Liquid State – Chemistry Quick Revision Notes 2025

Liquid state chemistry deals with the behaviour, structure and properties of liquids and solutions. In school-level chapters, we learn about concentration terms, Raoult’s law, vapour pressure, colligative properties and ideal/non-ideal solutions. But graduate-level discussions expand these ideas into molecular interactions, activity coefficients, deviations from ideality, solution thermodynamics, viscosity mechanisms, diffusion theory, electrolyte behaviour and modern applications such as osmotic drug delivery, polymer solutions, and ionic liquids. In simple terms, liquid chemistry tries to explain how molecules move in a liquid, why mixing changes properties, and how liquids behave differently from solids and gases.

1. Nature of the Liquid State

Definition: A liquid is a condensed phase of matter where particles remain close together but have enough freedom to move past each other. Because of this movement, liquids take the shape of the container but retain a fixed volume. Intermolecular forces in liquids are strong enough to hold particles close but not strong enough to keep them rigid (as in solids). Liquids show unique properties like viscosity, surface tension and diffusion, which arise from molecular interactions.

Key Properties of Liquids

• Fluidity – Molecules slide over each other.
• Incompressibility – Very little change in volume on applying pressure.
• Surface tension – Tend to minimise their surface area.
• Viscosity – Internal resistance to flow.
• Diffusion – Random motion leading to mixing.

2. Vapour Pressure – Foundation of Raoult’s Law

Definition: Vapour pressure is the pressure exerted by vapour molecules in equilibrium with its liquid at a definite temperature. When temperature rises, vapour pressure increases because more molecules have enough energy to escape the liquid surface. A liquid boils when its vapour pressure becomes equal to external pressure. Vapour pressure is strongly affected by intermolecular forces: stronger the force, lower the vapour pressure.

Raoult’s Law

For an ideal solution, the partial vapour pressure of each component is directly proportional to its mole fraction in the solution.$$p_i = x_i p_i^\circ$$pi​=x_i​p_i^∘​

and total pressure:$$p = p_A + p_B = x_A p_A^\circ + x_B p_B^\circ$$p= p_A ​+ p_B​ = x_A​p_A^∘​+x_B​p_B^∘​

Assumptions of Ideal Solutions

• Enthalpy of mixing (ΔH_mix) = 0
• Volume of mixing (ΔV_mix) = 0
• Similar intermolecular forces between A-A, B-B and A-B

Examples of Nearly Ideal Systems

• Benzene + toluene
• n-hexane + n-heptane

Deviations from Raoult’s Law

Positive Deviation

• A-B interactions < A-A or B-B
• Vapour pressure ↑
• ΔH_mix > 0 (endothermic)
• Example: ethanol + acetone

Negative Deviation

• A-B interactions > A-A or B-B
• Vapour pressure ↓
• ΔH_mix < 0 (exothermic)
• Example: chloroform + acetone

Comparison Table: Ideal vs Non-ideal Solutions

Azeotropes

Azeotropes are mixtures that boil at a constant temperature and behave like pure liquids because their vapour has the same composition as the liquid.

PG Question: Why are azeotropes difficult to separate by simple distillation?
→ Because liquid and vapour compositions are identical.

3. Concentration Terms

PG Insight: Molarity changes with temperature because volume changes; molality does not, since mass remains constant.

4. Colligative Properties

Colligative properties depend on number of particles, not their nature.

1. Relative Lowering of Vapour Pressure

$$\frac{p^0 – p}{p^0} = x_{\text{solute}}$$

2. Elevation of Boiling Point

$$\Delta T_b = K_b m$$ΔT_b​=K_b​m

3. Depression of Freezing Point

$$\Delta T_f = K_f m$$ΔT_f​=K_f​m

4. Osmotic Pressure

$$\pi = MRT$$π=MRT

Applications

• Blood pressure replacement fluids
• Determination of molar mass
• Antifreeze in automobile engines
• Salt spreading on ice roads in winter

5. Abnormal Molar Mass: van’t Hoff Factor (i)

The van’t Hoff factor accounts for association or dissociation of solute particles.

Example Questions:

• Why does KNO₃ show i > 1? → Dissociates into ions.
• Why does acetic acid in benzene show i < 1? → Dimerises (association).

6. Electrolytic and Nonelectrolytic Solutions

Electrolytes: Produce ions in solution → Conduct electricity.

Non-Electrolytes: Do not ionise → Do not conduct.

