---
title: "Positive Deviations in Thermodynamics II"
description: "Positive deviations are cases where a real solution has higher vapor pressure than Raoult's Law predicts, showing weaker liquid interactions in Thermodynamics II."
canonical: "https://fiveable.me/thermodynamics-ii/key-terms/positive-deviations"
type: "key-term"
subject: "Thermodynamics II"
unit: "Unit 10"
---

# Positive Deviations in Thermodynamics II

## Definition

Positive deviations in Thermodynamics II happen when a real solution's vapor pressure is higher than Raoult's Law predicts. That means molecules escape the liquid more easily than they would in an ideal mixture.

## What It Is

Positive deviations are a non-ideal solution behavior in Thermodynamics II where the measured vapor pressure sits above the Raoult's Law prediction. In plain terms, the liquid mixture lets molecules escape into the vapor phase more easily than an ideal mixture would.

This usually happens when unlike molecules attract each other less strongly than like molecules do. If A-A and B-B attractions are stronger than A-B attractions, each component feels a weaker pull holding it in the liquid. That makes it easier for molecules to leave the liquid surface, so the total vapor pressure rises.

A common way to picture this is to compare a mixture of a polar liquid with a nonpolar one, or two liquids whose molecular sizes and attractions do not match well. The liquid structure is less stable than you would expect from an ideal model, so the mixture behaves as if it were more volatile. The result is not just a small math correction, it changes how the phase equilibrium itself looks.

In phase equilibrium problems, positive deviations matter because they shift the liquid-vapor balance. Since vapor pressure is higher, the vapor phase becomes richer than ideal models predict, and the fugacity calculations need the real behavior instead of the ideal shortcut. That is why this term shows up right alongside VLE and fugacity in Thermodynamics II.

This behavior can also lead to azeotrope formation in some mixtures. A minimum-boiling azeotrope is one common result of strong positive deviation, where the mixture boils at a lower temperature than either pure component. In separation problems, that matters because simple distillation may stop working once the azeotropic composition is reached.

## Why It Matters

Positive deviations are one of the clearest signs that a liquid mixture is not ideal, so they show up whenever you move past simple Raoult's Law calculations. In Thermodynamics II, that means you cannot just plug mole fractions into a clean ideal equation and expect the answer to match a real distillation or flash calculation.

This term matters because it changes three things at once: vapor pressure, vapor composition, and equilibrium temperature. If a mixture has positive deviations, the more volatile component may appear even more volatile than expected, which affects how you interpret phase diagrams and how you size separation equipment.

It also connects directly to fugacity. When you use fugacity-based equilibrium conditions, positive deviation tells you the liquid-phase behavior is pushing the system away from ideality, so the correction is not cosmetic. You are adjusting the math to match how the molecules actually interact.

For engineering problems, that difference can affect whether a mixture can be separated by ordinary distillation or whether an azeotrope gets in the way. For homework and exams, it often shows up as a graph-reading or concept-check question: does the observed vapor pressure lie above or below the Raoult's Law line, and what does that say about molecular interactions?

## Connections

### Raoult's Law

Raoult's Law is the ideal baseline that positive deviations are compared against. If a solution follows Raoult's Law closely, each component's partial pressure is proportional to its mole fraction and pure-component vapor pressure. Positive deviations mean the real mixture sits above that ideal line, so the law underpredicts the vapor pressure.

### Fugacity

Fugacity is the phase-equilibrium quantity you use when ideal pressure comparisons are not enough. Positive deviations change the liquid-phase behavior, which means fugacity-based calculations need a real-solution correction. In problems, this often appears when you move from a simple Raoult's Law estimate to a more accurate equilibrium condition.

### [Non-ideal solutions](/thermodynamics-ii/key-terms/non-ideal-solutions)

Positive deviations are one type of non-ideal solution behavior. They tell you the mixture does not behave like the simple model where interactions are balanced and uniform. In Thermodynamics II, spotting positive deviation is one way to diagnose which direction the non-ideality is going.

### [negative deviations](/thermodynamics-ii/key-terms/negative-deviations)

Negative deviations are the opposite pattern, where the vapor pressure falls below Raoult's Law prediction. Comparing the two helps you think in terms of molecular attractions. Stronger-than-expected A-B attractions tend to give negative deviations, while weaker-than-expected A-B attractions lead to positive deviations.

## On the AP Exam

A quiz or problem set will usually ask you to identify whether a plotted vapor pressure curve is above or below Raoult's Law and explain what that says about intermolecular forces. You may also be asked to predict whether a binary mixture will form a minimum-boiling azeotrope or whether its vapor phase will be richer than the ideal calculation suggests.

In a calculation problem, the key move is to compare the real behavior to the ideal baseline, then decide whether the system needs a non-ideal correction. If the prompt gives you VLE data or a phase diagram, look for vapor pressure values above the Raoult's Law line, then connect that to weaker unlike-molecule attractions and increased volatility. That is the reasoning instructors usually want, not just the phrase itself.

## positive deviations vs negative deviations

These are easy to mix up because both describe non-ideal solution behavior. Positive deviations mean higher vapor pressure than Raoult's Law predicts, while negative deviations mean lower vapor pressure. The shortcut is to ask whether unlike molecules attract each other more weakly or more strongly than the like molecules in the pure liquids.

## Key Takeaways

- Positive deviations mean a real solution has a higher vapor pressure than Raoult's Law predicts.
- They usually happen when unlike molecules attract each other more weakly than like molecules do.
- In Thermodynamics II, this is a non-ideal solution effect that changes VLE and fugacity calculations.
- Positive deviations can push a mixture toward minimum-boiling azeotrope behavior.
- When you see a vapor pressure curve above the ideal line, think stronger volatility and weaker liquid-phase attraction.

## FAQs

### What is positive deviations in Thermodynamics II?

Positive deviations are when a real liquid mixture has a vapor pressure higher than the value predicted by Raoult's Law. In Thermodynamics II, that points to non-ideal solution behavior and weaker attractions between unlike molecules. It matters most in vapor-liquid equilibrium and fugacity problems.

### What causes positive deviations?

They usually happen when A-B interactions are weaker than A-A and B-B interactions. That makes molecules leave the liquid more easily, so the mixture becomes more volatile than the ideal model predicts. Mixed polar and nonpolar liquids are a common place to see this pattern.

### How are positive deviations different from negative deviations?

Positive deviations sit above the Raoult's Law vapor pressure line, while negative deviations sit below it. The difference comes from intermolecular forces: weaker unlike-molecule attraction gives positive deviation, and stronger unlike-molecule attraction gives negative deviation. That comparison is a common exam and homework check.

### Why do positive deviations matter in VLE problems?

Because they change the pressure and composition at equilibrium. If you assume ideal behavior when the solution has positive deviation, you can miscalculate vapor composition, boiling point, and separation behavior. That can also matter when a mixture forms an azeotrope.

## Related Study Guides

- [10.2 Vapor-Liquid Equilibrium and Fugacity](/thermodynamics-ii/unit-10/vapor-liquid-equilibrium-fugacity/study-guide/xEsrblcJepwd3Tnn)

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