Introduction
A solution is a homogeneous mixture of two or more substances where the components are uniformly distributed at the molecular level. The substance present in smaller quantity is called the solute, while the substance present in larger quantity is called the solvent.
Solubility is defined as the maximum amount of solute that can dissolve in 100g of solvent at a given temperature to form a saturated solution.
Types of Solutions
| Solvent | Solute | Example |
|---|---|---|
| Gas | Gas | Air |
| Gas | Liquid | Humid air |
| Gas | Solid | Smoke |
| Liquid | Gas | Aerated drinks |
| Liquid | Liquid | Alcohol in water |
| Liquid | Solid | Sugar in water |
| Solid | Gas | H₂ in Pd |
| Solid | Liquid | Hg in Zn |
| Solid | Solid | Alloys |
Concentration Units
Important Relationships
Colligative Properties
Colligative properties depend only on the number of solute particles in solution, not on their identity. The four main colligative properties are:
Raoult's Law
Raoult's Law states that the partial vapor pressure of each component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution: PA = P⁰A × XA
Ideal vs Non-Ideal Solutions
| Property | Ideal Solution | Positive Deviation | Negative Deviation |
|---|---|---|---|
| Raoult's Law | Obeys | Does not obey | Does not obey |
| ΔHmix | = 0 | > 0 (Endothermic) | < 0 (Exothermic) |
| ΔVmix | = 0 | > 0 | < 0 |
| Molecular Interactions | A-A ≈ A-B ≈ B-B | A-B < A-A, B-B | A-B > A-A, B-B |
| Examples | Benzene + Toluene | Acetone + Ethanol | Acetone + Chloroform |
Azeotropic Mixtures
Azeotropes are constant boiling mixtures that have the same composition in liquid and vapor phases. They cannot be separated by simple distillation.
Osmosis and Osmotic Pressure
Osmosis is the spontaneous flow of solvent molecules through a semipermeable membrane from a pure solvent to a solution or from a dilute to a concentrated solution.
Types of Osmosis
Types of Solutions Based on Osmotic Pressure
| Type | Osmotic Pressure | Solvent Flow | Example |
|---|---|---|---|
| Isotonic | π1 = π2 | No net flow | 0.9% NaCl (physiological saline) |
| Hypertonic | π1 > π2 | Into solution 1 | Concentrated salt solution |
| Hypotonic | π1 < π2 | Out of solution 1 | Distilled water |
Abnormal Molecular Masses
When solutes dissociate or associate in solution, the observed colligative properties differ from expected values, leading to abnormal molecular masses.
Van't Hoff Factor (i)
Degree of Dissociation/Association
Modified Colligative Properties
- Relative lowering of vapor pressure = iXB
- Elevation in boiling point: ΔTb = iKbm
- Depression in freezing point: ΔTf = iKfm
- Osmotic pressure: π = iCRT
Important Points to Remember
Key Points for JEE Main
- Molality is temperature independent, while molarity is temperature dependent
- Osmotic pressure is the most sensitive colligative property for determining molecular mass
- For electrolytes, colligative properties depend on the number of ions produced
- Raoult's law is applicable only to ideal solutions and very dilute solutions
- Azeotropes cannot be separated by simple distillation
- Reverse osmosis is used in water purification and desalination
- Van't Hoff factor i > 1 for dissociation, i < 1 for association
Do's
Don'ts
JEE Main Weightage
This chapter typically carries 2-3 questions in JEE Main, making it a high-weightage chapter. Questions often focus on concentration calculations, colligative properties, Raoult's law, and Van't Hoff factor.