Degree of Ionisation (α): For weak electrolytes:$$K_a = \frac{c\alpha^2}{1-\alpha}$$Ostwald’s Dilution Law: As dilution increases (c decreases), α increases.
This explains why weak electrolytes ionise more in dilute solutions.

Activity and Activity Coefficients

Ideal solutions assume concentration = effective concentration. Real solutions behave differently, so we use activity (a) instead of concentration.$$a = \gamma c$$

Where γ = activity coefficient.

Key Points

• γ = 1 → Ideal behaviour
• γ < 1 or > 1 → Non-ideal behaviour
• Used heavily in thermodynamics, electrochemistry and ionic equilibria

7. Henry’s Law

Statement

The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid.$$p = k_H x$$

Applications

• Carbonated drinks
• Scuba diving (decompression sickness)
• Industrial gas absorption

B.Sc Insight: Gaseous solubility decreases as temperature increases because gas molecules escape more easily.

Solutions of Gases in Liquids

8. Diffusion in Liquids

Definition: Diffusion in liquids is the spontaneous movement of molecules from a high concentration region to a lower concentration region. The rate of diffusion is slower than gases because liquid molecules face more resistance. Temperature increases diffusion because kinetic energy increases. Diffusion explains mixing of liquids, perfume smell spreading in water, and many biological processes.

Fick’s Laws (B.Sc/ PG Level)

1. Rate of diffusion ∝ concentration gradient.
2. Change in concentration with time relates to second derivative of concentration.

9. Surface Tension

Definition: Surface tension is the force acting per unit length that pulls the surface molecules inward, reducing surface area. It arises because molecules at the surface experience unbalanced attractive forces. Liquids with strong intermolecular attraction, like water, have high surface tension. Temperature reduces surface tension.

Applications

• Capillary rise
• Floating of needle on water
• Formation of droplets
• Detergent action

10. Viscosity – Flow Behaviour of Liquids

Definition: Viscosity is the measure of a liquid’s internal resistance to flow. A liquid with high viscosity, like honey or glycerin, flows slowly. Viscosity depends strongly on intermolecular forces and temperature. Higher temperature reduces viscosity because molecules move faster.

Poiseuille’s Equation

$$\eta = \frac{\pi r^4 \Delta p}{8VL}$$

Comparison of Liquids

11. Structure of Liquid Water

Water forms a dynamic hydrogen-bonded network. Although bonds continuously break and reform, on an average, each molecule is connected to four neighbours in near-tetrahedral arrangement. This explains:

• High boiling point
• High heat capacity
• High surface tension
• Density anomaly (ice less dense than water)

12. Solutions of Solids in Liquids

Factors Affecting Solubility

Solubility Curves explain crystallisation, recrystallisation and separation techniques.

13. Solutions of Liquids in Liquids

Types

14. Thermodynamics of Solutions (PGT Upgrade)

Enthalpy of Solution

$$\Delta H_{\text{solution}} = \Delta H_{\text{solvent}} + \Delta H_{\text{solute}} + \Delta H_{\text{mix}}$$

Negative → exothermic → dissolution favoured
Positive → endothermic → depends on entropy

Gibbs Free Energy

$$\Delta G = \Delta H – T\Delta S$$ΔG=ΔH−TΔS

For spontaneous dissolution → ΔG < 0.

15. Osmosis and Reverse Osmosis (RO)

Definition: Osmosis is the movement of solvent molecules through a semipermeable membrane from low solute concentration to high solute concentration. The pressure required to stop this flow is called osmotic pressure. Reverse osmosis occurs when pressure greater than osmotic pressure is applied, forcing the solvent to move in the opposite direction. RO is widely used in water purification.

16. Polymers in Solution

Polymer solutions behave differently from small-molecule solutions.

Features

• High viscosity
• Coil-shaped molecules
• Show anomalous diffusion
• Exhibit theta temperature behaviour

17. Important Comparison Tables

Table: Liquid vs Gas

Table: Colligative Properties Comparison

18. Common PGT INTERVIEW Questions (with mini answers)

Q1. Why does salt increase boiling point of water?

→ Because solute lowers vapour pressure, requiring higher temperature to reach atmospheric pressure.

Q2. Why does ice float on water?

→ Ice has open tetrahedral structure with hydrogen bonding → lower density.

Q3. What type of deviation is shown by ethanol and water?

→ Positive deviation at higher ethanol concentration; hydrogen bonding complicates behaviour.

Q4. Why do gases become less soluble at higher temperature?

→ Increased kinetic energy causes molecules to escape.

Q5. Why is osmotic pressure preferred for molar mass measurement?

→ Measurable even for very dilute solutions and macromolecules.