Concept 01 / 07Solutions
Solubility of Solids and Gases in Liquids and Henry's Law
Types of solutions, factors affecting solubility of solids (nature, temperature, Le Chatelier) and of gases (pressure and temperature), Henry's law with its constant and units, applications to carbonated drinks, scuba diving and high altitudes, and limitations.
A solution is a homogeneous mixture of two or more components. The component present in the largest amount is usually the solvent; the others are solutes. Binary solutions can be gaseous (air), liquid (gas in liquid: oxygen in water; liquid in liquid: ethanol in water; solid in liquid: glucose in water) or solid (hydrogen in palladium, amalgams, alloys such as brass).
Solubility of a solid in a liquid. "Like dissolves like": polar and ionic solutes dissolve in polar solvents, non-polar solutes in non-polar solvents. A saturated solution is in dynamic equilibrium with undissolved solute. Temperature affects solubility according to Le Chatelier: if dissolution is endothermic (), solubility rises with temperature (most salts, ); if exothermic, it falls (such as , ). Pressure has little effect, since solids and liquids are nearly incompressible.
Solubility of a gas in a liquid. Increases greatly with pressure and decreases with temperature (dissolution of gases is exothermic, and gas molecules escape more easily when hot, which is why aquatic life prefers cold water and why bubbles form in water before it boils).
Henry's law. At constant temperature, the solubility of a gas in a liquid is directly proportional to its partial pressure above the liquid. Stated with mole fraction of the gas in solution:
(see figure).
(Henry's constant, unit of pressure, often kbar) depends on the gas, solvent and temperature. A larger means lower solubility; increases with temperature. At about to in water: , , , , .
Applications.
Soft drinks and soda water are bottled under high pressure; opening the bottle lowers the pressure and the gas bubbles out.
Scuba divers breathing compressed air at depth dissolve more in blood; on rapid ascent it forms bubbles that block capillaries (the bends). Tanks are filled with air diluted by helium (about He, , ), since helium is much less soluble.
At high altitudes the low partial pressure of oxygen reduces its concentration in blood, causing anoxia (weakness, inability to think clearly).
Limitations. Henry's law holds for dilute solutions, low pressures, moderate temperatures, and gases that do not react with or dissociate in the solvent ( and HCl in water deviate strongly).
Key Takeaways:
- Solid solubility follows Le Chatelier with ; pressure has little effect.
- Gas solubility increases with pressure and decreases with temperature.
- ; higher means lower solubility.
- Applications: carbonated drinks, bends, altitude anoxia.
- Pitfall: in , a large means a gas is less soluble, not more.
Brain-Teaser 1: at a partial pressure of is bubbled through water at (). How many millimoles of dissolve in of water?
Solution: . of water is , and , so .
Brain-Teaser 2: The solubility of in water at STP () is . Calculate Henry's constant.
Solution: In of water (): . . Its is far smaller than that of (), so is much more soluble.
Brain-Teaser 3: Why do fish struggle in warm, still water on a summer afternoon?
Solution: Dissolving oxygen is exothermic, so its solubility falls as temperature rises ( increases). Warm water holds less dissolved oxygen, while the fish's metabolic need rises, so they may suffocate.
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