Types of Solutions

A solution is a homogeneous mixture of two or more substances, consisting of a solute dissolved in a solvent. Depending on whether the solute and solvent are solids, liquids, or gases, solutions are categorized into nine distinct types.

Classification Based on the Concentration of Solute

Solutions vary based on the quantitative proportion of the solute relative to the maximum capacity of the solvent at a given temperature.

Unsaturated Solution

A solution in which more solute can be dissolved at a specific, unchanged temperature. The amount of solute present is below its equilibrium solubility limit.

Saturated Solution

A solution that has dissolved the maximum possible amount of solute at a given temperature. If any additional solute is added, it will not dissolve and will remain as a precipitate at the bottom.

Supersaturated Solution

A metastable solution that contains more dissolved solute than a saturated solution would contain at the same temperature. It is prepared by dissolving solute at an elevated temperature and then cooling the solution carefully without disturbance.

• Property: Highly unstable; any slight agitation or addition of a tiny “seed crystal” causes the excess solute to crystallize out immediately.

Classification Based on the Nature of Solvent

The choice of liquid medium determines the broader chemical behavior and naming convention of the solution.

Aqueous Solution

A solution where water acts as the solvent.

• Examples: Sugar solution, mineral acids dissolved in water, and seawater.

Non-Aqueous Solution

A solution where the solvent is any liquid other than water.

• Examples: Iodine dissolved in carbon tetrachloride (CCl₄), sulfur dissolved in carbon disulfide (CS₂), and naphthalene dissolved in benzene. Common non-aqueous solvents include ether, alcohol, and acetone.

Methods of Expressing Concentration of Solutions

Concentration defines the amount of solute present in a given quantity of solution or solvent. UPSC questions frequently touch upon temperature-dependent vs. temperature-independent parameters.

Mass Percentage (w/w)

The mass of a solute component per 100 grams of the solution.

• Temperature Dependency: Independent of temperature.

Volume Percentage (v/v)

The volume of a solute component per 100 volume units of the solution.

• Temperature Dependency: Dependent on temperature due to thermal expansion of liquids.

Mass by Volume Percentage (w/v)

The mass of solute dissolved in 100 milliliters of the final solution. It is widely used in medicine and pharmacy.

• Temperature Dependency: Dependent on temperature.

Parts Per Million (ppm)

Used when a solute is present in trace quantities. It represents the parts of solute per million (10⁶) parts of the solution.

• Application: Expressing water hardness or pollutants like sulfur dioxide in the atmosphere.

Mole Fraction (x)

The ratio of the number of moles of a particular component to the total number of moles of all the components present in the solution. For a binary solution containing components A and B:

• Key Fact: The sum of all mole fractions in a solution is always equal to 1 (x_A + x_B = 1). It is independent of temperature.

Molarity (M)

The number of moles of solute dissolved in one liter (or cubic decimeter) of solution.

• Temperature Dependency: Dependent on temperature because the volume of a liquid expands or contracts with temperature variations.

Molality (m)

The number of moles of solute dissolved per kilogram (1000 g) of the solvent.

• Temperature Dependency: Independent of temperature because mass does not alter with thermal fluctuations. Molality is preferred over molarity in precise thermodynamic experiments.

Factors Affecting Solubility of Solutions

Solubility is the maximum amount of a solute that can be dissolved in a specified amount of solvent at a specific temperature.

Nature of Solute and Solvent

Follows the universal rule of “like dissolves like”. Polar solutes (e.g., sodium chloride) dissolve easily in polar solvents (e.g., water), whereas non-polar solutes (e.g., naphthalene) dissolve readily in non-polar solvents (e.g., benzene).

Effect of Temperature

• Solid in Liquid: If the dissolution process is endothermic (Δ H > 0), solubility increases with a rise in temperature. If the process is exothermic (Δ H < 0), solubility decreases with an increase in temperature (Le Chatelier’s Principle).
• Gas in Liquid: Dissolution of gas in liquid is universally an exothermic process. Therefore, gas solubility decreases continuously with an increase in temperature. This is why aquatic species are more comfortable in cold waters than in warm waters.

Effect of Pressure

• Solid in Liquid: Practically no effect, as solids and liquids are highly incompressible.
• Gas in Liquid: Highly significant. The solubility of a gas increases directly with an increase in pressure, governed quantitatively by Henry’s Law.

Henry’s Law and Real-world Applications

Henry’s Law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid or solution.

• Deep-Sea Diving: Scuba divers breathe compressed air at high pressure underwater, increasing the solubility of atmospheric gases in their blood. Upon rising rapidly, the pressure drops, causing dissolved nitrogen to release as bubbles in the blood vessels, leading to a painful and dangerous medical condition called the bends. To avoid this, diving cylinders are diluted with helium (11.7% Helium, 56.2% Nitrogen, 32.1% Oxygen).
• Anoxia in Mountain Climbers: At high altitudes, the partial pressure of oxygen is low. This leads to low concentrations of oxygen in the blood and tissues of climbers, causing anoxia, characterized by physical weakness and decreased mental clarity.
• Carbonated Beverages: To maximize the solubility of carbon dioxide, soda and soft drink bottles are sealed under exceptionally high pressure.