Class 12 Chemistry Chapter 2 Notes: Solutions | Important Questions & PYQs

Class 12 Chemistry – Chapter 2: Solutions (NCERT Notes)

1. What is a Solution?

A solution is a homogeneous mixture of two or more non-reacting substances.

• Solute → present in smaller amount

• Solvent → present in larger amount

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2. Types of Solutions (Based on Physical State)

(A) Gaseous Solutions

• Gas in gas → Air (O₂ + N₂)

• Liquid in gas → Water vapour in air

• Solid in gas → Smoke, camphor vapours

(B) Liquid Solutions

• Gas in liquid → CO₂ in water (soda)

• Liquid in liquid → Alcohol + water

• Solid in liquid → Sugar in water

(C) Solid Solutions

• Gas in solid → H₂ in Pd

• Liquid in solid → Amalgams

• Solid in solid → Alloys (brass, gold ornaments)

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3. Concentration of Solutions

3.1 Mass Percentage (% w/w)

\text{Mass %} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100

3.2 Mole Fraction (χ)

$${\chi }_A=\frac{n_A}{n_A+n_B}$$

• No unit

• Used in vapour pressure calculations

3.3 Molality (m)

$$m=\frac{\text{moles of solute}}{\text{kg of solvent}}$$

✔ Temperature independent (very important for exams)

3.4 Molarity (M)

$$M=\frac{\text{moles of solute}}{\text{litre of solution}}$$

✖ Changes with temperature

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4. Solubility

• Depends on nature of solute & solvent

• “Like dissolves like”

• Gas solubility decreases with increase in temperature

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5. Vapour Pressure of Liquid Solutions

Raoult’s Law

$$P_A={\chi }_A{P}_A^{\circ }$$

• Valid for ideal solutions

• Relative lowering of vapour pressure depends only on number of particles

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6. Ideal vs Non-Ideal Solutions

Ideal Solutions

• Obey Raoult’s law

• ΔHₛₒₗ = 0, ΔVₛₒₗ = 0

• Example: Benzene + Toluene

Non-Ideal Solutions

• Show positive or negative deviation

Positive deviation

• A–B interactions weaker

• Vapour pressure ↑

• ΔHₛₒₗ positive

Negative deviation

• A–B interactions stronger

• Vapour pressure ↓

• ΔHₛₒₗ negative

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7. Colligative Properties

Depend only on number of solute particles, not their nature.

Four Colligative Properties

1. Relative lowering of vapour pressure

2. Elevation of boiling point

$$\mathrm{\Delta }{T}_b=K_{b}m$$

3. Depression of freezing point

$$\mathrm{\Delta }{T}_f=K_{f}m$$

4. Osmotic pressure

$$\pi =MRT$$

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8. Abnormal Molar Mass

Van’t Hoff Factor (i)

$$i=\frac{\text{Observed colligative property}}{\text{Calculated value}}$$

• i > 1 → Dissociation

• i < 1 → Association

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9. Henry’s Law (Gas Solubility)

$$p=K_H\chi$$

• Solubility of gas ∝ pressure

• Higher pressure → more solubility

Applications

• Soda water bottles

• Deep-sea diving

• Oxygen transport in blood

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10. Osmotic Pressure

• Important for polymers & biomolecules

• Used to find molar mass of macromolecules

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11. Intermolecular Interactions

Pair	Interaction
Non-polar + Non-polar	London forces
Ionic + Polar	Ion–dipole
Polar + Polar	Dipole–dipole
–OH / –NH	Hydrogen bonding

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12. Solubility Trend in Water

• Highly soluble → Formic acid, ethylene glycol

• Partially soluble → Phenol, pentanol

• Insoluble → Toluene, chloroform

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13. Important Exam Tips

✔ Molality preferred over molarity
✔ Colligative properties → number of particles
✔ Henry’s law → gas solubility
✔

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What is a solution?
   A solution is a homogeneous mixture of two or more non-reacting substances.

2. Define solute and solvent.
   Solute is present in smaller amount, while solvent is present in larger amount.

3. What is molality? Why is it preferred over molarity?
   Molality is moles of solute per kg of solvent; it is temperature independent.

