---
title: "Engineers test a real gas for use in a high-pressure storage cylinder. A fixed amount of the gas is maintained at a constant temperature of \\(320\\ \\text{K}\\), and no phase change occurs over the pressure range investigated. The compressibility factor, defined as \\(Z=\\dfrac{PV}{nRT}\\), is plotted as the volume of the gas is progressively decreased.  Which statement best explains why \\(Z\\) first decreases below the ideal-gas value and then increases above the ideal-gas value?"
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url: "https://nerd-notes.com/ubq/120031/"
date_modified: "2026-08-21T08:40:40+00:00"
---

# Engineers test a real gas for use in a high-pressure storage cylinder. A fixed amount of the gas is maintained at a constant temperature of \(320\ \text{K}\), and no phase change occurs over the pressure range investigated. The compressibility factor, defined as \(Z=\dfrac{PV}{nRT}\), is plotted as the volume of the gas is progressively decreased.

Which statement best explains why \(Z\) first decreases below the ideal-gas value and then increases above the ideal-gas value?

Engineers test a real gas for use in a high-pressure storage cylinder. A fixed amount of the gas is maintained at a constant temperature of \(320\ \text{K}\), and no phase change occurs over the pressure range investigated. The compressibility factor, defined as \(Z=\dfrac{PV}{nRT}\), is plotted as the volume of the gas is progressively decreased.

Which statement best explains why \(Z\) first decreases below the ideal-gas value and then increases above the ideal-gas value?

![Draw a grayscale Cartesian graph with a horizontal axis labeled \(P\) increasing from left to right and a vertical axis labeled \(Z=\dfrac{PV}{nRT}\) increasing upward. Use no gridlines. Add a thin dashed horizontal reference line at \(Z=1.00\). Draw one thick solid curve for the measured gas. The curve begins essentially on the reference line in the low-pressure region, falls smoothly to a single minimum at \(Z=0.80\) in the intermediate-pressure region, then rises steeply, crosses the reference line in the high-pressure region, and ends at \(Z=1.20\) in the extreme-pressure region. Place the qualitative region labels low pressure, intermediate pressure, high pressure, and extreme pressure once beneath the corresponding portions of the horizontal axis. Use uniform high-contrast strokes and text on an unshaded background. No other curves, points, labels, text, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787301640-OHkriZ.jpg)

- **A.** At intermediate pressures, \(Z<1\) because the finite sizes of the gas particles reduce the volume available for translational motion; at extreme pressures, \(Z>1\) because intermolecular attractions become increasingly important.
- **B.** At intermediate pressures, \(Z<1\) because intermolecular attractions reduce the pressure exerted on the container walls; at extreme pressures, \(Z>1\) because compression increases the average kinetic energy of the particles even though the temperature is constant.
- **C.** At intermediate pressures, \(Z<1\) because collisions between gas particles permanently remove translational kinetic energy; at extreme pressures, \(Z>1\) because excluded-volume and short-range repulsive effects become dominant.
- **D.** At intermediate pressures, \(Z<1\) because intermolecular attractions reduce the pressure exerted on the container walls; at extreme pressures, \(Z>1\) because excluded-volume and short-range repulsive effects become dominant.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120031/*