--- title: "Real Gases, Fugacity and Activity" description: "Every thermodynamic expression so far assumed ideal behaviour. Fugacity and activity are the corrected quantities that keep the same equations working for real gases and real solutions." canonical: https://lightmysky.com/learn/science/real-gases-fugacity-and-activity-mt_HzRqV-KXfN source: https://lightmysky.com/learn/science/real-gases-fugacity-and-activity-mt_HzRqV-KXfN.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Real Gases, Fugacity and Activity Every thermodynamic expression so far assumed ideal behaviour. Fugacity and activity are the corrected quantities that keep the same equations working for real gases and real solutions. Subject: Science · Area: Chemistry · Ages 20 to 21 Page: https://lightmysky.com/learn/science/real-gases-fugacity-and-activity-mt_HzRqV-KXfN ## Ready when they can - Reads a compression factor plot and says where attraction dominates and where repulsion does - Explains what each term of the van der Waals equation is correcting - Defines fugacity as an effective pressure and activity as an effective concentration, and states the standard state each refers to - Says which measurements go wrong if activities are replaced by concentrations in a concentrated solution - Reads a Z chart plotted against reduced pressure and reduced temperature and says why one chart serves many gases ## Lesson: Fixing the gas laws for real gases Gas tanks and pipes push gases far from ideal behaviour, so chemists track the gap with the compression factor Z through Z = PV over nRT. Where attraction dominates, molecules pull closer and Z dips below 1. Where size dominates, Z climbs above 1. **Example.** Take 1.00 mol at 400 K in a 0.0300 cubic metre tank at 100000 Pa. First nRT is 1.00 times 8.314 times 400, which is 3325.6 J. Then PV is 100000 times 0.0300, which is 3000 J. Divide 3000 by 3325.6 and Z is about 0.90, so attraction is winning at this state. The van der Waals equation corrects both faults: one term for attraction pull, one for occupied space. Fugacity is the effective pressure that keeps the ideal equations working for real gases, and activity is the effective concentration for solutions, each measured against its standard state. You met the chemical potential sliding with the log of a fraction in an ideal mixture; in real gases and solutions it slides with fugacity or activity instead. In concentrated solutions, swapping activities for plain concentrations corrupts equilibrium constants and cell potentials. At very high pressure almost every gas tops 1 as packed size resists squeeze. Near the critical point both faults act at once, so Z swings furthest from 1 there. Reduced pressure and temperature divide by critical values, so one Z chart fits many gases. **Recap.** Read Z to see which fault wins, then use fugacity and activity to keep the old equations honest. ## Practice 18 questions on this page, each with its working shown. ## Needs first - [Gas Volumes and the Ideal Gas Equation](https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD) - [Chemical Potential and Gibbs Energy That Depends on Composition](https://lightmysky.com/learn/science/chemical-potential-and-gibbs-energy-that-depends-on-composition-mt_Tdl6K-gFO3) ## Opens up - [Phase Diagrams, the Clapeyron Equation and the Phase Rule](https://lightmysky.com/learn/science/phase-diagrams-the-clapeyron-equation-and-the-phase-rule-mt_U-zj6r4WFM)