Hydrogen gas is generated when acids react with metals.

In this context, a chemical phenomenon known as a displacement reaction occurs. During this reaction, the hydrogen ions from the acid are displaced by the metal. The end products of this reaction are a salt and hydrogen gas. The general equation representing this reaction can be expressed as:

For instance, when hydrochloric acid ($\text{HCl}$) reacts with magnesium ($\text{Mg}$), the resulting products are magnesium chloride ($\text{MgCl}_2$), which is a salt, and hydrogen gas ($\text{H}_2$). The balanced chemical equation for this specific reaction is:

The presence of hydrogen gas can be verified using the ‘pop’ test. In this procedure, a lit splint is brought close to the opening of the test tube where the reaction occurs. If hydrogen gas is indeed present, it will ignite, producing a characteristic ‘pop’ sound. This method serves as a practical and effective means of confirming the presence of hydrogen gas.

It’s essential to understand that not all metals react with acids to produce hydrogen gas. For example, metals such as copper do not react with acids under standard conditions because they are less reactive than hydrogen. The reactivity series of metals is a useful tool for predicting which metals will engage in reactions with acids to produce hydrogen gas. Metals that are positioned higher in the reactivity series than hydrogen will effectively displace hydrogen from the acid, leading to the generation of hydrogen gas.