Chem notesPhysicalGases & Liquids
Independent topic page
Identify ideal, mixture, kinetic-theory or real-gas mode in 12 seconds; then one state equation plus a physical limit check controls the stem.
01. PV=nRT is a model whose strongest domain is low pressure and high temperature; van der Waals corrections explain finite molecular size (b) and attractions (a).
02. Dalton works with gas-phase partial pressures. Vapour collected over liquid requires subtracting that liquid's equilibrium vapour pressure.
03. Speed order is v_mp < v_avg < v_rms for one gas at one temperature; every characteristic speed scales as √(T/M). Diffusion/effusion follows Graham’s square-root mass law.
04. Official syllabus also tests intermolecular interactions: type, distance dependence, and effect on properties. Stronger cohesion generally lowers vapour pressure and raises boiling point, viscosity and surface tension.
05. Pick one model and stay inside it: ideal P–V–T, Dalton partial pressures, kinetic speeds, or van der Waals — hybrid formulas without matching assumptions lose marks.
06. Gas collected over water is never total pressure of the dry gas; subtract the liquid’s vapour pressure before using PV=nRT.
07. Liquid-property trends are intermolecular-force stories: stronger cohesion lowers vapour pressure and raises boiling point, viscosity and surface tension.
Official · 2026
Official syllabus points for Gaseous & Liquid States only — not the full Physical portion map. Portion hub: Physical · Physical analysis.
No topic-specific exclusions flagged — finish the unit points below for Gases & Liquids.
Source: https://jeeadv.ac.in/documents/jee-advanced-2026-syllabus.pdf
Exam focusvan der Waals, rms velocities, vapour pressure/viscosity ideas.
Syllabus points
Do / check
2020–2026 paper weight
Topic match inside parent portion Physical (49.8% of Chemistry tags). Stats below are portion backdrop; matched subtopics show where Gaseous & Liquid States sits.
Portion-wide type/difficulty mix lives on the Physical analysis — not repeated here so this page stays on Gases & Liquids.
01. Dominant subtopics: Mixed (36), Mole/Gaseous (24), Electrochemistry (14), Thermodynamics (11).
Cross-topic stems
Advanced rarely asks a chapter in isolation. These are tagged multi-idea Chemistry stems that touch Gaseous & Liquid States — practise the knowledge bridge, not only the single-chapter drill.
Step 1: from the linked ideal-solution data, compute the vapour-to-liquid molar volume ratio for pure B. Step 2: find liquid mole fractions (e.g. from molality). Step 3: apply Raoult to get y_B in the vapour (numerical ~0.16 style).
Two numericals share solution data; unit slips (molality vs mole fraction) are common.
Convert every molality stem to x_i before touching vapour pressures.
Step 1: parse which Physical sub-ideas are fused (mole + thermo, gas + equilibrium, etc.). Step 2: write governing equations for each piece. Step 3: combine numerically without dropping stoichiometric factors.
‘Mixed’ Physical tags mean the stem will not stay in one textbook chapter.
Practise deliberately cross-chapter Physical sets (thermo×gases×equilibrium).
Why methods matter
Gas and liquid questions are bookkeeping problems once partial pressure, molar volume and phase composition are separated.
Dalton applies to what remains in the gas phase, not blindly to initial labels.
Do thisConvert each gas to moles, remove reacted/condensed species, then calculate p_i.
Dalton applies to what remains in the gas phase, not blindly to initial labels.
Wet gas: P_dry=P_total−P_H₂O before using PV=nRT for dry-gas moles.
Cancels units and exposes √T/M scaling directly.
Do thisDivide two gas-state or speed equations before substituting R.
Cancels units and exposes √T/M scaling directly.
v_rms(A)/v_rms(B)=√(T_A M_B/(T_B M_A)).
Attraction lowers observed P (Z<1); hard-core repulsion dominates high P (Z>1).
Do thisAt moderate P ask whether attraction or excluded volume dominates.
Attraction lowers observed P (Z<1); hard-core repulsion dominates high P (Z>1).
A negative deviation cannot be explained by b alone; it signals net attractive correction.
One causal map replaces disconnected memorisation.
Do thisTranslate liquid trend to intermolecular forces before ranking.
One causal map replaces disconnected memorisation.
Stronger cohesion generally lowers vapour pressure and raises boiling point, viscosity and surface tension.
Processed examples
Read the impact line first. Then notice which family/model was chosen. The solution is written the way your rough sheet should look in the hall.
2 mol N₂ and 3 mol H₂ occupy a vessel at total pressure 10 bar. Find p(H₂).
This stem rewards early recognition of the gas mixture and partial pressure model.
Model chosenApply Dalton’s law.
Answer6 bar
Takeaway: Partial pressure follows mole fraction.
Find r(H₂)/r(O₂) when H₂ is at 300 K and O₂ at 1200 K.
This stem rewards early recognition of the kinetic theory and effusion model.
Model chosenUse r∝√(T/M).
Answer2
Takeaway: Temperature and molar mass enter under one square root.
One mole of a van der Waals gas is at its critical point. Express Zc using the van der Waals critical constants.
This Advanced-level variation couples hidden constraints with the real-gas equation model.
Model chosenInsert Pc, Vc and Tc into Z=PV/RT.
Answer3/8
Takeaway: Critical constants collapse dimensionless expressions.
Reusable models
Advanced stems cluster. Name the family in 10–15s, write the fast model, then run the same path. Do not invent a new plan each time.
Looks likeSeveral gases mixed, collected over water or selectively removed
Fast modelUse mole bookkeeping with Dalton’s law.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likevan der Waals constants, compression factor or critical data
Fast modelInsert molar volume consistently before interpreting deviation.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeRMS speed, temperature change or porous-wall escape
Fast modelCompare squared speeds or use Graham’s square-root ratio.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeVapour pressure, boiling point or nonvolatile solute
Fast modelWork with mole fraction and relative lowering.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Hall timing
Underline data, units, phase/standard-state labels and the exact quantity asked. Reject any formula whose assumptions are already violated.
Write the governing relation symbolically before substitution; define one basis, reference state, or extent variable if the stem mixes quantities.
Keep ratios and logarithms exact as long as possible. Carry signs, powers of ten and units on the rough sheet instead of repairing them later.
Check dimensions, limiting cases and chemical sense. For multi-correct, test every option independently rather than forcing one common conclusion.
After the model is locked
PV=nRT; ρ=PM/RT
Dilute gas and molar-mass/density stems
P=Σp_i; p_i=x_iP=n_iRT/V
Non-reacting ideal-gas mixtures
v_mp=√(2RT/M); v_avg=√(8RT/πM); v_rms=√(3RT/M)
Speed comparison; M in kg mol⁻¹ for SI speed
Ē_trans=(3/2)RT per mole; (3/2)k_BT per molecule
Ideal-gas molecular energy at T
r₁/r₂=√(M₂/M₁)=√(ρ₂/ρ₁) at same T,P
Effusion/diffusion comparison
(P+a n²/V²)(V−nb)=nRT
Attraction and excluded-volume corrections
Z=PV/(nRT); Z→1 as P→0
Measure and direction of non-ideality
ln(P₂/P₁)=−ΔH_vap/R(1/T₂−1/T₁)
Approximate vapour-pressure change over a modest range
Make it automatic