NEET Notes: Solutions

Introduction to Solutions

A solution is a homogeneous mixture of two or more substances. It consists of a solute and a solvent, where the solute is dissolved in the solvent to form a uniform composition. The study of solutions is essential in chemistry as it helps understand various properties such as concentration, colligative properties, and solubility, which are significant for NEET aspirants.

Types of Solutions

Based on the Physical State of Components

• Solid in Solid: Alloys like brass (zinc in copper).

• Solid in Liquid: Salt or sugar dissolved in water.

• Liquid in Liquid: Alcohol in water.

• Gas in Liquid: Carbon dioxide in soft drinks.

• Gas in Gas: Air (a mixture of gases like oxygen, nitrogen, etc.).

Based on the Amount of Solute Present

• Saturated Solution: Contains the maximum amount of solute that can dissolve at a given temperature.

• Unsaturated Solution: Can dissolve more solute at a given temperature.

• Supersaturated Solution: Contains more solute than the solvent can theoretically dissolve.

Concentration Terms of Solutions

Mass Percentage (w/w)

The mass of the solute per 100 g of solution.

Volume Percentage (v/v)

The volume of solute per 100 mL of solution.

Molarity (M)

The number of moles of solute per liter of solution.

Molality (m)

The number of moles of solute per kg of solvent.

Normality (N)

The number of gram-equivalents of solute per liter of solution.

Mole Fraction (X)

The ratio of moles of one component to the total moles in the solution.

Solubility and Factors Affecting It

Solubility of Solids in Liquids

• Temperature: Solubility generally increases with temperature for most solids.

• Nature of Solute and Solvent: "Like dissolves like" – polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.

Solubility of Gases in Liquids

• Effect of Pressure (Henry’s Law): The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid.

• Effect of Temperature: Solubility of gases decreases with an increase in temperature.

Colligative Properties

Colligative properties depend only on the number of solute particles and not their identity. These properties include:

Relative Lowering of Vapor Pressure

• When a non-volatile solute is added to a solvent, the vapor pressure of the solution decreases compared to the pure solvent.

Elevation of Boiling Point

• The boiling point of a solution is higher than that of the pure solvent due to the presence of a solute.

Depression of Freezing Point

• The freezing point of a solution is lower than that of the pure solvent due to the solute particles interfering with the crystallization process.

Osmotic Pressure

• The pressure required to stop the flow of solvent molecules through a semipermeable membrane is known as osmotic pressure.

• Important in biological processes like water absorption by plant roots.

Raoult’s Law and Its Applications

• For a solution containing two volatile components: The total vapor pressure is the sum of the partial vapor pressures of each component.

• For a solution with a non-volatile solute: The relative lowering of vapor pressure is proportional to the mole fraction of the solute.

• Applications: Used in determining molecular masses and studying non-ideal behavior in solutions.

Ideal and Non-Ideal Solutions

Ideal Solutions

• Obey Raoult’s Law at all concentrations and temperatures.

• No volume or enthalpy change upon mixing.

• Example: Benzene and Toluene.

Non-Ideal Solutions

• Do not obey Raoult’s Law due to interactions between solute and solvent molecules.

• Show positive deviation (higher vapor pressure than expected) or negative deviation (lower vapor pressure than expected).

• Example of positive deviation: Ethanol and Acetone.

• Example of negative deviation: Chloroform and Acetone.

Abnormal Molar Mass and Van’t Hoff Factor

Abnormal Molar Mass

• When solutes undergo association (combination of solute molecules) or dissociation (splitting into ions) in solution, the calculated molar mass deviates from the expected value.

Van’t Hoff Factor (i)

• Introduced to account for the effect of dissociation or association in solutions.

• For dissociation: i > 1 (e.g., NaCl in water).

• For association: i < 1 (e.g., acetic acid in benzene).

Applications of Solutions in Daily Life

• Electrolytes in the Human Body: Body fluids contain ions essential for physiological functions.

• Industrial Applications: Used in the pharmaceutical industry to prepare medicines and intravenous (IV) fluids.

• Food and Beverages: Solutions play a role in soft drinks, sugar syrups, and alcoholic beverages.

Conclusion

Understanding solutions and their properties is crucial for NEET aspirants as it forms the foundation for physical chemistry concepts. Knowledge of concentration terms, colligative properties, Raoult’s Law, and ideal vs. non-ideal solutions is essential for solving numerical and theoretical problems in competitive exams.