NEET Chemistry · States of Matter
Ideal Gas Laws
Change P, V, or T — watch the molecules respond. Boyle's, Charles's, and Gay-Lussac's laws in one interactive lab.
Section 1 of 5 · Foundations
What you'll master here
Three separate labs, each isolating one gas law. In each you control two variables and watch the third respond — with live molecule animations, formula strips, and NEET trap callouts.
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Kinetic pictureWatch molecules bounce — their speed reflects T, their density reflects P/V. The simulation responds to every slider move.
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Graphical intuitionEach lab shows the live curve (hyperbola, linear) so you know exactly which graph NEET will show.
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Formula walkthroughEvery change updates a live calculation strip — the exact working NEET expects you to reproduce.
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6 NEET traps flaggedKelvin vs Celsius, STP vs NTP, and more — the errors that cost marks every year.
Pressure P (atm / Pa)
Volume V (L / m³)
Temperature T (K)
Moles n (mol)
Ideal Gas Equation
PV = nRT
The master equation. R = 8.314 J mol⁻¹ K⁻¹ (or 0.0821 L·atm mol⁻¹ K⁻¹). T must always be in Kelvin. All three gas laws are special cases of this equation.
Boyle's Law (T, n constant)
P₁V₁ = P₂V₂
At constant temperature, pressure and volume are inversely proportional. P ∝ 1/V. The P-V graph is a rectangular hyperbola. PV = constant = nRT.
Charles's Law (P, n constant)
V₁/T₁ = V₂/T₂
At constant pressure, volume is directly proportional to absolute temperature. V ∝ T (in Kelvin!). The V-T graph is a straight line through origin (in K).
Gay-Lussac's Law (V, n constant)
P₁/T₁ = P₂/T₂
At constant volume, pressure is directly proportional to absolute temperature. P ∝ T (in Kelvin). The P-T graph is a straight line through the origin.
Characteristic Graph Shapes — Memorise for NEET
Boyle's Law · P vs V
Charles's Law · V vs T
Gay-Lussac · P vs T
Section 1 of 5
Section 2 of 5 · Boyle's Law
Boyle's Law — P × V = constant
Temperature T is held constant (isothermal process). Change pressure or volume — the other adjusts so their product stays the same. The molecules get squeezed closer or spread out.
1
Set initial stateUse P₁ and V₁ sliders to set the starting conditions. The product P₁V₁ is your constant.
2
Change one variableDrag P₂ or V₂. Watch the piston move and the molecule density change. The other quantity auto-calculates.
3
Verify P₁V₁ = P₂V₂The formula strip shows the full calculation. The constant k = nRT is highlighted.
Boyle's Law Lab — T constant 🔒 T fixed
Piston Compression
Drag P₂ slider — watch volume respond
INITIAL STATE
Pressure P₁
1.0 atm
Volume V₁
4.0 L
CHANGE PRESSURE → V₂ CALCULATED
New Pressure P₂
2.0 atm
P₁V₁ const
—
L·atm
V₂ result
—
Litres
Ratio P₂/P₁
—
× change
V change
—
× factor
—
Boyle's Law — key concepts
Inverse proportionality: P ∝ 1/V at constant T. Double the pressure → half the volume. Triple the pressure → one-third the volume.
Graph shape: P vs V gives a rectangular hyperbola. PV vs V gives a horizontal line. P vs 1/V gives a straight line through origin.
Molecular picture: same number of molecules, smaller space → more frequent wall collisions → higher pressure. Temperature (kinetic energy) unchanged.
The constant: PV = nRT = k. Different temperatures give different hyperbolas on the P-V diagram — each is called an isotherm.
NEET TIPBoyle's Law applies only if T and n are constant. The P-V graph is a hyperbola — NEET often asks you to identify this. A common wrong answer is a straight line. Also: if P doubles, V halves — not V reduces by 2.
Section 2 of 5
Section 3 of 5 · Charles's Law
Charles's Law — V / T = constant
Pressure P is held constant (isobaric process). As temperature rises, molecules move faster and push the container walls out — volume increases. Temperature must be in Kelvin.
1
Set T₁ and V₁Establish the initial state at constant pressure. The ratio V₁/T₁ is your constant.
2
Change temperature T₂Drag the T₂ slider. Watch the balloon expand or contract, and molecule speeds change (faster = hotter).
3
Notice the linear relationshipV₂ = V₁ × T₂/T₁. The V-T graph is always a straight line through the origin when T is in Kelvin.
Charles's Law Lab — P constant 🔒 P fixed
Flexible Container (Balloon)
Drag T₂ slider — watch volume respond
INITIAL STATE
Volume V₁
4.0 L
Temperature T₁
300 K
CHANGE TEMPERATURE → V₂ CALCULATED
New Temp T₂
450 K
V₁/T₁ const
—
L/K
V₂ result
—
Litres
T₂/T₁ ratio
—
× change
V₂ in °C
—
°C equiv.
—
Charles's Law — key concepts
Direct proportionality: V ∝ T (Kelvin only!). Double the absolute temperature → double the volume. Halve T → halve V.
Graph shape: V vs T (in Kelvin) gives a straight line through origin. V vs °C gives a straight line that extrapolates to −273.15 °C (absolute zero) at V = 0.
Molecular picture: higher T → faster molecules → stronger collisions with piston → piston pushed out → volume increases until pressure re-equalises.
Absolute zero: At 0 K (−273.15 °C), volume extrapolates to zero — the basis of the Kelvin scale. Charles's law doesn't hold near 0 K in practice.
