Raoult's Law & Vapor Pressure Why vapor pressure and Raoult’s law matter Open a perfume bottle and the fragrance spreads — that is molecules escaping from liquid to vapor. Their tendency to escape is measured by vapor pressure. When we mix liquids, vapor pressure changes in a predictable way (Raoult’s law) if the mixture is ideal; if not, we see deviations and azeotropes that limit how far distillation can separate them. These ideas power petroleum refining and alcohol concentration — high-yield for NEET. Pure solvent has more escaping molecules (higher vapor pressure) than a solution containing a non-volatile solute — vapor pressure is lowered in solution. Vapor pressure of a pure liquid — the start point In a closed container at a fixed temperature, a liquid evaporates until the rate of evaporation equals the rate of condensation. The pressure exerted by the vapor at this dynamic equilibrium is the vapor pressure of the pure liquid. It rises steeply with temperature because more molecules have enough energy to escape. Every volatile liquid has its own vapor pressure curve vs temperature (more volatile = higher vapor pressure at the same T). Equilibrium pressure of a liquid’s vapor above its surface at a given temperature in a closed system. Vapor pressure remember For a given pure liquid, vapor pressure depends strongly on temperature and the liquid’s identity — not on the amount of liquid (as long as some liquid is present). Raoult’s Law for two volatile liquids (ideal solutions) Consider an ideal binary solution of two volatile liquids A and B. Let x A and x B be their mole fractions in the liquid ( x A + x B = 1). Each component contributes a partial pressure in the vapor according to its mole fraction and its own pure-liquid vapor pressure at that temperature. Add the two partial pressures to get total pressure. Raoult’s Law (component A) Partial pressure of A equals its mole fraction times its pure vapor pressure at the same temperature. Add the two partial pressures (Dalton’s law) to get the total vapor pressure. Total pressure of an ideal binary solution For an ideal solution, P A and P B vary linearly with x A and x B , and P total is a straight line between P B 0 (at x A = 0) and P A 0 (at x A = 1). Write x A and x B (remember x A + x B = 1). Compute P A = P A 0 x A and P B = P B 0 x B . Add to get P total = P A + P B . More volatile component (higher P 0 ) will dominate the vapor composition. How to compute P total quickly (NEET-style) What is an ideal solution? An ideal solution obeys Raoult’s law over the entire composition range. On mixing, the average A–B attraction is the same as A–A and B–B, so nothing special happens energetically or volumetrically on mixing. Common near-ideal pairs are chemically similar liquids, like benzene + toluene or n-hexane + n-heptane. No heat is absorbed/evolved and no volume change occurs on mixing for an ideal solution. Thermodynamic signature of ideality Ideal A–B ≈ A–A ≈ B–B (similar strength) None (obeys over entire range) Benzene + toluene; n-hexane + n-heptane; chlorobenzene + bromobenzene Non-ideal (positive deviation) A–B weaker than A–A or B–B (more escaping tendency) > 0 (endothermic mixing) ≥ 0 often Observed P > Raoult line Ethanol (CCO) + acetone (CC(=O)C); ethanol (CCO) + water (O); n-hexane (CCCCCC) + ethanol (CCO) Non-ideal (negative deviation) A–B stronger than A–A or B–B (less escaping tendency) < 0 (exothermic mixing) ≤ 0 often Observed P < Raoult line Hydrogen chloride ( [H]Cl) + water (O); chloroform (ClC(Cl)Cl) + acetone (CC(=O)C); nitric acid (O=[N+](O)[O-]) + water (O) Feature Type Intermolecular interactions (A–B vs A–A/B–B) Sign of ΔH mix ΔV mix Deviation from Raoult Typical examples Ideal vs Non-ideal solutions — quick comparison Non-ideal solutions: positive vs negative deviation If unlike molecules (A–B) attract each other less than like molecules, they escape more easily, raising vapor pressure above Raoult’s line — positive deviation. If unlike molecules attract more strongly (e.g., new hydrogen bonding or specific dipole–dipole pairing), they escape less, lowering vapor pressure — negative deviation. gpt-image-2 Overlay graph: P vs x A at fixed T. Base: straight Raoult line between P B 0 ( x A =0) and P A 0 ( x A =1). Curve 1 (positive deviation) bows upward; Curve 2 (negative deviation) bows downward. Label A, B, P B 0 , P A 0 , and 'positive'/'negative deviation'. Clean 2D vector style, red arrows for deviations. 2026-05-26T17:04:29.033Z Positive and negative deviations from Raoult’s law: observed P total curve lies above (positive) or below (negative) the straight Raoult line. Binary system Type Molecular reasoning (short) System Positive vs Negative deviation — examples and reasoning Ethanol (CCO) + water (O) Positive H-bond networks in pure liquids get disrupted on mixing; A–B on average weaker than original networks. Ethanol (CCO) + acetone (CC(=O)C) Positive Disruption of ethanol–ethanol H-bonds; limited specific interactions with acetone. n-Hexane (CCCCCC) + ethanol (CCO) Positive Nonpolar hexane can’t H-bond with ethanol; weak A–B dispersive contacts. Hydrogen chloride ([H]Cl) + water (O) Negative Strong ion–dipole/H-bonding after dissolution; A–B interactions stronger. Chloroform (ClC(Cl)Cl) + acetone (CC(=O)C) Negative H-bond-like C–H···O interactions plus dipole pairing. Nitric acid (O=[N+](O)[O-]) + water (O) Negative Strong hydration and specific interactions; escaping tendency reduced. Raoult’s law holds for ideal solutions (and often near-ideally for similar liquids). Many real systems show positive or negative deviations because A–B interactions differ from A–A and B–B. All solutions obey Raoult’s law