4. Write the formula for mole fraction.
   χₐ = nₐ / (nₐ + nᵦ)

5. State Raoult’s law.
   Vapour pressure of a component is proportional to its mole fraction.

6. What are ideal solutions?
   Solutions that obey Raoult’s law over the entire range of concentration.

7. What are colligative properties?
   Properties that depend only on number of solute particles.

8. Write Henry’s law.
   p = Kᴴ χ

9. What is van’t Hoff factor?
   It is the ratio of observed to calculated colligative property.

10. What happens when i > 1?
    Dissociation of solute occurs.

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📝 Long Answer Questions

1. Explain different methods of expressing concentration of solutions.

(a) Mass Percentage (% w/w)

• Ratio of mass of solute to mass of solution × 100

(b) Mole Fraction (χ)

• Ratio of moles of one component to total moles

• Unitless, used in vapour pressure calculations

(c) Molality (m)

• Moles of solute per kg of solvent

• Temperature independent

(d) Molarity (M)

• Moles of solute per litre of solution

• Changes with temperature

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2. Differentiate between ideal and non-ideal solutions.

Ideal Solutions

• Obey Raoult’s law

• ΔHₛₒₗ = 0 and ΔVₛₒₗ = 0

• Example: benzene + toluene

Non-Ideal Solutions

• Show positive or negative deviation

• Positive deviation: weaker A–B interactions, vapour pressure increases

• Negative deviation: stronger A–B interactions, vapour pressure decreases

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3. Explain colligative properties of solutions.

Colligative properties depend only on number of solute particles:

• Relative lowering of vapour pressure

• Elevation of boiling point
  ΔTᵦ = Kᵦ m

• Depression of freezing point
  ΔT𝒻 = K𝒻 m

• Osmotic pressure
  π = MRT

These properties are used to calculate molar mass.

📝 PYQs (Previous Years’ Questions) – Board Exam Focused

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🔹 Very Short Answer / 1 Mark PYQs

1. Define a solution.

2. What is mole fraction?

3. Which concentration term is temperature independent?

4. Write the formula of molarity.

5. State Raoult’s law.

6. What is the unit of osmotic pressure?

7. Write the expression for elevation of boiling point.

8. What happens when van’t Hoff factor is less than 1?

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🔹 Short Answer / 2 Marks PYQs

1. Why is molality preferred over molarity in colligative properties?

2. Define ideal solution with one example.

3. Write two differences between ideal and non-ideal solutions.

4. What is Henry’s law? Write one application.

5. Define osmotic pressure.

6. Why does gas solubility decrease with rise in temperature?

7. What is positive deviation from Raoult’s law?

8. State two applications of osmotic pressure.

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🔹 Short Answer / 3 Marks PYQs

1. Explain Raoult’s law for a binary liquid solution.

2. Describe different types of solutions based on physical state.

3. Explain elevation in boiling point with suitable expression.

4. What are colligative properties? Name any three.

5. Explain van’t Hoff factor and its significance.

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🔹 Long Answer / 5 Marks PYQs

1. (a) What are ideal solutions?
   (b) Explain non-ideal solutions showing positive and negative deviations from Raoult’s law with reasons.

2. (a) Define colligative properties.
   (b) Derive expression for depression in freezing point.

3. (a) State Henry’s law.
   (b) Write its applications in daily life.

4. (a) What is osmotic pressure?
   (b) How is it used to determine molar mass of macromolecules?

5. (a) Define abnormal molar mass.
   (b) Explain van’t Hoff factor for association and dissociation.

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🔹 Numerical PYQs (Most Repeated)

1. Calculate molality of a solution containing given mass of solute in water.

2. Find elevation in boiling point using ΔTᵦ = Kᵦ m.

3. Calculate molar mass using osmotic pressure data.

4. Determine van’t Hoff factor for electrolytes.

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🔑 High-Weightage Exam Areas

✔ Colligative properties (formulas + numericals)
✔ Raoult’s law & deviations
✔ Henry’s law applications
✔ Van’t Hoff factor
✔ Osmotic pressure

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❓ FAQs (Concept Clarity)

1. Why is molality temperature independent?
   Because mass of solvent does not change with temperature.

2. Why does gas solubility decrease with increase in temperature?
   Because dissolution of gas is exothermic.

3. Why do ideal solutions show no heat change on mixing?
   Because intermolecular interactions remain unchanged.

4. Why are colligative properties independent of nature of solute?
   They depend only on number of solute particles.

5. Why is osmotic pressure preferred for molar mass determination of polymers?
   Because it can be measured at room temperature.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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