Question

Separation of two substances by fractional crystallization depends upon their difference in: (a) Viscosity (b) Density (c) Solubility (d) Crystalline shape

Short answer

The separation by fractional crystallization depends on solubility differences (c).

Step-by-step solution

Identifying Key Concepts

To solve this problem, we first recognize that fractional crystallization is a method primarily used to separate substances based on their physical properties. The process involves forming solid crystals from a solution and relies on differences in these properties.

Understanding Fractional Crystallization

Fractional crystallization typically involves cooling a saturated solution slowly so that one substance crystallizes out before the other. This process is based on the principle that different substances have different solubilities at different temperatures.

Analyzing Each Option

- **Viscosity**: Not typically related to how substances crystallize. - **Density**: While it affects layering in solutions, it is not a direct factor in crystallization. - **Solubility**: This governs how a substance dissolves and crystallizes from a solution as conditions change. - **Crystalline Shape**: Related to how crystals form under certain conditions, but not a separating factor itself.

Choosing the Best Option

Given the explanation, the separation based on solubility differences is the fundamental principle. Therefore, the correct choice is the option that aligns with solubility.

Key concepts

Solubility

Solubility is a key concept when understanding processes like fractional crystallization. It refers to the ability of a substance (solute) to dissolve in a solvent, forming a homogeneous solution. The degree of solubility can vary greatly depending on the temperature and the substances involved. For example, sugar dissolves more readily in hot water than in cold water.

In fractional crystallization, solubility differences between substances are crucial. This technique takes advantage of how solubility changes with temperature to separate substances. As a solution is cooled, the substance with the lower solubility crystallizes out first. This process allows us to separate and purify components from a mixture.

Understanding solubility and its dependence on factors like temperature and solvent choice is essential in many fields, from chemistry to environmental science. It affects not only crystallization but also reactions, mixing processes, and even biological functions.

Separating Techniques

Separating techniques are fundamental in chemistry for purifying substances or analyzing mixtures. These techniques rely on the physical and chemical properties of the components. Fractional crystallization is one such method, particularly useful when differences in solubility are evident between substances.

There are various separating techniques, including:

• Filtration: Separates solids from liquids based on particle size.
• Distillation: Utilizes boiling point differences to separate liquids.
• Magnetic Separation: Uses magnetic properties to separate materials.
• Centrifugation: Separates based on density by spinning mixtures rapidly.

Each method has its own applications and limitations, but selecting the right technique depends on the properties of the substances involved. Fractional crystallization specifically focuses on solubility and is often used in purification and separation of mixtures containing components with varied solubilities.

Crystallization Process

The crystallization process is a fascinating natural and laboratory technique where a solid forms from a homogeneous solution or melt. This process can occur as solutions cool or solvents evaporate.

In fractional crystallization, the goal is to form crystals selectively to separate components in a mixture. Here's how it typically works:

• A solution is prepared where the desired solid is dissolved.
• The solution is slowly cooled, encouraging the formation of crystals.
• As temperature decreases, different substances crystallize out based on their solubility.
• The resulting crystals are then filtered and isolated for use.

Crystallization relies heavily on controlled cooling and sometimes even stochastic nucleation processes. It is a pivotal technique in industries for purifying products such as pharmaceuticals, chemicals, and food. Mastery of the crystallization process allows chemists to obtain high-purity substances and understand more about molecular interactions within solutions.