Question

If chemical reactivity increases on going down in group 1 then we can say that Francium is the most reactive in group 1 but Potassium is the most reactive element​

Answer 1

The actual fact is that Francium is the most reactive metal. Potassium isn't actually as reactive as Francium(not even close). Second most reactive metal is Ceasium and so on. The Periodic table trends will help you understand this.

Note:

Francium has more tendency to give off electrons than potassium. Thus it is considered more reactive.