Henry's law states that, at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution. It is applicable only to non-reacting mixtures of liquid and gas. On dissolution, ammonia gas reacts with water to form ammonium hydroxide. Oxygen gas dissolves in unsaturated blood to form oxyhaemoglobin. Therefore, they do not follow Henry's law. O2 gas and CO2 gas do not react with water on dissolution. Therefore, they follow Henry's law.