Video narration
How do we know it is hydrogen? The teacher collects the gas safely in a test tube and brings a burning splint to its mouth. A small pop is heard. Hydrogen burns with this characteristic pop. The test is controlled-never a large explosion. Now compare magnesium. It reacts more rapidly with dilute hydrochloric acid, effervescing strongly as the ribbon dissolves: Mg + 2HCl → MgCl2 + H2. The speed is different, but the gas is the same: hydrogen. Here is the key turn: the bubbles are not just fizz. They are hydrogen displaced from the acid by a metal above hydrogen in the reactivity series. That is why metals above hydrogen generally react with dilute acids: Metal plus acid gives salt plus hydrogen. Copper lies below hydrogen, so it does not react with dilute hydrochloric acid under these conditions. So why were the bubbles appearing? Zinc and magnesium displaced hydrogen, forming salts, and the burning-splint test confirmed the gas by its pop.