When metals react with acids, they produce hydrogen gas (H2). This is because most metals are more reactive than hydrogen, and so they displace hydrogen from the acid. For example, when iron (Fe) reacts with hydrochloric acid (HCl), the following reaction occurs:

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)

In this reaction, the iron atoms displace the hydrogen atoms from the hydrochloric acid, forming iron chloride (FeCl2) and hydrogen gas (H2). The hydrogen gas is released as bubbles, which can be seen if the reaction is carried out in a test tube.

There are a few exceptions to this rule. For example, when aluminium (Al) reacts with concentrated nitric acid (HNO3), it produces nitrogen dioxide gas (NO2) instead of hydrogen gas. This is because aluminium is less reactive than hydrogen, and so it cannot displace hydrogen from nitric acid. However, most metals react with acids to produce hydrogen gas.