NEET TIPAlways convert °C to Kelvin before applying Charles's Law. T(K) = T(°C) + 273. NEET frequently gives temperatures in Celsius to trap you. V ∝ T holds only for Kelvin — V ∝ t(°C) is WRONG.
Section 3 of 5
Section 4 of 5 · Gay-Lussac's Law
Gay-Lussac's Law — P / T = constant
Volume V is held constant (isochoric process). As temperature rises, molecules hit the rigid walls faster and harder — pressure rises. The container cannot expand, so all energy goes into pressure.
1
Set rigid container stateV is fixed (rigid flask). Set P₁ and T₁ to establish the initial ratio P₁/T₁ = constant.
2
Heat or cool itDrag T₂. Watch molecule speeds change and the pressure gauge needle move. Volume stays the same.
3
Real-world linkThis is why sealed tyre pressure rises in summer and drops in winter — and why pressure cookers work.
Gay-Lussac's Law Lab — V constant 🔒 V fixed
Rigid Vessel (Fixed Volume)
Drag T₂ slider — watch pressure respond
INITIAL STATE
Pressure P₁
1.0 atm
Temperature T₁
300 K
CHANGE TEMPERATURE → P₂ CALCULATED
New Temp T₂
450 K
P₁/T₁ const
—
atm/K
P₂ result
—
atm
T₂/T₁ ratio
—
× change
ΔP change
—
atm
—
Gay-Lussac's Law — key concepts
Direct proportionality: P ∝ T (Kelvin only). Double the absolute temperature → double the pressure. The constant P/T = nR/V.
Graph shape: P vs T (Kelvin) gives a straight line through origin. Slope = nR/V. Different volumes give different slopes — larger V means smaller slope.
Molecular picture: higher T → molecules move faster (KE ∝ T) → hit walls harder and more often → pressure rises. Volume cannot change (rigid container).
Real world: Tyre pressure rises in summer (T↑ → P↑), pressure cookers work by raising T above 100°C at P > 1 atm, gas cylinders must not be heated excessively.
NEET TIPGay-Lussac's is most confused with Charles's. Remember: Gay-Lussac = rigid container (V fixed), P changes. Charles = flexible container (P fixed), V changes. Both involve T. Both are linear — but with different y-axes.
Section 4 of 5
Section 5 of 5 · Revision Sheet
Quick Reference — All Gas Law Formulae
Every formula, constant, condition, and NEET trap for Ideal Gas Laws in one place.
All Gas Law Formulae
| Law | Condition | Formula | Graph | Constant |
|---|---|---|---|---|
| Boyle's | T, n fixed | P₁V₁ = P₂V₂ | P-V: hyperbola · PV-V: horizontal · P-1/V: straight | PV = nRT = k |
| Charles's | P, n fixed | V₁/T₁ = V₂/T₂ | V-T(K): straight through origin · slope = nR/P | V/T = nR/P = k |
| Gay-Lussac's | V, n fixed | P₁/T₁ = P₂/T₂ | P-T(K): straight through origin · slope = nR/V | P/T = nR/V = k |
| Avogadro's | P, T fixed | V ∝ n | V-n: straight through origin | V/n = RT/P = k |
| Ideal Gas | all vary | PV = nRT | — | R = 8.314 J mol⁻¹ K⁻¹ |
| Combined Gas | n fixed | P₁V₁/T₁ = P₂V₂/T₂ | — | PV/T = nR = k |
Key Constants & Standard Conditions
| Quantity | Value | Note |
|---|---|---|
| R (SI) | 8.314 J mol⁻¹ K⁻¹ | Use when P in Pa, V in m³ |
| R (litre-atm) | 0.0821 L·atm mol⁻¹ K⁻¹ | Use when P in atm, V in litres |
| STP (IUPAC 1982) | 0 °C (273.15 K), 1 bar | Molar volume = 22.7 L |
| STP (old / NEET) | 0 °C (273.15 K), 1 atm | Molar volume = 22.4 L ← use this in NEET |
| NTP | 25 °C (298.15 K), 1 atm | Molar volume ≈ 24.5 L |
| Absolute zero | 0 K = −273.15 °C | T(K) = T(°C) + 273 |
| 1 atm | 101325 Pa = 760 mmHg | = 760 torr = 1.01325 bar |
| Avogadro's N_A | 6.022 × 10²³ mol⁻¹ | Molecules per mole |
NEET Traps — Memorise These
TRAP 1Always use Kelvin, never Celsius. T(K) = T(°C) + 273. Using °C in V/T = k or P/T = k gives the wrong answer every time. Charles's and Gay-Lussac's laws only work in Kelvin.
TRAP 2STP molar volume = 22.4 L, not 22.7 L. NEET still uses old STP (0 °C, 1 atm). The IUPAC 1982 revision to 1 bar gives 22.7 L — but NEET questions use 22.4 L.
TRAP 3Boyle's graph is a hyperbola, not a straight line. P vs V = curved (rectangular hyperbola). P vs 1/V = straight line through origin. PV vs V = horizontal line. Know all three.
TRAP 4Real gases deviate from ideal behaviour. At high pressure and low temperature, real gases deviate most (intermolecular forces become significant). Ideal gas: no intermolecular forces, zero molecular volume.
TRAP 5Compressibility factor Z: Z = PV/nRT. For ideal gas Z = 1. Z > 1: repulsive forces dominate. Z < 1: attractive forces dominate. Z = 1 does NOT mean gas is ideal at all pressures.
TRAP 6Effusion vs Diffusion: Graham's Law — rate ∝ 1/√M. Lighter gases effuse faster. Rate₁/Rate₂ = √(M₂/M₁). Effusion = escape through a pinhole. Diffusion = spreading through space.
Section 5 of 5 · Complete!