Chapter 13 · Solutions

13.4Solutions of Gases in Water

9 min · two checks

Predict

What does Henry’s law say about pressure and dissolved gas?

The idea

Predict gas solubility from pressure and temperature.
Contrast gases with most solids.

Gas solubility rises with the partial pressure of that gas above the liquid. Henry’s law writes concentration = k × P, where k depends on the gas, the solvent, and the temperature. Soda is bottled under high CO₂ pressure. Open it, drop the pressure, and the new equilibrium concentration is lower, so bubbles form. Divers care because nitrogen forced into blood under pressure can come out as bubbles if the ascent is too fast.

Temperature runs the other way from most solids. Warm water holds less dissolved gas than cold water. Fish kills in hot shallow water are partly an oxygen-solubility problem. Do not apply the solid’s “heat it to dissolve more” rule to a gas.

Keep these

Gas solubility is proportional to partial pressure.
Gases dissolve less as temperature rises.
Opening a pressurized bottle drops the equilibrium concentration.

Check yourself

1. Warming a glass of cold tap water makes bubbles appear because

air becomes more solubledissolved air becomes less soluble and leaveswater decomposes into hydrogenthe glass catalyzes boiling at 20 °C

2. Henry’s law says that doubling the partial pressure of a gas above a liquid

halves the dissolved concentrationdoubles the dissolved concentration, ideallyhas no effectfreezes the solvent