Solutions in Chemistry: Class 12 NCERT Complete Guide

Understanding Solutions: Definition and Types

A solution is a homogeneous mixture of two or more substances. In Chemistry, the substance present in the larger amount is called the solvent, and the substance dissolved is the solute.

Types of Solutions:

• Solid Solutions: Example: Brass (solid in solid)
• Liquid Solutions: Example: Sugar dissolved in water
• Gaseous Solutions: Example: Air (oxygen in nitrogen)

Solutions can also be classified based on solute and solvent phases:

Understanding these types helps in analyzing their properties and behavior in different conditions.

Concentration Terms in Solutions

Concentration expresses how much solute is present in a given amount of solvent or solution. Common concentration units in Class 12 NCERT Chemistry include:

• Molarity (M): Moles of solute per litre of solution. Formula: $M = \frac{\text{moles of solute}}{\text{volume of solution in litres}}$

• Molality (m): Moles of solute per kilogram of solvent. Formula: $m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}}$

• Mole Fraction (x): Ratio of moles of one component to total moles.

• Mass Percentage: $\frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

Important Note: Molality is independent of temperature because it depends on mass, not volume, which changes with temperature.

Colligative Properties of Solutions

Colligative properties depend only on the number of solute particles, not their nature. They are important for understanding solution behavior:

• Vapour Pressure Lowering: Adding solute lowers vapour pressure.
• Boiling Point Elevation: Solution boils at a higher temperature.
• Freezing Point Depression: Solution freezes at a lower temperature.
• Osmotic Pressure: Pressure required to stop solvent flow through a semipermeable membrane.

These properties help determine molar masses and purity of substances.

Formula for Boiling Point Elevation: $$ \Delta T_b = K_b \times m $$ where $\Delta T_b$ is boiling point elevation, $K_b$ is molal boiling point elevation constant, and $m$ is molality.

Conductance of Electrolytic Solutions

Conductance in electrolytic solutions occurs due to ion movement under an electric field. Key points:

• Resistance (R): Opposition to current, $R = \rho \frac{l}{A}$ where $\rho$ is resistivity, $l$ length, $A$ cross-sectional area.
• Conductance (G): Inverse of resistance, $G = \frac{1}{R}$, unit Siemens (S).
• Conductivity ($\kappa$): Inverse of resistivity, indicates how well a solution conducts electricity.

Comparison of Conductivity:

Conductivity depends on electrolyte nature, ion size, solvent, concentration, and temperature. It increases with temperature due to enhanced ion mobility.

Example: Calculate resistance of a copper wire 2 m long and 1 mm$^2$ in cross-section if resistivity $\rho = 1.7 \times 10^{-8} \Omega m$.

$$ R = \rho \frac{l}{A} = 1.7 \times 10^{-8} \times \frac{2}{1 \times 10^{-6}} = 0.034 \Omega $$

Factors Affecting Solubility and Solution Formation

Solubility is the maximum amount of solute that can dissolve in a solvent at a given temperature.

Factors influencing solubility:

• Nature of Solute and Solvent: 'Like dissolves like' principle.
• Temperature: Solubility of solids usually increases with temperature; gases decrease.
• Pressure: Mainly affects gas solubility (Henry's law).

Henry's Law: $$ C = kP $$ where $C$ is gas solubility, $k$ is Henry's constant, and $P$ is gas pressure.

Understanding these factors helps in predicting solution behavior in various chemical processes.

Ideal and Non-Ideal Solutions

Solutions can be ideal or non-ideal based on interactions between molecules.

• Ideal Solutions: Enthalpy of mixing ($\Delta H_{mix}$) is zero; volume change is zero. Example: Benzene and Toluene.
• Non-Ideal Solutions: Show positive or negative deviations from Raoult's law due to stronger or weaker intermolecular forces.

Raoult’s Law: $$ P_{solution} = x_{solvent} P^0_{solvent} $$ where $P_{solution}$ is vapour pressure of solution, $x_{solvent}$ mole fraction of solvent, and $P^0_{solvent}$ vapour pressure of pure solvent.

Ideal solutions obey Raoult’s law at all concentrations, important for predicting vapor pressures.