across the full composition range. Raoult’s law for a non-volatile solute — vapor-pressure lowering If B is a non-volatile solute (does not contribute to vapor), only the solvent A evaporates. In a dilute solution, the solvent’s partial pressure equals its mole fraction times its pure vapor pressure. The drop in vapor pressure (Δp) is proportional to the solute mole fraction — the bridge to colligative properties. Solute mole fraction equals the relative lowering of the solvent’s vapor pressure. Relative lowering of vapor pressure (dilute solution) Solving steps (non-volatile solute) Find x B = n B / ( n A + n B ) from moles (or molar mass + mass). Compute P A = P A 0 (1 − x B ). Relative lowering ( P A 0 − P A )/ P A 0 gives x B directly. Azeotropes — constant-boiling mixtures that cap distillation At a particular composition, some non-ideal mixtures boil at a constant temperature and the liquid and vapor have the same composition — an azeotrope. A positive-deviation system forms a minimum-boiling azeotrope (boils at a lower T than either pure liquid). A negative-deviation system forms a maximum-boiling azeotrope (boils at a higher T than either pure liquid). Types of azeotropes and canonical examples Type Deviation type Characteristic Example (approximate composition; b.p.) Azeotrope Minimum-boiling Positive deviation Boils at a lower temperature than either pure component Ethanol (CCO) + water (O): ≈95% ethanol; b.p. ≈78.15 °C (1 atm) Maximum-boiling Negative deviation Boils at a higher temperature than either pure component Hydrogen chloride ([H]Cl) + water (O): 20.2% HCl; b.p. 108.5 °C (1 atm) gpt-image-2 P vs x A diagram with Raoult baseline and an upward-bowing curve (positive deviation) touching a maximum at x az . Mark x A (liquid) and y A (vapor) tie-line intersecting at the extremum where x = y. Clean vector plot; labels: P, x A , azeotrope. 2026-05-26T17:04:29.069Z P–x diagram at fixed T showing an azeotrope where the observed P total curve touches an extremum; at this point liquid and vapor compositions are equal. T–x diagram for a minimum-boiling azeotrope: liquidus and vaporus curves meet at a minimum; at that composition, vapor and liquid are the same, so composition cannot change by simple distillation. 2026-05-26T17:04:29.023Z T vs x A (0 to 1). Two curves: lower vapor line and upper liquid line meeting at a sharp minimum at x az . Arrows indicate simple distillation path failing to enrich beyond x az . Labels: T, x A , 'liquid', 'vapor', 'minimum-boiling azeotrope'. Clean 2D style. gpt-image-2 Fractional distillation separates components based on vapor pressure (boiling point) differences — but stops at an azeotrope because vapor and liquid then have the same composition. Fractional distillation in petroleum refining and in alcohol production uses vapor pressure differences (Raoult + volatility). Components with higher vapor pressure (lower b.p.) enrich in vapor and are collected first — until an azeotrope is reached, where composition locks. remember neet-alert Ethanol–water (≈95% ethanol) forms a minimum-boiling azeotrope near 78.15 °C. You cannot get absolute ethanol by simple fractional distillation. It is obtained industrially by azeotropic distillation (e.g., with benzene/toluene historically) or by drying (e.g., quicklime CaO) or by molecular sieves. Henry’s law vs Raoult’s law — which to apply? Raoult’s law describes a solvent (or components of a liquid mixture) when it is the major component and behaves ideally. Henry’s law describes a sparingly soluble gas or volatile solute at low concentration, where its partial pressure is proportional to its mole fraction via Henry’s constant. Choose based on which species is the solute at low x and which is the solvent matrix. Henry’s law applies to the solute at low mole fraction; Raoult’s law applies to the solvent in dilute solutions (and to both components if the solution is ideal). Do not swap them. Use Henry’s law constants to compute solvent partial pressures, or apply Raoult’s law directly to a volatile solute just because the solution is dilute. Crude oil is separated by fractional distillation across trays as more volatile (higher vapor pressure) fractions rise and condense higher up. 95% ethanol–water azeotrope limits simple distillation; absolute alcohol is produced via azeotropic distillation (historically benzene/toluene), drying with CaO (quicklime), or modern molecular sieves. Sodium metal dries alcohols in labs but is hazardous. Industrial notes (NEET-friendly facts) For an ideal solution, the partial vapor pressure of each component equals its mole fraction times its pure vapor pressure at that temperature. Raoult’s law Solution obeying Raoult’s law over the full composition range with Δ H mix = 0 and Δ V mix = 0. Ideal solution Shows positive or negative deviation from Raoult’s law due to A–B interactions differing from A–A and B–B. Non-ideal solution Observed vapor pressure is higher than Raoult’s prediction; A–B attractions are weaker; Δ H mix > 0. Positive deviation Observed vapor pressure is lower than Raoult’s prediction; A–B attractions are stronger; Δ H mix < 0. Negative deviation Constant-boiling mixture where liquid and vapor compositions are equal at a specific composition and pressure. Azeotrope Separation method exploiting volatility differences; repeated vaporization–condensation in a column enriches more volatile components. Fractional distillation ( P 0 − P)/ P 0 for the solvent; equals solute mole fraction for dilute solutions with non-volatile solute. Relative lowering of vapor pressure Equilibrium pressure of vapor above a liquid at a given temperature. Vapor pressure Core terms at a glance