Henry’s Law vs. Raoult’s Law: Key Differences & Applications Explained
Henry’s Law states the dissolved gas concentration in a liquid is proportional to the gas’s partial pressure above it. Raoult’s Law says the vapor pressure of a solvent above a solution equals its pure vapor pressure multiplied by its mole fraction in the solution.
Mix-up happens because both predict partial pressures, but Henry’s Law is for trace gases like CO₂ in soda, while Raoult’s Law is for the main solvent like water in antifreeze. The same lab bench, two very different jobs.
Key Differences
Henry: gas → liquid, constant kH, independent of solvent identity. Raoult: liquid → vapor, constant P*, dependent on solvent-solvent bonds. One is linear with pressure; the other is linear with mole fraction. Temperature sensitivity also differs—kH drops as water warms, P* rises exponentially.
Which One Should You Choose?
Carbonating drinks or analyzing blood gases? Use Henry. Distilling vodka or calculating antifreeze boiling points? Pick Raoult. When both solute and solvent are volatile, blend the laws—Raoult for the solvent, Henry for the solute—to get total vapor pressure.
Examples and Daily Life
Pop fizz obeys Henry: open the can, pressure drops, CO₂ leaves. Salted pasta water obeys Raoult: salt lowers water’s vapor pressure, so it boils hotter. Brewers toggle both laws—CO₂ dissolution follows Henry, ethanol evaporation follows Raoult.
Can a single solution follow both laws?
Yes. In champagne, CO₂ follows Henry while ethanol-water follows Raoult.
Why does Henry’s constant vary with temperature?
Warmer liquid holds less gas; kH decreases, so CO₂ escapes faster from warm soda.
What happens if Raoult’s Law fails?
Non-ideal mixtures like acetone-chloroform form extra bonds, vapor pressure drops below prediction, and azeotropes can form.