Unit 1 of Physical Chemistry sets the mathematical groundwork for the Class 12 syllabus. This guide breaks down the behavior of homogeneous mixtures, focusing on how solutes alter the physical properties of solvents. From measuring concentration using Molarity and Molality to predicting Vapour Pressure changes with Raoult’s Law, we connect theoretical formulas with practical simulations.

Students often struggle with the Van’t Hoff factor and the thermodynamic logic behind Azeotropes. This module addresses those specific challenges through interactive charts and real-time calculators. Whether preparing for CBSE board exams or competitive tests, this resource aligns strictly with the 2025-26 academic curriculum, offering clear explanations of Henry’s Law, ideal vs. non-ideal solutions, and the four key colligative properties.

Solutions: The Complete Guide

A resource for the 2025-26 CBSE Syllabus. Master concentration terms, vapour pressure laws, azeotropes, and colligative properties with interactive tools.

Physical Chemistry 7 Marks Interactive

1. Measuring Concentration

Understanding solution chemistry starts with quantification. A primary distinction exists between units that change with temperature and those that remain constant. Mass does not fluctuate with thermal changes. Volume does.

Temperature Independent (Mass-based)
Molality (m): Moles solute / kg solvent. 
Essential for boiling/freezing points.
Mole Fraction (χ): Ratio of moles. 
Essential for vapour pressure calculations.
Mass % (w/w): Mass solute / Total mass.
ppm: Parts per million (Pollution analysis).

Temperature Dependent (Volume-based)

Molarity (M): Moles solute / Litre solution.
Volume % (V/V): Liquid in liquid mixtures (e.g. Alcohol in water).
Mass/Vol % (w/V): Pharmacy standard (e.g. Saline drips).

Insight: As temperature rises, liquid expands (Volume ↑), so Molarity (n/V) decreases.

2. Solubility Dynamics

Solubility is not static. It depends heavily on the nature of the solute, temperature, and pressure.

🍬 Solid in Liquid

Like sugar in tea.

Increase Temperature (Endothermic) Solubility ↑

Increase Temperature (Exothermic) Solubility ↓

Increase Pressure No Effect

🥤 Gas in Liquid

Like CO₂ in soda.

Increase Temperature Solubility ↓

Increase Pressure (Henry’s Law) Solubility ↑

Henry’s Law Visualizer

Formula: p = K_H · χ

Higher K_H means lower solubility at the same pressure. K_H increases with temperature, which is why aquatic species are more comfortable in cold water (more oxygen).

2.1 The Surface Barrier Effect

Why does adding a solid solute lower the Vapour Pressure?
Evaporation is a surface phenomenon. Non-volatile solute particles occupy surface area, reducing the number of solvent molecules that can escape.

Solute Amount

Pure Solvent High Solute

Blue Dots: Volatile Solvent

Gray Dots: Non-volatile Solute (Blocker)

Arrows: Vapour Pressure

Vapour Pressure: 100% (P°)

3. Raoult’s Law & Deviations

Relation to Henry’s Law: If the solute is a gas, Raoult’s law becomes a special case of Henry’s law where K_H equals P°.

Type	Interaction	ΔH_mix	ΔV_mix	Example
Ideal	A-B ≈ A-A	Zero	Zero	Benzene + Toluene
Positive Dev.	A-B < A-A	+ve (Absorbs Heat)	+ve (Expands)	Ethanol + Acetone
Negative Dev.	A-B > A-A	-ve (Releases Heat)	-ve (Shrinks)	Chloroform + Acetone

3.1 Azeotropes

Binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. Fractional distillation cannot separate them.

Minimum Boiling Azeotrope

Occurs in solutions showing Large Positive Deviation. The mixture boils at a temperature lower than either component.

Example: Ethanol (95%) + Water (5%).

Maximum Boiling Azeotrope

Occurs in solutions showing Large Negative Deviation. The mixture boils at a temperature higher than either component.

Example: Nitric Acid (68%) + Water (32%).

4. Colligative Properties

Properties that depend only on the number of solute particles, not their identity.

1. Relative Lowering of Vapour Pressure

ΔP/P° = χ_solute

2. Elevation in Boiling Point

Solute lowers VP -> Higher Temp needed to boil.

ΔT_b = K_b · m

3. Depression in Freezing Point

Used in car radiators (Anti-freeze).

ΔT_f = K_f · m

4. Osmotic Pressure (π)

Best for calculating molar mass of proteins.

π = C R T

Reference: Common Constants

Solvent	BP (°C)	K_b	FP (°C)	K_f
Water	100	0.52	0	1.86
Benzene	80.1	2.53	5.5	5.12

⚡ Interactive: Osmosis Lab

Simulation

Red blood cells behave differently depending on the concentration of the salt solution surrounding them. Select a solution type to observe the effect.

Select Solution

📊 Interactive: Visualizing the ‘i’ Factor Effect

Comparing the Elevation in Boiling Point ($\Delta T_b$) for different solutes at the same Molality (m). Observe how the number of particles (i) multiplies the effect.

Adjust Molality (m)

1.0 m

Logic: $\Delta T_b \propto i \times m$.
Since $K_b$ is constant (Water), the height of the bar depends entirely on the product of $i$ and $m$.

