

Separation Techniques

Separation techniques are vital processes in chemistry that help to isolate and purify substances from mixtures. Many methods exist, and choosing the correct one depends on the properties of the components to be separated.

Some common separation techniques include:

• Distillation: Used to separate liquids based on their boiling points.
• Filtration: Removes solids from liquids using a porous barrier.
• Chromatography: Separates substances based on their movement through a medium.
• Fractional Crystallization: Relies on differences in solubility, which we will discuss in more depth in the crystallization process section.

Each technique has its own applications and is selected based on the specific physical and chemical characteristics of the substances to be separated.

Crystallization Process

Crystallization is a separation process used to form solid crystals from a homogeneous solution. In fractional crystallization, this process is used to separate substances based on their differing solubilities. Here's how it works:

• Cooling the Solution: When a solution containing multiple dissolved substances is slowly cooled, the substances with lower solubility at the reduced temperature begin to crystallize out first.
• Nucleation and Growth: As the solution cools, small nuclei or clusters of molecules form. These act as seeds for the growth of larger crystals.
• Selection Based on Solubility: The solute with the lowest solubility at the given temperature crystallizes first, allowing it to be separated from the remaining solution.

Fractional crystallization is particularly useful for purifying substances or separating mixtures where the components have a significant difference in solubility. This method is highly effective in the chemical industry for producing pure compounds from complex mixtures.