A student investigates how four metals react with dilute hydrochloric acid. Potassium reacts violently, magnesium reacts quickly, iron reacts slowly, and copper shows no visible reaction. Explain what these observations tell us about the reactivity of these metals and how their reactivity is related to the formation of positive ions.

The observations show that the metals can be ranked in a reactivity series: potassium > magnesium > iron > copper. Potassium reacts violently because it is highly reactive and readily loses an electron to form a +1 ion, so it displaces hydrogen from the acid. Magnesium reacts quickly for the same reason – it forms a +2 ion and is more reactive than hydrogen, so it also displaces H⁺. Iron reacts more slowly because, although it can form a +2 ion, its tendency to lose electrons is weaker than that of magnesium, so it displaces hydrogen but at a slower rate. Copper shows no visible reaction because it is less reactive than hydrogen; it does not form a positive ion readily enough to displace H⁺ from the acid.

This demonstrates that a metal’s reactivity is directly related to its ability to form positive ions: the more readily a metal forms a cation, the higher its position in the reactivity series and the more vigorous its reaction with dilute acid.