Metals in acid. Four test tubes containing different metals in hydrochloric acid, showing their relative reactivity. Metals react in acids to form a metal salt and hydrogen gas (bubbles). From left to right: calcium - bubbling wildly, magnesium - bubbling rapidly, zinc - bubbling slowly and copper - not bubbling. Calcium is the most reactive and therefore the reaction is the most vigorous. Copper is far less reactive and therefore does not react readily at room temperature.