NEET Chemistry - Chapter 3

States of Matter

Original NEET physical chemistry notes on gases, gas laws, kinetic theory, real gases, liquids, intermolecular forces, phase changes, and introductory solid-state ideas used in NEET.

NEET Chemistry States of Matter Notes Ad

States of Matter Notes Sponsor

Premium placement inside the NEET chemistry chapter notes for States of Matter.

Concept Block

1. Gas Laws and the Ideal Gas Equation

Four experimental gas laws combine into one master equation. Know each law independently because NEET asks "which law says X" type questions.

Law	Constant	Relationship
Boyle's	$T, n$	$PV =$ const → $P \propto 1/V$
Charles's	$P, n$	$V/T =$ const → $V \propto T$
Gay-Lussac's	$V, n$	$P/T =$ const → $P \propto T$
Avogadro's	$P, T$	$V \propto n$ (equal volumes, equal moles)

PV=nRT,\quad R=8.314\text{ J mol}^{-1}\text{K}^{-1}=0.0821\text{ L atm mol}^{-1}\text{K}^{-1}

\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}\quad(\text{Combined gas law})

Dalton's Law: In a mixture, $P_{total}=P_A+P_B+\ldots$ and $P_A=x_A\cdot P_{total}$ where $x_A$ is mole fraction.

NEET trap: Always use absolute temperature in Kelvin (

T = t°C + 273

). Using °C directly in

PV=nRT

gives completely wrong answers.

Concept Block

2. Kinetic Theory of Gases, Molecular Speeds, and Graham's Law

The kinetic theory derives gas pressure from molecular collisions and links molecular kinetic energy to temperature. Its key assumptions: point-mass molecules, elastic collisions, no intermolecular forces, negligible molecular volume.

\text{Average KE per molecule}=\frac{3}{2}k_BT,\qquad\text{per mole}=\frac{3}{2}RT

u_{rms}=\sqrt{\frac{3RT}{M}},\quad u_{avg}=\sqrt{\frac{8RT}{\pi M}},\quad u_{mp}=\sqrt{\frac{2RT}{M}}

Ratio: $u_{rms}:u_{avg}:u_{mp}=\sqrt{3}:\sqrt{8/\pi}:\sqrt{2}\approx 1.225:1.128:1.000$

Graham's Law of Diffusion: Lighter gases diffuse faster. The rate is inversely proportional to the square root of molar mass.

\frac{r_A}{r_B}=\sqrt{\frac{M_B}{M_A}}=\sqrt{\frac{d_B}{d_A}}

Worked example: H

_2

(

M=2

) vs O

_2

(

M=32

r_{H_2}/r_{O_2}=\sqrt{32/2}=4

. Hydrogen diffuses 4 times faster than oxygen.

Concept Block

3. Real Gases, van der Waals Equation, and Compressibility Factor

Real gases deviate from ideal behavior because: (1) molecules have finite volume and (2) molecules attract each other. The van der Waals equation corrects both.

\left(P+\frac{an^2}{V^2}\right)(V-nb)=nRT

$a$ = intermolecular attraction correction; $b$ = volume correction (co-volume per mole). High $a$ → easily liquefied. High $b$ → large molecule.

The compressibility factor $Z = PV/nRT$ . For ideal gas, $Z=1$ .

$Z < 1$ : attractive forces dominate (gas is more compressible than ideal)
$Z > 1$ : repulsive forces / finite volume dominate (less compressible than ideal)

Real gases approach ideal behavior at low pressure and high temperature (Boyle temperature for each gas).

NEET tip: At very low pressures, all gases have

Z\approx1

. At moderate pressures,

Z&lt;1

for most gases. At very high pressures,

Z&gt;1

always.

Concept Block

4. Liquid State: Vapour Pressure, Surface Tension, and Viscosity

Liquids have defined volume but not defined shape. The three key liquid properties tested in NEET all depend on the strength of intermolecular forces.

Property	Definition	Effect of stronger IMF	Effect of ↑ Temperature
Vapour pressure	Pressure of vapour over liquid at equilibrium	Decreases	Increases
Surface tension	Force per unit length at surface	Increases	Decreases
Viscosity	Resistance to flow	Increases	Decreases

Boiling point = temperature where vapour pressure = external pressure. At high altitude, external pressure is lower, so water boils below 100°C.

Capillary rise: $h = 2T\cos\theta/(r\rho g)$ . For water (concave meniscus), capillary rise occurs. For mercury (convex meniscus), capillary depression occurs.

NEET tip: Water has anomalously high BP, surface tension, and viscosity for its low molar mass — all because of extensive hydrogen bonding.

Concept Block

5. Phase Diagrams, Critical Point, Triple Point, and Solid-State Overview

A phase diagram maps which state of matter is stable at each temperature-pressure combination.

Triple point: unique $T$ and $P$ where solid, liquid, and gas coexist in equilibrium. For water: 0.006 atm and 0.0075°C.
Critical point ( $T_c$ , $P_c$ ): above this, liquid and gas phases are indistinguishable — it becomes a supercritical fluid. The liquid-gas boundary ends here.
Sublimation curve: solid ⇌ gas equilibrium. CO $_2$ sublimes at 1 atm (triple point is above 1 atm).

For solids, two types matter in NEET:

Property	Crystalline Solid	Amorphous Solid
Melting	Sharp melting point	Softens over a range
Order	Long-range 3D order	Short-range or no order
Anisotropy	Anisotropic	Isotropic
Examples	NaCl, diamond, quartz	Glass, rubber, plastic

NEET trap: Glass is an amorphous solid — it is NOT a supercooled liquid (an older, incorrect term still seen in some textbooks). NCERT now classifies glass as amorphous solid.

Practice Tests

5 Chapter Tests of 25 Questions Each

Each test is original, NEET-aligned, and answer-backed. Use them as sectional revision instead of a single long mock so your weak subtopics become easier to identify quickly.

Test 1: Gas Laws and Kinetic Theory

Gas laws, ideal gas equation, partial pressure, and kinetic theory basics.

Test 2: Liquid State

Vapour pressure, boiling point, surface tension, viscosity, capillarity, and phase change.

Test 3: Real Gases

van der Waals corrections, compressibility factor, diffusion, and liquefaction.

Test 4: Solids and Phase Ideas

Crystalline vs amorphous solids, defects, packing, and state-property comparisons.

Test 5: Mixed NEET Drill

Numerical and conceptual integration across gases, liquids, and solids.

Open Practice Tests

Finished this topic?

Keep the practice loop moving

Move straight from chapter-wise questions into a subject test, then loop back into weaker areas instead of ending the session here.

Try Full NEET Mock