Final answer:
Hydrogen gas is not evolved when metals react with water or nitric acid due to the particular reactions that occur. Metals like platinum do not react with water to produce hydrogen gas, and nitric acid, being an oxidizing acid, yields nitrogen oxides instead of hydrogen gas. Moreover, gases that react chemically with water, such as HCl and NH3, do not follow Henry's law.
Step-by-step explanation:
The reason hydrogen gas is not evolved when metals react with both water and nitric acid can be explained by their specific chemical behaviors. When metals react with water, they often produce hydrogen gas, especially if the metal is active enough, like sodium or zinc. However, not all metals react with water to release hydrogen gas, and platinum is an example of a metal that does not react with water to produce hydrogen gas.
Regarding nitric acid, it is an oxidizing acid that typically does not release hydrogen gas when it reacts with metals. Instead of producing hydrogen gas, the reaction with nitric acid tends to yield various nitrogen oxides (NO, NO2, N2O, etc.) as byproducts. This is due to the oxidation state of hydrogen in nitric acid, which is already at its maximum and as such, it is more favorable for the acid to release oxygen to oxidize the metal, rather than hydrogen.
Additionally, the concept of Henry's law, which describes the solubility of gases in a liquid, indicates that gases that react with water do not obey this law. Gases such as HCl, H2S, and NH3, when dissolved in water, react chemically rather than existing as mere dissolved gases. Therefore, their solubility is not predicted by Henry's law, because they form ions instead of remaining as intact molecules. This concept further illustrates why hydrogen gas is not evolved in such cases.