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

Fractional crystallization depends on solubility differences (option c).

Step 1: Understanding Fractional Crystallization

Fractional crystallization is a process used to separate substances based on differences in certain properties. It is especially effective for substances with varying solubility properties.

Step 2: Evaluating the Options

Consider how fractional crystallization might be influenced by each of the given options: (a) viscosity, (b) density, (c) solubility, and (d) crystalline shape. Look for which property allows selective separation during crystallization.

Step 3: Identifying Key Property

Fractional crystallization primarily relies on the solubility differences between substances. As a solution cools, the substance with lower solubility will crystallize out first, facilitating separation.

Key Concepts

Solubility

Solubility is the maximum amount of a substance that can dissolve in a solvent at a given temperature. This property plays a vital role in fractional crystallization. Different substances have different solubility levels. When attempting to separate two substances using fractional crystallization, their differences in solubility become the key.

As a solution containing multiple substances begins to cool, the substance with lower solubility will start to crystallize out first. For example, if you have a mixture of two salts with different solubility in water, as you lower the temperature, the salt with a lower solubility will come out of the solution in the form of crystals.

Knowing the solubility curve is essential. This curve shows how the solubility of a substance varies with temperature. By manipulating the temperature, one can control which substance crystallizes first, thus achieving separation.

Selective Separation

Selective separation is a process in which one component of a mixture is isolated from the others, based on specific properties such as solubility. In fractional crystallization, selective separation occurs when the conditions are adjusted so that one compound crystallizes out of a solution, leaving the other components dissolved.

The choice of solvent is crucial for effective selective separation. A good solvent should dissolve both substances when hot but only one substance when cooled. By carefully controlling the temperature and stirring, it is possible to selectively separate one component from the mixture.

This method is widely used in chemistry and industry to purify compounds. It is especially valuable when a single compound needs to be isolated from a solution containing multiple solutes.

Crystallization Process

The crystallization process is a sequence of steps that result in the formation of solid crystals from a homogeneous solution. This is the driving force behind fractional crystallization.

The process starts with creating a supersaturated solution, where more solute is dissolved in the solvent than it can typically hold at a certain temperature. Cooling this solution allows the solute to form crystals. This phase change from a dissolved state to a solid crystal is what achieves separation in fractional crystallization.

Factors such as temperature, rate of cooling, and impurities in the solution can influence the crystallization process. Rapid cooling might lead to small, impure crystals, whereas slow cooling tends to produce larger and more pure crystals. The aim is to find the ideal balance that maximizes purity while efficiently separating the desired substance from the solution.