States of Matter – Comprehensive Study Notes for NEET (Medical)
Why this topic matters
In competitive exams such as NEET, questions on states of matter test your conceptual understanding of physical chemistry, ability to derive and manipulate equations, and your capacity to solve quantitative problems involving phase equilibria and thermodynamics. A solid grasp of these fundamentals will also aid in understanding physiological processes (e.g., water transport, blood viscosity, gas exchange).
1. Overview
| State | Key Features | Typical Energy Scale (kJ mol⁻¹) | Example |
|---|---|---|---|
| Solid | Long‑range order, fixed volume & shape | 10–1000 | Ice, graphite |
| Liquid | Short‑range order, fixed volume, free shape | 10–100 | Water, ethanol |
| Gas | No order, variable volume & shape | 0–10 | N₂, O₂ |
| Plasma | Ionized gas, free electrons & ions | >10 | Sun, neon lights |
Tip: In NEET, only solid, liquid, gas and plasma are asked. Bose–Einstein condensate and fermionic superfluid are rarely included.
2. Solid – Crystalline & Amorphous
| Property | Explanation | Key Equation |
|---|---|---|
| Lattice & unit cell | Repeating arrangement of atoms | ( V = N a^{3} ) (cubic lattice) |
| Crystal defects | Vacancies, dislocations, interstitials | ( n_{\text{vac}} = e^{-E_{\text{vac}}/kT} ) |
| Heat capacity | ( C_{V} \approx 3R ) (Dulong–Petit) | |
| Phase transitions | Melting: ( \Delta H_{\text{fus}} ) | ( T_{m} = \frac{\Delta H_{\text{fus}}}{\Delta S_{\text{fus}}} ) |
| Hardness | Interatomic bonding |
Tricky Point: Amorphous solids (e.g., glass) have no long‑range order but still possess a fixed volume. Their glass transition temperature (T_g) is not a true phase transition.
3. Liquid
| Concept | Details | Formula/Relation |
|---|---|---|
| Diffusion | Random thermal motion | ( D = \frac{1}{3} \lambda \bar{v} ) |
| Viscosity | Internal friction | ( \eta = \frac{1}{3} \rho \bar{v} \lambda ) |
| Surface tension | Energy per unit area | ( \gamma = \frac{E_{\text{surf}}}{A} ) |
| Compressibility | Relative volume change | ( \kappa_T = -\frac{1}{V}\left(\frac{\partial V}{\partial P}\right)_T ) |
| Surface phenomena | Capillary rise, wetting | ( h = \frac{2\gamma \cos\theta}{\rho g r} ) |
Tricky Point: Liquids have higher density than gases because intermolecular forces are strong enough to keep molecules closer, but weaker than solids allowing flow.
4. Gas – Ideal vs Real
4.1 Ideal Gas
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Assumptions: Point particles, no intermolecular forces, elastic collisions.
-
Equation of State (EOS):
[ PV = nRT = Nk_{B}T ] -
Kinetic Theory Derivation
- Pressure from collisions
( P = \frac{1}{3}\rho \bar{v^2} ) (where ( \rho = m n/V )) - Average kinetic energy
( \frac{1}{2}m\bar{v^2} = \frac{3}{2}k_{B}T ) - Combine → ( PV = nRT )
- Pressure from collisions
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Internal Energy
[ U = \frac{3}{2}nRT \quad (\text{monatomic}),\qquad U = \frac{5}{2}nRT \quad (\text{diatomic}) ] -
Speed of Sound
[ v_s = \sqrt{\frac{\gamma RT}{M}} ] with ( \gamma = \frac{C_P}{C_V} ).
4.2 Real Gas – Van der Waals Equation
[ \left(P + \frac{n^2 a}{V^2}\right)(V - nb) = nRT ]
- (a