Here's why:
* Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. This assumes that the dissolved gas remains chemically unchanged in the solution.
* Chemical reactions can alter the concentration of the dissolved gas, making its solubility deviate from what Henry's Law predicts. For instance:
* Acidic or basic gases: Gases like CO₂ or NH₃ can react with water to form carbonic acid (H₂CO₃) or ammonium hydroxide (NH₄OH), respectively. These reactions consume the dissolved gas, leading to lower solubility than expected by Henry's Law.
* Gases forming complexes: Certain gases, like O₂ or CO, can form complexes with water molecules or dissolved ions. These complexes can affect the free gas concentration in the solution, influencing its solubility.
* Reactions with dissolved species: The dissolved gas may react with other solutes in the water, reducing its concentration and deviating from Henry's Law.
Examples of substances that may not follow Henry's Law:
* Carbon dioxide (CO₂): Reacts with water to form carbonic acid.
* Ammonia (NH₃): Reacts with water to form ammonium hydroxide.
* Sulfur dioxide (SO₂): Reacts with water to form sulfurous acid (H₂SO₃).
* Hydrogen sulfide (H₂S): Can react with water to form hydrosulfuric acid (H₂S).
Note: While many substances can deviate from Henry's Law to some extent, the substances that undergo significant chemical reactions with water are most likely to show the most significant deviations.