Glucose
(i=1)

NaCl
(i=2)

AlCl₃
(i=4)

Relative increase in Boiling Point

🧮 Calculator: Degree of Dissociation (α)

Calculate the relationship between the Van’t Hoff factor (i) and the Degree of Dissociation (α) for electrolytes.
Formula: α = (i – 1) / (n – 1)

Number of Ions (n)

2 (NaCl) 3 (MgCl2) 4 (FeCl3) 5

n = 2

Observed Van’t Hoff Factor (i)

i = 1.50

Degree of Dissociation

50%

Percentage of molecules split

⚠️ Common Exam Pitfalls

Temperature Units

In Osmotic Pressure (π = CRT), Temperature (T) must ALWAYS be in Kelvin.
Mistake: Using Celsius directly.

Boiling Point vs Change

The formula ΔT_b = K_bm gives the change, not the new boiling point.
Correct: T_solution = T°_solvent + ΔT_b

Van’t Hoff Factor omission

Forgetting ‘i’ for ionic compounds (NaCl, K2SO4) is the #1 error.
Rule: If it’s a salt, acid, or base, check ‘i’.

Student Q&A Deep Dive

Q1: Why is Molality (m) preferred over Molarity (M) for calculating Elevation in Boiling Point?

Hint: Think about what happens to the volume of a liquid when you heat it up. Does mass change?

Answer: Molality depends on the mass of the solvent, which does not change with temperature. Molarity depends on the volume of the solution. Since liquids expand when heated (volume increases), Molarity decreases as temperature rises. Colligative property experiments involve temperature changes, making Molarity an unstable unit for accurate constants (Kb/Kf).

Q2: Why do deep-sea divers use Helium in their air tanks instead of pure Nitrogen?

Hint: Look at Henry’s Law constant (KH) for Helium vs Nitrogen. Is Helium more or less soluble in blood?

Answer: According to Henry’s Law, high pressure increases gas solubility. Nitrogen dissolves in blood at depth. If a diver surfaces quickly, pressure drops, and N₂ bubbles out, causing “The Bends.” Helium is used because it has a very high Henry’s Law constant (KH), meaning it has very low solubility in blood even at high pressure, and its small atomic size allows it to diffuse out of tissues safely.

Q3: Why does a mixture of Ethanol and Acetone show positive deviation from Raoult’s Law?

Hint: Consider the Hydrogen bonds in pure Ethanol. What does Acetone do to them?

Answer: Pure ethanol molecules are held together by strong Hydrogen bonds. When acetone is added, its molecules insert themselves between ethanol molecules, breaking these strong H-bonds. The new interaction between ethanol and acetone is weaker than the original ethanol-ethanol attraction. This weaker holding force allows molecules to escape into the vapour phase more easily, increasing vapour pressure.

Q4: Ethanoic acid has a molar mass of 60 g/mol. However, when determined by cryoscopy in benzene, the value approaches 120 g/mol. Explain.

Hint: Does Ethanoic acid break apart or come together in a non-polar solvent like benzene? Check the Van’t Hoff factor (i).

Answer: In non-polar solvents like benzene, ethanoic acid molecules associate to form dimers due to intermolecular hydrogen bonding. This reduces the number of particles in the solution by half (Van’t Hoff factor i ≈ 0.5). Since colligative properties are inversely proportional to molar mass, halving the particles results in a calculated molar mass that is double the actual value (60 × 2 = 120 g/mol).

Comprehensive FAQs

Why is Ethylene Glycol added to car radiators?

Ethylene glycol acts as an anti-freeze. It lowers the freezing point of water, preventing the coolant from freezing in winter. Interestingly, it also raises the boiling point, preventing the engine from overheating in summer.

Why can’t Azeotropes be separated by Fractional Distillation?

Fractional distillation works on the principle that different components boil at different temperatures. An azeotrope behaves like a pure liquid because the vapour phase has the exact same composition as the liquid phase. When you boil it, both components evaporate together in the same ratio, so no separation occurs.

What happens to the solubility of solids in liquids when temperature increases?

It depends on the nature of the dissolution process (Le Chatelier’s Principle). If dissolution is Endothermic (ΔsolH > 0, like Sugar), solubility increases. If dissolution is Exothermic (ΔsolH < 0, like Calcium Acetate), solubility decreases.

What material is used for the semi-permeable membrane in Reverse Osmosis?

Cellulose Acetate is commonly used. It is placed over a suitable support and allows solvent molecules (water) to pass through while blocking larger solute particles/ions.

Final Learning Checklist

Concepts Mastered

Differentiating Molarity (Vol-dependent) vs Molality (Mass-dependent).
Interpreting graphs for Henry’s Law and Raoult’s Law.
Identifying Positive/Negative deviations based on interaction strength (A-B vs A-A).
Understanding that Colligative Properties depend on particle count (i), not mass.

Critical Formulae

P = K_H · x (Henry’s Law)
P_total = P_A + P_B (Raoult’s Law)
ΔT = i · K · m (Elevation/Depression)
π = i · C · R · T (Osmotic Pressure)

Class 12 Chemistry Unit 1: Solutions – Notes, Formulas & Visuals (2025-26)