Gas Solubility and Pressure

Introduction

Sealed sparkling water holds more dissolved carbon dioxide than the same drink after it has been open for a while. The pressure of CO₂ above the liquid changes when the container opens. Gas solubility therefore depends on the pressure of the particular gas in contact with the liquid, as well as temperature and chemical interactions.

Core explanation

At equilibrium, gas molecules enter a liquid from the gas phase while others leave the liquid. Raising the partial pressure of a gas increases the rate of molecular encounters with the liquid surface and often raises the equilibrium dissolved concentration. For many dilute gas–liquid systems at fixed temperature, dissolved concentration is approximately proportional to the gas's partial pressure. This is the form of Henry's law used in the next page. It is a conditional relation, not an unlimited prediction at every pressure.

The relevant pressure is the gas's own partial pressure. If a gas mixture at total pressure 1.0 atm contains 20% oxygen by mole fraction and behaves approximately ideally, oxygen partial pressure is about 0.20 atm. Doubling total pressure by adding nitrogen while leaving oxygen partial pressure unchanged does not double the equilibrium oxygen concentration. Conversely, compressing the same mixture so every gas partial pressure doubles can increase dissolved oxygen under suitable conditions.

Opening a carbonated drink lowers the CO₂ partial pressure in contact with the liquid. The concentration initially present was established under the higher sealed pressure and is now above the new equilibrium value. CO₂ molecules leave, often forming bubbles at nucleation sites, until a new state is approached. The hissing is not evidence that all dissolved CO₂ was chemically created upon opening; pressure changed the balance between dissolved and gaseous material.

Temperature must be held fixed when applying a simple pressure comparison. For many gases, warmer water holds less dissolved gas at a given partial pressure. A sealed container warming may also change gas pressure, creating two simultaneous effects. The gas identity, solvent and chemical reactions matter too. Carbon dioxide participates in acid-base equilibria in water, so a precise total-carbon calculation may be more complicated than treating every dissolved species as unchanged CO₂ molecules.

Pressure has a much smaller effect on the solubility of ordinary solid solutes in liquid water in many school examples. Do not apply gas-pressure reasoning to a sugar crystal. The reason is not simply that solids are “heavy”; it is that the relevant equilibrium and compressibility differ. Gas solubility is especially sensitive to the gas chemical potential controlled by partial pressure.

Step-by-step reasoning

1. Identify the named gas and its partial pressure above the liquid. 2. Hold temperature, solvent and gas identity fixed for a pressure comparison. 3. Use the supplied proportional relation if the problem permits it. 4. Predict gas entering or leaving when the actual concentration differs from the new equilibrium value. 5. Check whether chemical reaction or another gas mixture component complicates the simple model.

Visual explanation

Draw a sealed bottle with many CO₂ molecules in the headspace and dissolved in the liquid. After opening, draw fewer CO₂ molecules above and bubbles leaving the liquid. Label the decreased CO₂ partial pressure and the resulting lower equilibrium dissolved amount.

Real-world analogy

If a crowded hallway feeds people into a room while people also leave, increasing the hallway crowd can increase entries. The analogy helps picture a changed balance of exchange, though gas dissolution is a molecular equilibrium governed by pressure and interactions.

Real-world example

Aquariums need dissolved oxygen, which is supplied from air at an oxygen partial pressure rather than at the total atmospheric pressure. Temperature and water movement also matter. A calculation that uses total pressure as if it were all oxygen would overestimate the available driving pressure.

Why?

Why does a carbonated beverage foam after opening? Its dissolved CO₂ amount reflects the earlier higher CO₂ pressure. At the lower outside pressure, the liquid can temporarily hold more CO₂ than its new equilibrium amount, so gas leaves.

Common misconception

“Only total pressure matters.” In a gas mixture, the solubility of a named gas is related principally to that gas's partial pressure under the simple Henry-law model, not the sum of unrelated gas pressures.

Worked example

At a fixed temperature, suppose a gas has equilibrium dissolved concentration 0.0040 mol L⁻¹ when its partial pressure is 0.20 atm. If the system follows proportional Henry behavior, at 0.50 atm its concentration is 0.0040 × (0.50/0.20) = 0.010 mol L⁻¹. This uses the same gas and solvent at the same temperature; it would be invalid to scale by total pressure if the gas's own partial pressure stayed at 0.20 atm.

Quick check

1. Which pressure should be used for oxygen dissolving from air in the simple gas-solubility model? Answer: Use oxygen's partial pressure in the air mixture, not the full atmospheric pressure contributed by all gases.

Exam focus

Write a partial-pressure ratio explicitly. Check that the gas, solvent and temperature are unchanged before using proportionality.

Advanced insight

Henry-law constants use several different conventions. A constant in c = kP has units of concentration per pressure, while a constant defined by P = kc has reciprocal units. Read the equation attached to any given number.

Summary

Gas solubility commonly rises with the gas's partial pressure at fixed temperature. A pressure drop can leave liquid temporarily supersaturated and cause degassing. Use the pressure of the named gas, and treat chemical reactions or changed temperature separately.

Practice questions

1. A gas has c = 0.006 M at partial pressure 0.30 atm. Predict c at 0.15 atm under proportional conditions. Answer: Halving partial pressure halves concentration to 0.003 M at the same temperature and solvent. 2. Air pressure doubles only because nitrogen is added, while oxygen partial pressure remains fixed. Does the simple model double dissolved oxygen? Answer: No. The oxygen pressure driving its dissolution has not changed. 3. Why is a newly opened fizzy drink temporarily above its new gas-solubility limit? Answer: It initially retains dissolved CO₂ established under a higher sealed CO₂ pressure, while the outside pressure is now lower.